linux-stable/block/bfq-iosched.h

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/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
* Header file for the BFQ I/O scheduler: data structures and
* prototypes of interface functions among BFQ components.
*/
#ifndef _BFQ_H
#define _BFQ_H
#include <linux/blktrace_api.h>
#include <linux/hrtimer.h>
#include "blk-cgroup-rwstat.h"
#define BFQ_IOPRIO_CLASSES 3
#define BFQ_CL_IDLE_TIMEOUT (HZ/5)
#define BFQ_MIN_WEIGHT 1
#define BFQ_MAX_WEIGHT 1000
#define BFQ_WEIGHT_CONVERSION_COEFF 10
#define BFQ_DEFAULT_QUEUE_IOPRIO 4
#define BFQ_WEIGHT_LEGACY_DFL 100
#define BFQ_DEFAULT_GRP_IOPRIO 0
#define BFQ_DEFAULT_GRP_CLASS IOPRIO_CLASS_BE
#define MAX_BFQQ_NAME_LENGTH 16
/*
* Soft real-time applications are extremely more latency sensitive
* than interactive ones. Over-raise the weight of the former to
* privilege them against the latter.
*/
#define BFQ_SOFTRT_WEIGHT_FACTOR 100
struct bfq_entity;
/**
* struct bfq_service_tree - per ioprio_class service tree.
*
* Each service tree represents a B-WF2Q+ scheduler on its own. Each
* ioprio_class has its own independent scheduler, and so its own
* bfq_service_tree. All the fields are protected by the queue lock
* of the containing bfqd.
*/
struct bfq_service_tree {
/* tree for active entities (i.e., those backlogged) */
struct rb_root active;
/* tree for idle entities (i.e., not backlogged, with V < F_i)*/
struct rb_root idle;
/* idle entity with minimum F_i */
struct bfq_entity *first_idle;
/* idle entity with maximum F_i */
struct bfq_entity *last_idle;
/* scheduler virtual time */
u64 vtime;
/* scheduler weight sum; active and idle entities contribute to it */
unsigned long wsum;
};
/**
* struct bfq_sched_data - multi-class scheduler.
*
* bfq_sched_data is the basic scheduler queue. It supports three
* ioprio_classes, and can be used either as a toplevel queue or as an
block, bfq: consider also in_service_entity to state whether an entity is active Groups of BFQ queues are represented by generic entities in BFQ. When a queue belonging to a parent entity is deactivated, the parent entity may need to be deactivated too, in case the deactivated queue was the only active queue for the parent entity. This deactivation may need to be propagated upwards if the entity belongs, in its turn, to a further higher-level entity, and so on. In particular, the upward propagation of deactivation stops at the first parent entity that remains active even if one of its child entities has been deactivated. To decide whether the last non-deactivation condition holds for a parent entity, BFQ checks whether the field next_in_service is still not NULL for the parent entity, after the deactivation of one of its child entity. If it is not NULL, then there are certainly other active entities in the parent entity, and deactivations can stop. Unfortunately, this check misses a corner case: if in_service_entity is not NULL, then next_in_service may happen to be NULL, although the parent entity is evidently active. This happens if: 1) the entity pointed by in_service_entity is the only active entity in the parent entity, and 2) according to the definition of next_in_service, the in_service_entity cannot be considered as next_in_service. See the comments on the definition of next_in_service for details on this second point. Hitting the above corner case causes crashes. To address this issue, this commit: 1) Extends the above check on only next_in_service to controlling both next_in_service and in_service_entity (if any of them is not NULL, then no further deactivation is performed) 2) Improves the (important) comments on how next_in_service is defined and updated; in particular it fixes a few rather obscure paragraphs Reported-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Reported-by: Rick Yiu <rick_yiu@htc.com> Reported-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Tested-by: Rick Yiu <rick_yiu@htc.com> Tested-by: Laurentiu Nicola <lnicola@dend.ro> Tested-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-29 10:42:56 +00:00
* intermediate queue in a hierarchical setup.
*
* The supported ioprio_classes are the same as in CFQ, in descending
* priority order, IOPRIO_CLASS_RT, IOPRIO_CLASS_BE, IOPRIO_CLASS_IDLE.
* Requests from higher priority queues are served before all the
* requests from lower priority queues; among requests of the same
* queue requests are served according to B-WF2Q+.
block, bfq: consider also in_service_entity to state whether an entity is active Groups of BFQ queues are represented by generic entities in BFQ. When a queue belonging to a parent entity is deactivated, the parent entity may need to be deactivated too, in case the deactivated queue was the only active queue for the parent entity. This deactivation may need to be propagated upwards if the entity belongs, in its turn, to a further higher-level entity, and so on. In particular, the upward propagation of deactivation stops at the first parent entity that remains active even if one of its child entities has been deactivated. To decide whether the last non-deactivation condition holds for a parent entity, BFQ checks whether the field next_in_service is still not NULL for the parent entity, after the deactivation of one of its child entity. If it is not NULL, then there are certainly other active entities in the parent entity, and deactivations can stop. Unfortunately, this check misses a corner case: if in_service_entity is not NULL, then next_in_service may happen to be NULL, although the parent entity is evidently active. This happens if: 1) the entity pointed by in_service_entity is the only active entity in the parent entity, and 2) according to the definition of next_in_service, the in_service_entity cannot be considered as next_in_service. See the comments on the definition of next_in_service for details on this second point. Hitting the above corner case causes crashes. To address this issue, this commit: 1) Extends the above check on only next_in_service to controlling both next_in_service and in_service_entity (if any of them is not NULL, then no further deactivation is performed) 2) Improves the (important) comments on how next_in_service is defined and updated; in particular it fixes a few rather obscure paragraphs Reported-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Reported-by: Rick Yiu <rick_yiu@htc.com> Reported-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Tested-by: Rick Yiu <rick_yiu@htc.com> Tested-by: Laurentiu Nicola <lnicola@dend.ro> Tested-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-29 10:42:56 +00:00
*
* The schedule is implemented by the service trees, plus the field
* @next_in_service, which points to the entity on the active trees
* that will be served next, if 1) no changes in the schedule occurs
* before the current in-service entity is expired, 2) the in-service
* queue becomes idle when it expires, and 3) if the entity pointed by
* in_service_entity is not a queue, then the in-service child entity
* of the entity pointed by in_service_entity becomes idle on
* expiration. This peculiar definition allows for the following
* optimization, not yet exploited: while a given entity is still in
* service, we already know which is the best candidate for next
* service among the other active entities in the same parent
block, bfq: consider also in_service_entity to state whether an entity is active Groups of BFQ queues are represented by generic entities in BFQ. When a queue belonging to a parent entity is deactivated, the parent entity may need to be deactivated too, in case the deactivated queue was the only active queue for the parent entity. This deactivation may need to be propagated upwards if the entity belongs, in its turn, to a further higher-level entity, and so on. In particular, the upward propagation of deactivation stops at the first parent entity that remains active even if one of its child entities has been deactivated. To decide whether the last non-deactivation condition holds for a parent entity, BFQ checks whether the field next_in_service is still not NULL for the parent entity, after the deactivation of one of its child entity. If it is not NULL, then there are certainly other active entities in the parent entity, and deactivations can stop. Unfortunately, this check misses a corner case: if in_service_entity is not NULL, then next_in_service may happen to be NULL, although the parent entity is evidently active. This happens if: 1) the entity pointed by in_service_entity is the only active entity in the parent entity, and 2) according to the definition of next_in_service, the in_service_entity cannot be considered as next_in_service. See the comments on the definition of next_in_service for details on this second point. Hitting the above corner case causes crashes. To address this issue, this commit: 1) Extends the above check on only next_in_service to controlling both next_in_service and in_service_entity (if any of them is not NULL, then no further deactivation is performed) 2) Improves the (important) comments on how next_in_service is defined and updated; in particular it fixes a few rather obscure paragraphs Reported-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Reported-by: Rick Yiu <rick_yiu@htc.com> Reported-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Tested-by: Rick Yiu <rick_yiu@htc.com> Tested-by: Laurentiu Nicola <lnicola@dend.ro> Tested-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-29 10:42:56 +00:00
* entity. We can then quickly compare the timestamps of the
* in-service entity with those of such best candidate.
*
* All fields are protected by the lock of the containing bfqd.
*/
struct bfq_sched_data {
/* entity in service */
struct bfq_entity *in_service_entity;
/* head-of-line entity (see comments above) */
struct bfq_entity *next_in_service;
/* array of service trees, one per ioprio_class */
struct bfq_service_tree service_tree[BFQ_IOPRIO_CLASSES];
/* last time CLASS_IDLE was served */
unsigned long bfq_class_idle_last_service;
};
/**
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
* struct bfq_weight_counter - counter of the number of all active queues
* with a given weight.
*/
struct bfq_weight_counter {
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
unsigned int weight; /* weight of the queues this counter refers to */
unsigned int num_active; /* nr of active queues with this weight */
/*
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
* Weights tree member (see bfq_data's @queue_weights_tree)
*/
struct rb_node weights_node;
};
/**
* struct bfq_entity - schedulable entity.
*
* A bfq_entity is used to represent either a bfq_queue (leaf node in the
* cgroup hierarchy) or a bfq_group into the upper level scheduler. Each
* entity belongs to the sched_data of the parent group in the cgroup
* hierarchy. Non-leaf entities have also their own sched_data, stored
* in @my_sched_data.
*
* Each entity stores independently its priority values; this would
* allow different weights on different devices, but this
* functionality is not exported to userspace by now. Priorities and
* weights are updated lazily, first storing the new values into the
* new_* fields, then setting the @prio_changed flag. As soon as
* there is a transition in the entity state that allows the priority
* update to take place the effective and the requested priority
* values are synchronized.
*
* Unless cgroups are used, the weight value is calculated from the
* ioprio to export the same interface as CFQ. When dealing with
* "well-behaved" queues (i.e., queues that do not spend too much
* time to consume their budget and have true sequential behavior, and
* when there are no external factors breaking anticipation) the
* relative weights at each level of the cgroups hierarchy should be
* guaranteed. All the fields are protected by the queue lock of the
* containing bfqd.
*/
struct bfq_entity {
/* service_tree member */
struct rb_node rb_node;
/*
* Flag, true if the entity is on a tree (either the active or
* the idle one of its service_tree) or is in service.
*/
bool on_st_or_in_serv;
/* B-WF2Q+ start and finish timestamps [sectors/weight] */
u64 start, finish;
/* tree the entity is enqueued into; %NULL if not on a tree */
struct rb_root *tree;
/*
* minimum start time of the (active) subtree rooted at this
* entity; used for O(log N) lookups into active trees
*/
u64 min_start;
/* amount of service received during the last service slot */
int service;
/* budget, used also to calculate F_i: F_i = S_i + @budget / @weight */
int budget;
/* Number of requests allocated in the subtree of this entity */
int allocated;
/* device weight, if non-zero, it overrides the default weight of
* bfq_group_data */
int dev_weight;
/* weight of the queue */
int weight;
/* next weight if a change is in progress */
int new_weight;
/* original weight, used to implement weight boosting */
int orig_weight;
/* parent entity, for hierarchical scheduling */
struct bfq_entity *parent;
/*
* For non-leaf nodes in the hierarchy, the associated
* scheduler queue, %NULL on leaf nodes.
*/
struct bfq_sched_data *my_sched_data;
/* the scheduler queue this entity belongs to */
struct bfq_sched_data *sched_data;
/* flag, set to request a weight, ioprio or ioprio_class change */
int prio_changed;
block, bfq: fix decrement of num_active_groups Since commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")', if there are process groups with I/O requests waiting for completion, then BFQ tags the scenario as 'asymmetric'. This detection is needed for preserving service guarantees (for details, see comments on the computation * of the variable asymmetric_scenario in the function bfq_better_to_idle). Unfortunately, commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")' contains an error exactly in the updating of the number of groups with I/O requests waiting for completion: if a group has more than one descendant process, then the above number of groups, which is renamed from num_active_groups to a more appropriate num_groups_with_pending_reqs by this commit, may happen to be wrongly decremented multiple times, namely every time one of the descendant processes gets all its pending I/O requests completed. A correct, complete solution should work as follows. Consider a group that is inactive, i.e., that has no descendant process with pending I/O inside BFQ queues. Then suppose that num_groups_with_pending_reqs is still accounting for this group, because the group still has some descendant process with some I/O request still in flight. num_groups_with_pending_reqs should be decremented when the in-flight request of the last descendant process is finally completed (assuming that nothing else has changed for the group in the meantime, in terms of composition of the group and active/inactive state of child groups and processes). To accomplish this, an additional pending-request counter must be added to entities, and must be updated correctly. To avoid this additional field and operations, this commit resorts to the following tradeoff between simplicity and accuracy: for an inactive group that is still counted in num_groups_with_pending_reqs, this commit decrements num_groups_with_pending_reqs when the first descendant process of the group remains with no request waiting for completion. This simplified scheme provides a fix to the unbalanced decrements introduced by 2d29c9f89fcd. Since this error was also caused by lack of comments on this non-trivial issue, this commit also adds related comments. Fixes: 2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection") Reported-by: Steven Barrett <steven@liquorix.net> Tested-by: Steven Barrett <steven@liquorix.net> Tested-by: Lucjan Lucjanov <lucjan.lucjanov@gmail.com> Reviewed-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-12-06 18:18:18 +00:00
/* flag, set if the entity is counted in groups_with_pending_reqs */
bool in_groups_with_pending_reqs;
block, bfq: merge bursts of newly-created queues Many throughput-sensitive workloads are made of several parallel I/O flows, with all flows generated by the same application, or more generically by the same task (e.g., system boot). The most counterproductive action with these workloads is plugging I/O dispatch when one of the bfq_queues associated with these flows remains temporarily empty. To avoid this plugging, BFQ has been using a burst-handling mechanism for years now. This mechanism has proven effective for throughput, and not detrimental for service guarantees. This commit pushes this mechanism a little bit further, basing on the following two facts. First, all the I/O flows of a the same application or task contribute to the execution/completion of that common application or task. So the performance figures that matter are total throughput of the flows and task-wide I/O latency. In particular, these flows do not need to be protected from each other, in terms of individual bandwidth or latency. Second, the above fact holds regardless of the number of flows. Putting these two facts together, this commits merges stably the bfq_queues associated with these I/O flows, i.e., with the processes that generate these IO/ flows, regardless of how many the involved processes are. To decide whether a set of bfq_queues is actually associated with the I/O flows of a common application or task, and to merge these queues stably, this commit operates as follows: given a bfq_queue, say Q2, currently being created, and the last bfq_queue, say Q1, created before Q2, Q2 is merged stably with Q1 if - very little time has elapsed since when Q1 was created - Q2 has the same ioprio as Q1 - Q2 belongs to the same group as Q1 Merging bfq_queues also reduces scheduling overhead. A fio test with ten random readers on /dev/nullb shows a throughput boost of 40%, with a quadcore. Since BFQ's execution time amounts to ~50% of the total per-request processing time, the above throughput boost implies that BFQ's overhead is reduced by more than 50%. Tested-by: Jan Kara <jack@suse.cz> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Link: https://lore.kernel.org/r/20210304174627.161-7-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-03-04 17:46:27 +00:00
/* last child queue of entity created (for non-leaf entities) */
struct bfq_queue *last_bfqq_created;
};
struct bfq_group;
/**
* struct bfq_ttime - per process thinktime stats.
*/
struct bfq_ttime {
/* completion time of the last request */
u64 last_end_request;
/* total process thinktime */
u64 ttime_total;
/* number of thinktime samples */
unsigned long ttime_samples;
/* average process thinktime */
u64 ttime_mean;
};
/**
* struct bfq_queue - leaf schedulable entity.
*
* A bfq_queue is a leaf request queue; it can be associated with an
* io_context or more, if it is async or shared between cooperating
* processes. @cgroup holds a reference to the cgroup, to be sure that it
* does not disappear while a bfqq still references it (mostly to avoid
* races between request issuing and task migration followed by cgroup
* destruction).
* All the fields are protected by the queue lock of the containing bfqd.
*/
struct bfq_queue {
/* reference counter */
int ref;
block, bfq: merge bursts of newly-created queues Many throughput-sensitive workloads are made of several parallel I/O flows, with all flows generated by the same application, or more generically by the same task (e.g., system boot). The most counterproductive action with these workloads is plugging I/O dispatch when one of the bfq_queues associated with these flows remains temporarily empty. To avoid this plugging, BFQ has been using a burst-handling mechanism for years now. This mechanism has proven effective for throughput, and not detrimental for service guarantees. This commit pushes this mechanism a little bit further, basing on the following two facts. First, all the I/O flows of a the same application or task contribute to the execution/completion of that common application or task. So the performance figures that matter are total throughput of the flows and task-wide I/O latency. In particular, these flows do not need to be protected from each other, in terms of individual bandwidth or latency. Second, the above fact holds regardless of the number of flows. Putting these two facts together, this commits merges stably the bfq_queues associated with these I/O flows, i.e., with the processes that generate these IO/ flows, regardless of how many the involved processes are. To decide whether a set of bfq_queues is actually associated with the I/O flows of a common application or task, and to merge these queues stably, this commit operates as follows: given a bfq_queue, say Q2, currently being created, and the last bfq_queue, say Q1, created before Q2, Q2 is merged stably with Q1 if - very little time has elapsed since when Q1 was created - Q2 has the same ioprio as Q1 - Q2 belongs to the same group as Q1 Merging bfq_queues also reduces scheduling overhead. A fio test with ten random readers on /dev/nullb shows a throughput boost of 40%, with a quadcore. Since BFQ's execution time amounts to ~50% of the total per-request processing time, the above throughput boost implies that BFQ's overhead is reduced by more than 50%. Tested-by: Jan Kara <jack@suse.cz> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Link: https://lore.kernel.org/r/20210304174627.161-7-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-03-04 17:46:27 +00:00
/* counter of references from other queues for delayed stable merge */
int stable_ref;
/* parent bfq_data */
struct bfq_data *bfqd;
/* current ioprio and ioprio class */
unsigned short ioprio, ioprio_class;
/* next ioprio and ioprio class if a change is in progress */
unsigned short new_ioprio, new_ioprio_class;
block, bfq: tune service injection basing on request service times The processes associated with a bfq_queue, say Q, may happen to generate their cumulative I/O at a lower rate than the rate at which the device could serve the same I/O. This is rather probable, e.g., if only one process is associated with Q and the device is an SSD. It results in Q becoming often empty while in service. If BFQ is not allowed to switch to another queue when Q becomes empty, then, during the service of Q, there will be frequent "service holes", i.e., time intervals during which Q gets empty and the device can only consume the I/O already queued in its hardware queues. This easily causes considerable losses of throughput. To counter this problem, BFQ implements a request injection mechanism, which tries to fill the above service holes with I/O requests taken from other bfq_queues. The hard part in this mechanism is finding the right amount of I/O to inject, so as to both boost throughput and not break Q's bandwidth and latency guarantees. To this goal, the current version of this mechanism measures the bandwidth enjoyed by Q while it is being served, and tries to inject the maximum possible amount of extra service that does not cause Q's bandwidth to decrease too much. This solution has an important shortcoming. For bandwidth measurements to be stable and reliable, Q must remain in service for a much longer time than that needed to serve a single I/O request. Unfortunately, this does not hold with many workloads. This commit addresses this issue by changing the way the amount of injection allowed is dynamically computed. It tunes injection as a function of the service times of single I/O requests of Q, instead of Q's bandwidth. Single-request service times are evidently meaningful even if Q gets very few I/O requests completed while it is in service. As a testbed for this new solution, we measured the throughput reached by BFQ for one of the nastiest workloads and configurations for this scheduler: the workload generated by the dbench test (in the Phoronix suite), with 6 clients, on a filesystem with journaling, and with the journaling daemon enjoying a higher weight than normal processes. With this commit, the throughput grows from ~100 MB/s to ~150 MB/s on a PLEXTOR PX-256M5. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Francesco Pollicino <fra.fra.800@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:29 +00:00
/* last total-service-time sample, see bfq_update_inject_limit() */
u64 last_serv_time_ns;
/* limit for request injection */
unsigned int inject_limit;
/* last time the inject limit has been decreased, in jiffies */
unsigned long decrease_time_jif;
/*
* Shared bfq_queue if queue is cooperating with one or more
* other queues.
*/
struct bfq_queue *new_bfqq;
/* request-position tree member (see bfq_group's @rq_pos_tree) */
struct rb_node pos_node;
/* request-position tree root (see bfq_group's @rq_pos_tree) */
struct rb_root *pos_root;
/* sorted list of pending requests */
struct rb_root sort_list;
/* if fifo isn't expired, next request to serve */
struct request *next_rq;
/* number of sync and async requests queued */
int queued[2];
/* number of pending metadata requests */
int meta_pending;
/* fifo list of requests in sort_list */
struct list_head fifo;
/* entity representing this queue in the scheduler */
struct bfq_entity entity;
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
/* pointer to the weight counter associated with this entity */
struct bfq_weight_counter *weight_counter;
/* maximum budget allowed from the feedback mechanism */
int max_budget;
/* budget expiration (in jiffies) */
unsigned long budget_timeout;
/* number of requests on the dispatch list or inside driver */
int dispatched;
/* status flags */
unsigned long flags;
/* node for active/idle bfqq list inside parent bfqd */
struct list_head bfqq_list;
/* associated @bfq_ttime struct */
struct bfq_ttime ttime;
/* when bfqq started to do I/O within the last observation window */
u64 io_start_time;
/* how long bfqq has remained empty during the last observ. window */
u64 tot_idle_time;
/* bit vector: a 1 for each seeky requests in history */
u32 seek_history;
/* node for the device's burst list */
struct hlist_node burst_list_node;
/* position of the last request enqueued */
sector_t last_request_pos;
/* Number of consecutive pairs of request completion and
* arrival, such that the queue becomes idle after the
* completion, but the next request arrives within an idle
* time slice; used only if the queue's IO_bound flag has been
* cleared.
*/
unsigned int requests_within_timer;
/* pid of the process owning the queue, used for logging purposes */
pid_t pid;
/*
* Pointer to the bfq_io_cq owning the bfq_queue, set to %NULL
* if the queue is shared.
*/
struct bfq_io_cq *bic;
/* current maximum weight-raising time for this queue */
unsigned long wr_cur_max_time;
/*
* Minimum time instant such that, only if a new request is
* enqueued after this time instant in an idle @bfq_queue with
* no outstanding requests, then the task associated with the
* queue it is deemed as soft real-time (see the comments on
* the function bfq_bfqq_softrt_next_start())
*/
unsigned long soft_rt_next_start;
/*
* Start time of the current weight-raising period if
* the @bfq-queue is being weight-raised, otherwise
* finish time of the last weight-raising period.
*/
unsigned long last_wr_start_finish;
/* factor by which the weight of this queue is multiplied */
unsigned int wr_coeff;
/*
* Time of the last transition of the @bfq_queue from idle to
* backlogged.
*/
unsigned long last_idle_bklogged;
/*
* Cumulative service received from the @bfq_queue since the
* last transition from idle to backlogged.
*/
unsigned long service_from_backlogged;
block, bfq: limit sectors served with interactive weight raising To maximise responsiveness, BFQ raises the weight, and performs device idling, for bfq_queues associated with processes deemed as interactive. In particular, weight raising has a maximum duration, equal to the time needed to start a large application. If a weight-raised process goes on doing I/O beyond this maximum duration, it loses weight-raising. This mechanism is evidently vulnerable to the following false positives: I/O-bound applications that will go on doing I/O for much longer than the duration of weight-raising. These applications have basically no benefit from being weight-raised at the beginning of their I/O. On the opposite end, while being weight-raised, these applications a) unjustly steal throughput to applications that may truly need low latency; b) make BFQ uselessly perform device idling; device idling results in loss of device throughput with most flash-based storage, and may increase latencies when used purposelessly. This commit adds a countermeasure to reduce both the above problems. To introduce this countermeasure, we provide the following extra piece of information (full details in the comments added by this commit). During the start-up of the large application used as a reference to set the duration of weight-raising, involved processes transfer at most ~110K sectors each. Accordingly, a process initially deemed as interactive has no right to be weight-raised any longer, once transferred 110K sectors or more. Basing on this consideration, this commit early-ends weight-raising for a bfq_queue if the latter happens to have received an amount of service at least equal to 110K sectors (actually, a little bit more, to keep a safety margin). I/O-bound applications that reach a high throughput, such as file copy, get to this threshold much before the allowed weight-raising period finishes. Thus this early ending of weight-raising reduces the amount of time during which these applications cause the problems described above. Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-01-13 11:05:18 +00:00
/*
* Cumulative service received from the @bfq_queue since its
* last transition to weight-raised state.
*/
unsigned long service_from_wr;
/*
* Value of wr start time when switching to soft rt
*/
unsigned long wr_start_at_switch_to_srt;
unsigned long split_time; /* time of last split */
block, bfq: let a queue be merged only shortly after starting I/O In BFQ and CFQ, two processes are said to be cooperating if they do I/O in such a way that the union of their I/O requests yields a sequential I/O pattern. To get such a sequential I/O pattern out of the non-sequential pattern of each cooperating process, BFQ and CFQ merge the queues associated with these processes. In more detail, cooperating processes, and thus their associated queues, usually start, or restart, to do I/O shortly after each other. This is the case, e.g., for the I/O threads of KVM/QEMU and of the dump utility. Basing on this assumption, this commit allows a bfq_queue to be merged only during a short time interval (100ms) after it starts, or re-starts, to do I/O. This filtering provides two important benefits. First, it greatly reduces the probability that two non-cooperating processes have their queues merged by mistake, if they just happen to do I/O close to each other for a short time interval. These spurious merges cause loss of service guarantees. A low-weight bfq_queue may unjustly get more than its expected share of the throughput: if such a low-weight queue is merged with a high-weight queue, then the I/O for the low-weight queue is served as if the queue had a high weight. This may damage other high-weight queues unexpectedly. For instance, because of this issue, lxterminal occasionally took 7.5 seconds to start, instead of 6.5 seconds, when some sequential readers and writers did I/O in the background on a FUJITSU MHX2300BT HDD. The reason is that the bfq_queues associated with some of the readers or the writers were merged with the high-weight queues of some processes that had to do some urgent but little I/O. The readers then exploited the inherited high weight for all or most of their I/O, during the start-up of terminal. The filtering introduced by this commit eliminated any outlier caused by spurious queue merges in our start-up time tests. This filtering also provides a little boost of the throughput sustainable by BFQ: 3-4%, depending on the CPU. The reason is that, once a bfq_queue cannot be merged any longer, this commit makes BFQ stop updating the data needed to handle merging for the queue. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-12-20 11:38:33 +00:00
unsigned long first_IO_time; /* time of first I/O for this queue */
block, bfq: inject other-queue I/O into seeky idle queues on NCQ flash The Achilles' heel of BFQ is its failing to reach a high throughput with sync random I/O on flash storage with internal queueing, in case the processes doing I/O have differentiated weights. The cause of this failure is as follows. If at least two processes do sync I/O, and have a different weight from each other, then BFQ plugs I/O dispatching every time one of these processes, while it is being served, remains temporarily without pending I/O requests. This plugging is necessary to guarantee that every process enjoys a bandwidth proportional to its weight; but it empties the internal queue(s) of the drive. And this kills throughput with random I/O. So, if some processes have differentiated weights and do both sync and random I/O, the end result is a throughput collapse. This commit tries to counter this problem by injecting the service of other processes, in a controlled way, while the process in service happens to have no I/O. This injection is performed only if the medium is non rotational and performs internal queueing, and the process in service does random I/O (service injection might be beneficial for sequential I/O too, we'll work on that). As an example of the benefits of this commit, on a PLEXTOR PX-256M5S SSD, and with five processes having differentiated weights and doing sync random 4KB I/O, this commit makes the throughput with bfq grow by 400%, from 25 to 100MB/s. This higher throughput is 10MB/s lower than that reached with none. As some less random I/O is added to the mix, the throughput becomes equal to or higher than that with none. This commit is a very first attempt to recover throughput without losing control, and certainly has many limitations. One is, e.g., that the processes whose service is injected are not chosen so as to distribute the extra bandwidth they receive in accordance to their weights. Thus there might be loss of weighted fairness in some cases. Anyway, this loss concerns extra service, which would not have been received at all without this commit. Other limitations and issues will probably show up with usage. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-09-14 14:23:08 +00:00
block, bfq: merge bursts of newly-created queues Many throughput-sensitive workloads are made of several parallel I/O flows, with all flows generated by the same application, or more generically by the same task (e.g., system boot). The most counterproductive action with these workloads is plugging I/O dispatch when one of the bfq_queues associated with these flows remains temporarily empty. To avoid this plugging, BFQ has been using a burst-handling mechanism for years now. This mechanism has proven effective for throughput, and not detrimental for service guarantees. This commit pushes this mechanism a little bit further, basing on the following two facts. First, all the I/O flows of a the same application or task contribute to the execution/completion of that common application or task. So the performance figures that matter are total throughput of the flows and task-wide I/O latency. In particular, these flows do not need to be protected from each other, in terms of individual bandwidth or latency. Second, the above fact holds regardless of the number of flows. Putting these two facts together, this commits merges stably the bfq_queues associated with these I/O flows, i.e., with the processes that generate these IO/ flows, regardless of how many the involved processes are. To decide whether a set of bfq_queues is actually associated with the I/O flows of a common application or task, and to merge these queues stably, this commit operates as follows: given a bfq_queue, say Q2, currently being created, and the last bfq_queue, say Q1, created before Q2, Q2 is merged stably with Q1 if - very little time has elapsed since when Q1 was created - Q2 has the same ioprio as Q1 - Q2 belongs to the same group as Q1 Merging bfq_queues also reduces scheduling overhead. A fio test with ten random readers on /dev/nullb shows a throughput boost of 40%, with a quadcore. Since BFQ's execution time amounts to ~50% of the total per-request processing time, the above throughput boost implies that BFQ's overhead is reduced by more than 50%. Tested-by: Jan Kara <jack@suse.cz> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Link: https://lore.kernel.org/r/20210304174627.161-7-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-03-04 17:46:27 +00:00
unsigned long creation_time; /* when this queue is created */
block, bfq: inject other-queue I/O into seeky idle queues on NCQ flash The Achilles' heel of BFQ is its failing to reach a high throughput with sync random I/O on flash storage with internal queueing, in case the processes doing I/O have differentiated weights. The cause of this failure is as follows. If at least two processes do sync I/O, and have a different weight from each other, then BFQ plugs I/O dispatching every time one of these processes, while it is being served, remains temporarily without pending I/O requests. This plugging is necessary to guarantee that every process enjoys a bandwidth proportional to its weight; but it empties the internal queue(s) of the drive. And this kills throughput with random I/O. So, if some processes have differentiated weights and do both sync and random I/O, the end result is a throughput collapse. This commit tries to counter this problem by injecting the service of other processes, in a controlled way, while the process in service happens to have no I/O. This injection is performed only if the medium is non rotational and performs internal queueing, and the process in service does random I/O (service injection might be beneficial for sequential I/O too, we'll work on that). As an example of the benefits of this commit, on a PLEXTOR PX-256M5S SSD, and with five processes having differentiated weights and doing sync random 4KB I/O, this commit makes the throughput with bfq grow by 400%, from 25 to 100MB/s. This higher throughput is 10MB/s lower than that reached with none. As some less random I/O is added to the mix, the throughput becomes equal to or higher than that with none. This commit is a very first attempt to recover throughput without losing control, and certainly has many limitations. One is, e.g., that the processes whose service is injected are not chosen so as to distribute the extra bandwidth they receive in accordance to their weights. Thus there might be loss of weighted fairness in some cases. Anyway, this loss concerns extra service, which would not have been received at all without this commit. Other limitations and issues will probably show up with usage. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-09-14 14:23:08 +00:00
/* max service rate measured so far */
u32 max_service_rate;
block, bfq: detect wakers and unconditionally inject their I/O A bfq_queue Q may happen to be synchronized with another bfq_queue Q2, i.e., the I/O of Q2 may need to be completed for Q to receive new I/O. We call Q2 "waker queue". If I/O plugging is being performed for Q, and Q is not receiving any more I/O because of the above synchronization, then, thanks to BFQ's injection mechanism, the waker queue is likely to get served before the I/O-plugging timeout fires. Unfortunately, this fact may not be sufficient to guarantee a high throughput during the I/O plugging, because the inject limit for Q may be too low to guarantee a lot of injected I/O. In addition, the duration of the plugging, i.e., the time before Q finally receives new I/O, may not be minimized, because the waker queue may happen to be served only after other queues. To address these issues, this commit introduces the explicit detection of the waker queue, and the unconditional injection of a pending I/O request of the waker queue on each invocation of bfq_dispatch_request(). One may be concerned that this systematic injection of I/O from the waker queue delays the service of Q's I/O. Fortunately, it doesn't. On the contrary, next Q's I/O is brought forward dramatically, for it is not blocked for milliseconds. Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-06-25 05:12:47 +00:00
/*
* Pointer to the waker queue for this queue, i.e., to the
* queue Q such that this queue happens to get new I/O right
* after some I/O request of Q is completed. For details, see
* the comments on the choice of the queue for injection in
* bfq_select_queue().
*/
struct bfq_queue *waker_bfqq;
/* pointer to the curr. tentative waker queue, see bfq_check_waker() */
struct bfq_queue *tentative_waker_bfqq;
/* number of times the same tentative waker has been detected */
unsigned int num_waker_detections;
bfq: Limit waker detection in time Currently, when process A starts issuing requests shortly after process B has completed some IO three times in a row, we decide that B is a "waker" of A meaning that completing IO of B is needed for A to make progress and generally stop separating A's and B's IO much. This logic is useful to avoid unnecessary idling and thus throughput loss for cases where workload needs to switch e.g. between the process and the journaling thread doing IO. However the detection heuristic tends to frequently give false positives when A and B are fighting IO bandwidth and other processes aren't doing much IO as we are basically deemed to eventually accumulate three occurences of a situation where one process starts issuing requests after the other has completed some IO. To reduce these false positives, cancel the waker detection also if we didn't accumulate three detected wakeups within given timeout. The rationale is that if wakeups are really rare, the pointless idling doesn't hurt throughput that much anyway. This significantly reduces false waker detection for workload like: [global] directory=/mnt/repro/ rw=write size=8g time_based runtime=30 ramp_time=10 blocksize=1m direct=0 ioengine=sync [slowwriter] numjobs=1 fsync=200 [fastwriter] numjobs=1 fsync=200 Acked-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jan Kara <jack@suse.cz> Link: https://lore.kernel.org/r/20211125133645.27483-5-jack@suse.cz Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-11-25 13:36:38 +00:00
/* time when we started considering this waker */
u64 waker_detection_started;
block, bfq: detect wakers and unconditionally inject their I/O A bfq_queue Q may happen to be synchronized with another bfq_queue Q2, i.e., the I/O of Q2 may need to be completed for Q to receive new I/O. We call Q2 "waker queue". If I/O plugging is being performed for Q, and Q is not receiving any more I/O because of the above synchronization, then, thanks to BFQ's injection mechanism, the waker queue is likely to get served before the I/O-plugging timeout fires. Unfortunately, this fact may not be sufficient to guarantee a high throughput during the I/O plugging, because the inject limit for Q may be too low to guarantee a lot of injected I/O. In addition, the duration of the plugging, i.e., the time before Q finally receives new I/O, may not be minimized, because the waker queue may happen to be served only after other queues. To address these issues, this commit introduces the explicit detection of the waker queue, and the unconditional injection of a pending I/O request of the waker queue on each invocation of bfq_dispatch_request(). One may be concerned that this systematic injection of I/O from the waker queue delays the service of Q's I/O. Fortunately, it doesn't. On the contrary, next Q's I/O is brought forward dramatically, for it is not blocked for milliseconds. Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-06-25 05:12:47 +00:00
/* node for woken_list, see below */
struct hlist_node woken_list_node;
/*
* Head of the list of the woken queues for this queue, i.e.,
* of the list of the queues for which this queue is a waker
* queue. This list is used to reset the waker_bfqq pointer in
* the woken queues when this queue exits.
*/
struct hlist_head woken_list;
};
/**
* struct bfq_io_cq - per (request_queue, io_context) structure.
*/
struct bfq_io_cq {
/* associated io_cq structure */
struct io_cq icq; /* must be the first member */
/* array of two process queues, the sync and the async */
struct bfq_queue *bfqq[2];
/* per (request_queue, blkcg) ioprio */
int ioprio;
#ifdef CONFIG_BFQ_GROUP_IOSCHED
uint64_t blkcg_serial_nr; /* the current blkcg serial */
#endif
/*
* Snapshot of the has_short_time flag before merging; taken
* to remember its value while the queue is merged, so as to
* be able to restore it in case of split.
*/
bool saved_has_short_ttime;
/*
* Same purpose as the previous two fields for the I/O bound
* classification of a queue.
*/
bool saved_IO_bound;
u64 saved_io_start_time;
u64 saved_tot_idle_time;
/*
* Same purpose as the previous fields for the value of the
* field keeping the queue's belonging to a large burst
*/
bool saved_in_large_burst;
/*
* True if the queue belonged to a burst list before its merge
* with another cooperating queue.
*/
bool was_in_burst_list;
/*
* Save the weight when a merge occurs, to be able
* to restore it in case of split. If the weight is not
* correctly resumed when the queue is recycled,
* then the weight of the recycled queue could differ
* from the weight of the original queue.
*/
unsigned int saved_weight;
/*
* Similar to previous fields: save wr information.
*/
unsigned long saved_wr_coeff;
unsigned long saved_last_wr_start_finish;
unsigned long saved_service_from_wr;
unsigned long saved_wr_start_at_switch_to_srt;
unsigned int saved_wr_cur_max_time;
struct bfq_ttime saved_ttime;
/* Save also injection state */
u64 saved_last_serv_time_ns;
unsigned int saved_inject_limit;
unsigned long saved_decrease_time_jif;
block, bfq: merge bursts of newly-created queues Many throughput-sensitive workloads are made of several parallel I/O flows, with all flows generated by the same application, or more generically by the same task (e.g., system boot). The most counterproductive action with these workloads is plugging I/O dispatch when one of the bfq_queues associated with these flows remains temporarily empty. To avoid this plugging, BFQ has been using a burst-handling mechanism for years now. This mechanism has proven effective for throughput, and not detrimental for service guarantees. This commit pushes this mechanism a little bit further, basing on the following two facts. First, all the I/O flows of a the same application or task contribute to the execution/completion of that common application or task. So the performance figures that matter are total throughput of the flows and task-wide I/O latency. In particular, these flows do not need to be protected from each other, in terms of individual bandwidth or latency. Second, the above fact holds regardless of the number of flows. Putting these two facts together, this commits merges stably the bfq_queues associated with these I/O flows, i.e., with the processes that generate these IO/ flows, regardless of how many the involved processes are. To decide whether a set of bfq_queues is actually associated with the I/O flows of a common application or task, and to merge these queues stably, this commit operates as follows: given a bfq_queue, say Q2, currently being created, and the last bfq_queue, say Q1, created before Q2, Q2 is merged stably with Q1 if - very little time has elapsed since when Q1 was created - Q2 has the same ioprio as Q1 - Q2 belongs to the same group as Q1 Merging bfq_queues also reduces scheduling overhead. A fio test with ten random readers on /dev/nullb shows a throughput boost of 40%, with a quadcore. Since BFQ's execution time amounts to ~50% of the total per-request processing time, the above throughput boost implies that BFQ's overhead is reduced by more than 50%. Tested-by: Jan Kara <jack@suse.cz> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Link: https://lore.kernel.org/r/20210304174627.161-7-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-03-04 17:46:27 +00:00
/* candidate queue for a stable merge (due to close creation time) */
struct bfq_queue *stable_merge_bfqq;
bool stably_merged; /* non splittable if true */
};
/**
* struct bfq_data - per-device data structure.
*
* All the fields are protected by @lock.
*/
struct bfq_data {
/* device request queue */
struct request_queue *queue;
/* dispatch queue */
struct list_head dispatch;
/* root bfq_group for the device */
struct bfq_group *root_group;
/*
* rbtree of weight counters of @bfq_queues, sorted by
* weight. Used to keep track of whether all @bfq_queues have
* the same weight. The tree contains one counter for each
* distinct weight associated to some active and not
* weight-raised @bfq_queue (see the comments to the functions
* bfq_weights_tree_[add|remove] for further details).
*/
block, bfq: do not idle for lowest-weight queues In most cases, it is detrimental for throughput to plug I/O dispatch when the in-service bfq_queue becomes temporarily empty (plugging is performed to wait for the possible arrival, soon, of new I/O from the in-service queue). There is however a case where plugging is needed for service guarantees. If a bfq_queue, say Q, has a higher weight than some other active bfq_queue, and is sync, i.e., contains sync I/O, then, to guarantee that Q does receive a higher share of the throughput than other lower-weight queues, it is necessary to plug I/O dispatch when Q remains temporarily empty while being served. For this reason, BFQ performs I/O plugging when some active bfq_queue has a higher weight than some other active bfq_queue. But this is unnecessarily overkill. In fact, if the in-service bfq_queue actually has a weight lower than or equal to the other queues, then the queue *must not* be guaranteed a higher share of the throughput than the other queues. So, not plugging I/O cannot cause any harm to the queue. And can boost throughput. Taking advantage of this fact, this commit does not plug I/O for sync bfq_queues with a weight lower than or equal to the weights of the other queues. Here is an example of the resulting throughput boost with the dbench workload, which is particularly nasty for BFQ. With the dbench test in the Phoronix suite, BFQ reaches its lowest total throughput with 6 clients on a filesystem with journaling, in case the journaling daemon has a higher weight than normal processes. Before this commit, the total throughput was ~80 MB/sec on a PLEXTOR PX-256M5, after this commit it is ~100 MB/sec. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:28 +00:00
struct rb_root_cached queue_weights_tree;
block, bfq: fix decrement of num_active_groups Since commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")', if there are process groups with I/O requests waiting for completion, then BFQ tags the scenario as 'asymmetric'. This detection is needed for preserving service guarantees (for details, see comments on the computation * of the variable asymmetric_scenario in the function bfq_better_to_idle). Unfortunately, commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")' contains an error exactly in the updating of the number of groups with I/O requests waiting for completion: if a group has more than one descendant process, then the above number of groups, which is renamed from num_active_groups to a more appropriate num_groups_with_pending_reqs by this commit, may happen to be wrongly decremented multiple times, namely every time one of the descendant processes gets all its pending I/O requests completed. A correct, complete solution should work as follows. Consider a group that is inactive, i.e., that has no descendant process with pending I/O inside BFQ queues. Then suppose that num_groups_with_pending_reqs is still accounting for this group, because the group still has some descendant process with some I/O request still in flight. num_groups_with_pending_reqs should be decremented when the in-flight request of the last descendant process is finally completed (assuming that nothing else has changed for the group in the meantime, in terms of composition of the group and active/inactive state of child groups and processes). To accomplish this, an additional pending-request counter must be added to entities, and must be updated correctly. To avoid this additional field and operations, this commit resorts to the following tradeoff between simplicity and accuracy: for an inactive group that is still counted in num_groups_with_pending_reqs, this commit decrements num_groups_with_pending_reqs when the first descendant process of the group remains with no request waiting for completion. This simplified scheme provides a fix to the unbalanced decrements introduced by 2d29c9f89fcd. Since this error was also caused by lack of comments on this non-trivial issue, this commit also adds related comments. Fixes: 2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection") Reported-by: Steven Barrett <steven@liquorix.net> Tested-by: Steven Barrett <steven@liquorix.net> Tested-by: Lucjan Lucjanov <lucjan.lucjanov@gmail.com> Reviewed-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-12-06 18:18:18 +00:00
/*
block, bfq: fix decrement of num_active_groups Since commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")', if there are process groups with I/O requests waiting for completion, then BFQ tags the scenario as 'asymmetric'. This detection is needed for preserving service guarantees (for details, see comments on the computation * of the variable asymmetric_scenario in the function bfq_better_to_idle). Unfortunately, commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")' contains an error exactly in the updating of the number of groups with I/O requests waiting for completion: if a group has more than one descendant process, then the above number of groups, which is renamed from num_active_groups to a more appropriate num_groups_with_pending_reqs by this commit, may happen to be wrongly decremented multiple times, namely every time one of the descendant processes gets all its pending I/O requests completed. A correct, complete solution should work as follows. Consider a group that is inactive, i.e., that has no descendant process with pending I/O inside BFQ queues. Then suppose that num_groups_with_pending_reqs is still accounting for this group, because the group still has some descendant process with some I/O request still in flight. num_groups_with_pending_reqs should be decremented when the in-flight request of the last descendant process is finally completed (assuming that nothing else has changed for the group in the meantime, in terms of composition of the group and active/inactive state of child groups and processes). To accomplish this, an additional pending-request counter must be added to entities, and must be updated correctly. To avoid this additional field and operations, this commit resorts to the following tradeoff between simplicity and accuracy: for an inactive group that is still counted in num_groups_with_pending_reqs, this commit decrements num_groups_with_pending_reqs when the first descendant process of the group remains with no request waiting for completion. This simplified scheme provides a fix to the unbalanced decrements introduced by 2d29c9f89fcd. Since this error was also caused by lack of comments on this non-trivial issue, this commit also adds related comments. Fixes: 2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection") Reported-by: Steven Barrett <steven@liquorix.net> Tested-by: Steven Barrett <steven@liquorix.net> Tested-by: Lucjan Lucjanov <lucjan.lucjanov@gmail.com> Reviewed-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-12-06 18:18:18 +00:00
* Number of groups with at least one descendant process that
* has at least one request waiting for completion. Note that
* this accounts for also requests already dispatched, but not
* yet completed. Therefore this number of groups may differ
* (be larger) than the number of active groups, as a group is
* considered active only if its corresponding entity has
* descendant queues with at least one request queued. This
* number is used to decide whether a scenario is symmetric.
* For a detailed explanation see comments on the computation
* of the variable asymmetric_scenario in the function
* bfq_better_to_idle().
*
* However, it is hard to compute this number exactly, for
* groups with multiple descendant processes. Consider a group
* that is inactive, i.e., that has no descendant process with
* pending I/O inside BFQ queues. Then suppose that
* num_groups_with_pending_reqs is still accounting for this
* group, because the group has descendant processes with some
* I/O request still in flight. num_groups_with_pending_reqs
* should be decremented when the in-flight request of the
* last descendant process is finally completed (assuming that
* nothing else has changed for the group in the meantime, in
* terms of composition of the group and active/inactive state of child
* groups and processes). To accomplish this, an additional
* pending-request counter must be added to entities, and must
* be updated correctly. To avoid this additional field and operations,
* we resort to the following tradeoff between simplicity and
* accuracy: for an inactive group that is still counted in
* num_groups_with_pending_reqs, we decrement
* num_groups_with_pending_reqs when the first descendant
* process of the group remains with no request waiting for
* completion.
*
* Even this simpler decrement strategy requires a little
* carefulness: to avoid multiple decrements, we flag a group,
* more precisely an entity representing a group, as still
* counted in num_groups_with_pending_reqs when it becomes
* inactive. Then, when the first descendant queue of the
* entity remains with no request waiting for completion,
* num_groups_with_pending_reqs is decremented, and this flag
* is reset. After this flag is reset for the entity,
* num_groups_with_pending_reqs won't be decremented any
* longer in case a new descendant queue of the entity remains
* with no request waiting for completion.
*/
block, bfq: fix decrement of num_active_groups Since commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")', if there are process groups with I/O requests waiting for completion, then BFQ tags the scenario as 'asymmetric'. This detection is needed for preserving service guarantees (for details, see comments on the computation * of the variable asymmetric_scenario in the function bfq_better_to_idle). Unfortunately, commit '2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection")' contains an error exactly in the updating of the number of groups with I/O requests waiting for completion: if a group has more than one descendant process, then the above number of groups, which is renamed from num_active_groups to a more appropriate num_groups_with_pending_reqs by this commit, may happen to be wrongly decremented multiple times, namely every time one of the descendant processes gets all its pending I/O requests completed. A correct, complete solution should work as follows. Consider a group that is inactive, i.e., that has no descendant process with pending I/O inside BFQ queues. Then suppose that num_groups_with_pending_reqs is still accounting for this group, because the group still has some descendant process with some I/O request still in flight. num_groups_with_pending_reqs should be decremented when the in-flight request of the last descendant process is finally completed (assuming that nothing else has changed for the group in the meantime, in terms of composition of the group and active/inactive state of child groups and processes). To accomplish this, an additional pending-request counter must be added to entities, and must be updated correctly. To avoid this additional field and operations, this commit resorts to the following tradeoff between simplicity and accuracy: for an inactive group that is still counted in num_groups_with_pending_reqs, this commit decrements num_groups_with_pending_reqs when the first descendant process of the group remains with no request waiting for completion. This simplified scheme provides a fix to the unbalanced decrements introduced by 2d29c9f89fcd. Since this error was also caused by lack of comments on this non-trivial issue, this commit also adds related comments. Fixes: 2d29c9f89fcd ("block, bfq: improve asymmetric scenarios detection") Reported-by: Steven Barrett <steven@liquorix.net> Tested-by: Steven Barrett <steven@liquorix.net> Tested-by: Lucjan Lucjanov <lucjan.lucjanov@gmail.com> Reviewed-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-12-06 18:18:18 +00:00
unsigned int num_groups_with_pending_reqs;
/*
* Per-class (RT, BE, IDLE) number of bfq_queues containing
* requests (including the queue in service, even if it is
* idling).
*/
unsigned int busy_queues[3];
/* number of weight-raised busy @bfq_queues */
int wr_busy_queues;
/* number of queued requests */
int queued;
/* number of requests dispatched and waiting for completion */
int rq_in_driver;
block, bfq: do not merge queues on flash storage with queueing To boost throughput with a set of processes doing interleaved I/O (i.e., a set of processes whose individual I/O is random, but whose merged cumulative I/O is sequential), BFQ merges the queues associated with these processes, i.e., redirects the I/O of these processes into a common, shared queue. In the shared queue, I/O requests are ordered by their position on the medium, thus sequential I/O gets dispatched to the device when the shared queue is served. Queue merging costs execution time, because, to detect which queues to merge, BFQ must maintain a list of the head I/O requests of active queues, ordered by request positions. Measurements showed that this costs about 10% of BFQ's total per-request processing time. Request processing time becomes more and more critical as the speed of the underlying storage device grows. Yet, fortunately, queue merging is basically useless on the very devices that are so fast to make request processing time critical. To reach a high throughput, these devices must have many requests queued at the same time. But, in this configuration, the internal scheduling algorithms of these devices do also the job of queue merging: they reorder requests so as to obtain as much as possible a sequential I/O pattern. As a consequence, with processes doing interleaved I/O, the throughput reached by one such device is likely to be the same, with and without queue merging. In view of this fact, this commit disables queue merging, and all related housekeeping, for non-rotational devices with internal queueing. The total, single-lock-protected, per-request processing time of BFQ drops to, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz (time measured with simple code instrumentation, and using the throughput-sync.sh script of the S suite [1], in performance-profiling mode). To put this result into context, the total, single-lock-protected, per-request execution time of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7 us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ). Disabling merging provides a further, remarkable benefit in terms of throughput. Merging tends to make many workloads artificially more uneven, mainly because of shared queues remaining non empty for incomparably more time than normal queues. So, if, e.g., one of the queues in a set of merged queues has a higher weight than a normal queue, then the shared queue may inherit such a high weight and, by staying almost always active, may force BFQ to perform I/O plugging most of the time. This evidently makes it harder for BFQ to let the device reach a high throughput. As a practical example of this problem, and of the benefits of this commit, we measured again the throughput in the nasty scenario considered in previous commit messages: dbench test (in the Phoronix suite), with 6 clients, on a filesystem with journaling, and with the journaling daemon enjoying a higher weight than normal processes. With this commit, the throughput grows from ~150 MB/s to ~200 MB/s on a PLEXTOR PX-256M5 SSD. This is the same peak throughput reached by any of the other I/O schedulers. As such, this is also likely to be the maximum possible throughput reachable with this workload on this device, because I/O is mostly random, and the other schedulers basically just pass I/O requests to the drive as fast as possible. [1] https://github.com/Algodev-github/S Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Francesco Pollicino <fra.fra.800@gmail.com> Signed-off-by: Alessio Masola <alessio.masola@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:30 +00:00
/* true if the device is non rotational and performs queueing */
bool nonrot_with_queueing;
/*
* Maximum number of requests in driver in the last
* @hw_tag_samples completed requests.
*/
int max_rq_in_driver;
/* number of samples used to calculate hw_tag */
int hw_tag_samples;
/* flag set to one if the driver is showing a queueing behavior */
int hw_tag;
/* number of budgets assigned */
int budgets_assigned;
/*
* Timer set when idling (waiting) for the next request from
* the queue in service.
*/
struct hrtimer idle_slice_timer;
/* bfq_queue in service */
struct bfq_queue *in_service_queue;
/* on-disk position of the last served request */
sector_t last_position;
/* position of the last served request for the in-service queue */
sector_t in_serv_last_pos;
/* time of last request completion (ns) */
u64 last_completion;
block, bfq: detect wakers and unconditionally inject their I/O A bfq_queue Q may happen to be synchronized with another bfq_queue Q2, i.e., the I/O of Q2 may need to be completed for Q to receive new I/O. We call Q2 "waker queue". If I/O plugging is being performed for Q, and Q is not receiving any more I/O because of the above synchronization, then, thanks to BFQ's injection mechanism, the waker queue is likely to get served before the I/O-plugging timeout fires. Unfortunately, this fact may not be sufficient to guarantee a high throughput during the I/O plugging, because the inject limit for Q may be too low to guarantee a lot of injected I/O. In addition, the duration of the plugging, i.e., the time before Q finally receives new I/O, may not be minimized, because the waker queue may happen to be served only after other queues. To address these issues, this commit introduces the explicit detection of the waker queue, and the unconditional injection of a pending I/O request of the waker queue on each invocation of bfq_dispatch_request(). One may be concerned that this systematic injection of I/O from the waker queue delays the service of Q's I/O. Fortunately, it doesn't. On the contrary, next Q's I/O is brought forward dramatically, for it is not blocked for milliseconds. Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-06-25 05:12:47 +00:00
/* bfqq owning the last completed rq */
struct bfq_queue *last_completed_rq_bfqq;
block, bfq: merge bursts of newly-created queues Many throughput-sensitive workloads are made of several parallel I/O flows, with all flows generated by the same application, or more generically by the same task (e.g., system boot). The most counterproductive action with these workloads is plugging I/O dispatch when one of the bfq_queues associated with these flows remains temporarily empty. To avoid this plugging, BFQ has been using a burst-handling mechanism for years now. This mechanism has proven effective for throughput, and not detrimental for service guarantees. This commit pushes this mechanism a little bit further, basing on the following two facts. First, all the I/O flows of a the same application or task contribute to the execution/completion of that common application or task. So the performance figures that matter are total throughput of the flows and task-wide I/O latency. In particular, these flows do not need to be protected from each other, in terms of individual bandwidth or latency. Second, the above fact holds regardless of the number of flows. Putting these two facts together, this commits merges stably the bfq_queues associated with these I/O flows, i.e., with the processes that generate these IO/ flows, regardless of how many the involved processes are. To decide whether a set of bfq_queues is actually associated with the I/O flows of a common application or task, and to merge these queues stably, this commit operates as follows: given a bfq_queue, say Q2, currently being created, and the last bfq_queue, say Q1, created before Q2, Q2 is merged stably with Q1 if - very little time has elapsed since when Q1 was created - Q2 has the same ioprio as Q1 - Q2 belongs to the same group as Q1 Merging bfq_queues also reduces scheduling overhead. A fio test with ten random readers on /dev/nullb shows a throughput boost of 40%, with a quadcore. Since BFQ's execution time amounts to ~50% of the total per-request processing time, the above throughput boost implies that BFQ's overhead is reduced by more than 50%. Tested-by: Jan Kara <jack@suse.cz> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Link: https://lore.kernel.org/r/20210304174627.161-7-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
2021-03-04 17:46:27 +00:00
/* last bfqq created, among those in the root group */
struct bfq_queue *last_bfqq_created;
block, bfq: tune service injection basing on request service times The processes associated with a bfq_queue, say Q, may happen to generate their cumulative I/O at a lower rate than the rate at which the device could serve the same I/O. This is rather probable, e.g., if only one process is associated with Q and the device is an SSD. It results in Q becoming often empty while in service. If BFQ is not allowed to switch to another queue when Q becomes empty, then, during the service of Q, there will be frequent "service holes", i.e., time intervals during which Q gets empty and the device can only consume the I/O already queued in its hardware queues. This easily causes considerable losses of throughput. To counter this problem, BFQ implements a request injection mechanism, which tries to fill the above service holes with I/O requests taken from other bfq_queues. The hard part in this mechanism is finding the right amount of I/O to inject, so as to both boost throughput and not break Q's bandwidth and latency guarantees. To this goal, the current version of this mechanism measures the bandwidth enjoyed by Q while it is being served, and tries to inject the maximum possible amount of extra service that does not cause Q's bandwidth to decrease too much. This solution has an important shortcoming. For bandwidth measurements to be stable and reliable, Q must remain in service for a much longer time than that needed to serve a single I/O request. Unfortunately, this does not hold with many workloads. This commit addresses this issue by changing the way the amount of injection allowed is dynamically computed. It tunes injection as a function of the service times of single I/O requests of Q, instead of Q's bandwidth. Single-request service times are evidently meaningful even if Q gets very few I/O requests completed while it is in service. As a testbed for this new solution, we measured the throughput reached by BFQ for one of the nastiest workloads and configurations for this scheduler: the workload generated by the dbench test (in the Phoronix suite), with 6 clients, on a filesystem with journaling, and with the journaling daemon enjoying a higher weight than normal processes. With this commit, the throughput grows from ~100 MB/s to ~150 MB/s on a PLEXTOR PX-256M5. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Francesco Pollicino <fra.fra.800@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:29 +00:00
/* time of last transition from empty to non-empty (ns) */
u64 last_empty_occupied_ns;
/*
* Flag set to activate the sampling of the total service time
* of a just-arrived first I/O request (see
* bfq_update_inject_limit()). This will cause the setting of
* waited_rq when the request is finally dispatched.
*/
bool wait_dispatch;
/*
* If set, then bfq_update_inject_limit() is invoked when
* waited_rq is eventually completed.
*/
struct request *waited_rq;
/*
* True if some request has been injected during the last service hole.
*/
bool rqs_injected;
/* time of first rq dispatch in current observation interval (ns) */
u64 first_dispatch;
/* time of last rq dispatch in current observation interval (ns) */
u64 last_dispatch;
/* beginning of the last budget */
ktime_t last_budget_start;
/* beginning of the last idle slice */
ktime_t last_idling_start;
block, bfq: tune service injection basing on request service times The processes associated with a bfq_queue, say Q, may happen to generate their cumulative I/O at a lower rate than the rate at which the device could serve the same I/O. This is rather probable, e.g., if only one process is associated with Q and the device is an SSD. It results in Q becoming often empty while in service. If BFQ is not allowed to switch to another queue when Q becomes empty, then, during the service of Q, there will be frequent "service holes", i.e., time intervals during which Q gets empty and the device can only consume the I/O already queued in its hardware queues. This easily causes considerable losses of throughput. To counter this problem, BFQ implements a request injection mechanism, which tries to fill the above service holes with I/O requests taken from other bfq_queues. The hard part in this mechanism is finding the right amount of I/O to inject, so as to both boost throughput and not break Q's bandwidth and latency guarantees. To this goal, the current version of this mechanism measures the bandwidth enjoyed by Q while it is being served, and tries to inject the maximum possible amount of extra service that does not cause Q's bandwidth to decrease too much. This solution has an important shortcoming. For bandwidth measurements to be stable and reliable, Q must remain in service for a much longer time than that needed to serve a single I/O request. Unfortunately, this does not hold with many workloads. This commit addresses this issue by changing the way the amount of injection allowed is dynamically computed. It tunes injection as a function of the service times of single I/O requests of Q, instead of Q's bandwidth. Single-request service times are evidently meaningful even if Q gets very few I/O requests completed while it is in service. As a testbed for this new solution, we measured the throughput reached by BFQ for one of the nastiest workloads and configurations for this scheduler: the workload generated by the dbench test (in the Phoronix suite), with 6 clients, on a filesystem with journaling, and with the journaling daemon enjoying a higher weight than normal processes. With this commit, the throughput grows from ~100 MB/s to ~150 MB/s on a PLEXTOR PX-256M5. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Francesco Pollicino <fra.fra.800@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:29 +00:00
unsigned long last_idling_start_jiffies;
/* number of samples in current observation interval */
int peak_rate_samples;
/* num of samples of seq dispatches in current observation interval */
u32 sequential_samples;
/* total num of sectors transferred in current observation interval */
u64 tot_sectors_dispatched;
/* max rq size seen during current observation interval (sectors) */
u32 last_rq_max_size;
/* time elapsed from first dispatch in current observ. interval (us) */
u64 delta_from_first;
/*
* Current estimate of the device peak rate, measured in
* [(sectors/usec) / 2^BFQ_RATE_SHIFT]. The left-shift by
* BFQ_RATE_SHIFT is performed to increase precision in
* fixed-point calculations.
*/
u32 peak_rate;
/* maximum budget allotted to a bfq_queue before rescheduling */
int bfq_max_budget;
/* list of all the bfq_queues active on the device */
struct list_head active_list;
/* list of all the bfq_queues idle on the device */
struct list_head idle_list;
/*
* Timeout for async/sync requests; when it fires, requests
* are served in fifo order.
*/
u64 bfq_fifo_expire[2];
/* weight of backward seeks wrt forward ones */
unsigned int bfq_back_penalty;
/* maximum allowed backward seek */
unsigned int bfq_back_max;
/* maximum idling time */
u32 bfq_slice_idle;
/* user-configured max budget value (0 for auto-tuning) */
int bfq_user_max_budget;
/*
* Timeout for bfq_queues to consume their budget; used to
* prevent seeky queues from imposing long latencies to
* sequential or quasi-sequential ones (this also implies that
* seeky queues cannot receive guarantees in the service
* domain; after a timeout they are charged for the time they
* have been in service, to preserve fairness among them, but
* without service-domain guarantees).
*/
unsigned int bfq_timeout;
/*
* Force device idling whenever needed to provide accurate
* service guarantees, without caring about throughput
* issues. CAVEAT: this may even increase latencies, in case
* of useless idling for processes that did stop doing I/O.
*/
bool strict_guarantees;
/*
* Last time at which a queue entered the current burst of
* queues being activated shortly after each other; for more
* details about this and the following parameters related to
* a burst of activations, see the comments on the function
* bfq_handle_burst.
*/
unsigned long last_ins_in_burst;
/*
* Reference time interval used to decide whether a queue has
* been activated shortly after @last_ins_in_burst.
*/
unsigned long bfq_burst_interval;
/* number of queues in the current burst of queue activations */
int burst_size;
/* common parent entity for the queues in the burst */
struct bfq_entity *burst_parent_entity;
/* Maximum burst size above which the current queue-activation
* burst is deemed as 'large'.
*/
unsigned long bfq_large_burst_thresh;
/* true if a large queue-activation burst is in progress */
bool large_burst;
/*
* Head of the burst list (as for the above fields, more
* details in the comments on the function bfq_handle_burst).
*/
struct hlist_head burst_list;
/* if set to true, low-latency heuristics are enabled */
bool low_latency;
/*
* Maximum factor by which the weight of a weight-raised queue
* is multiplied.
*/
unsigned int bfq_wr_coeff;
/* maximum duration of a weight-raising period (jiffies) */
unsigned int bfq_wr_max_time;
/* Maximum weight-raising duration for soft real-time processes */
unsigned int bfq_wr_rt_max_time;
/*
* Minimum idle period after which weight-raising may be
* reactivated for a queue (in jiffies).
*/
unsigned int bfq_wr_min_idle_time;
/*
* Minimum period between request arrivals after which
* weight-raising may be reactivated for an already busy async
* queue (in jiffies).
*/
unsigned long bfq_wr_min_inter_arr_async;
/* Max service-rate for a soft real-time queue, in sectors/sec */
unsigned int bfq_wr_max_softrt_rate;
/*
* Cached value of the product ref_rate*ref_wr_duration, used
* for computing the maximum duration of weight raising
* automatically.
*/
u64 rate_dur_prod;
/* fallback dummy bfqq for extreme OOM conditions */
struct bfq_queue oom_bfqq;
spinlock_t lock;
/*
* bic associated with the task issuing current bio for
* merging. This and the next field are used as a support to
* be able to perform the bic lookup, needed by bio-merge
* functions, before the scheduler lock is taken, and thus
* avoid taking the request-queue lock while the scheduler
* lock is being held.
*/
struct bfq_io_cq *bio_bic;
/* bfqq associated with the task issuing current bio for merging */
struct bfq_queue *bio_bfqq;
2018-01-13 11:05:17 +00:00
/*
* Depth limits used in bfq_limit_depth (see comments on the
* function)
*/
unsigned int word_depths[2][2];
unsigned int full_depth_shift;
};
enum bfqq_state_flags {
BFQQF_just_created = 0, /* queue just allocated */
BFQQF_busy, /* has requests or is in service */
BFQQF_wait_request, /* waiting for a request */
BFQQF_non_blocking_wait_rq, /*
* waiting for a request
* without idling the device
*/
BFQQF_fifo_expire, /* FIFO checked in this slice */
BFQQF_has_short_ttime, /* queue has a short think time */
BFQQF_sync, /* synchronous queue */
BFQQF_IO_bound, /*
* bfqq has timed-out at least once
* having consumed at most 2/10 of
* its budget
*/
BFQQF_in_large_burst, /*
* bfqq activated in a large burst,
* see comments to bfq_handle_burst.
*/
BFQQF_softrt_update, /*
* may need softrt-next-start
* update
*/
BFQQF_coop, /* bfqq is shared */
block, bfq: detect wakers and unconditionally inject their I/O A bfq_queue Q may happen to be synchronized with another bfq_queue Q2, i.e., the I/O of Q2 may need to be completed for Q to receive new I/O. We call Q2 "waker queue". If I/O plugging is being performed for Q, and Q is not receiving any more I/O because of the above synchronization, then, thanks to BFQ's injection mechanism, the waker queue is likely to get served before the I/O-plugging timeout fires. Unfortunately, this fact may not be sufficient to guarantee a high throughput during the I/O plugging, because the inject limit for Q may be too low to guarantee a lot of injected I/O. In addition, the duration of the plugging, i.e., the time before Q finally receives new I/O, may not be minimized, because the waker queue may happen to be served only after other queues. To address these issues, this commit introduces the explicit detection of the waker queue, and the unconditional injection of a pending I/O request of the waker queue on each invocation of bfq_dispatch_request(). One may be concerned that this systematic injection of I/O from the waker queue delays the service of Q's I/O. Fortunately, it doesn't. On the contrary, next Q's I/O is brought forward dramatically, for it is not blocked for milliseconds. Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-06-25 05:12:47 +00:00
BFQQF_split_coop, /* shared bfqq will be split */
};
#define BFQ_BFQQ_FNS(name) \
void bfq_mark_bfqq_##name(struct bfq_queue *bfqq); \
void bfq_clear_bfqq_##name(struct bfq_queue *bfqq); \
int bfq_bfqq_##name(const struct bfq_queue *bfqq);
BFQ_BFQQ_FNS(just_created);
BFQ_BFQQ_FNS(busy);
BFQ_BFQQ_FNS(wait_request);
BFQ_BFQQ_FNS(non_blocking_wait_rq);
BFQ_BFQQ_FNS(fifo_expire);
BFQ_BFQQ_FNS(has_short_ttime);
BFQ_BFQQ_FNS(sync);
BFQ_BFQQ_FNS(IO_bound);
BFQ_BFQQ_FNS(in_large_burst);
BFQ_BFQQ_FNS(coop);
BFQ_BFQQ_FNS(split_coop);
BFQ_BFQQ_FNS(softrt_update);
#undef BFQ_BFQQ_FNS
/* Expiration reasons. */
enum bfqq_expiration {
BFQQE_TOO_IDLE = 0, /*
* queue has been idling for
* too long
*/
BFQQE_BUDGET_TIMEOUT, /* budget took too long to be used */
BFQQE_BUDGET_EXHAUSTED, /* budget consumed */
BFQQE_NO_MORE_REQUESTS, /* the queue has no more requests */
BFQQE_PREEMPTED /* preemption in progress */
};
struct bfq_stat {
struct percpu_counter cpu_cnt;
atomic64_t aux_cnt;
};
struct bfqg_stats {
/* basic stats */
struct blkg_rwstat bytes;
struct blkg_rwstat ios;
#ifdef CONFIG_BFQ_CGROUP_DEBUG
/* number of ios merged */
struct blkg_rwstat merged;
/* total time spent on device in ns, may not be accurate w/ queueing */
struct blkg_rwstat service_time;
/* total time spent waiting in scheduler queue in ns */
struct blkg_rwstat wait_time;
/* number of IOs queued up */
struct blkg_rwstat queued;
/* total disk time and nr sectors dispatched by this group */
struct bfq_stat time;
/* sum of number of ios queued across all samples */
struct bfq_stat avg_queue_size_sum;
/* count of samples taken for average */
struct bfq_stat avg_queue_size_samples;
/* how many times this group has been removed from service tree */
struct bfq_stat dequeue;
/* total time spent waiting for it to be assigned a timeslice. */
struct bfq_stat group_wait_time;
/* time spent idling for this blkcg_gq */
struct bfq_stat idle_time;
/* total time with empty current active q with other requests queued */
struct bfq_stat empty_time;
/* fields after this shouldn't be cleared on stat reset */
u64 start_group_wait_time;
u64 start_idle_time;
u64 start_empty_time;
uint16_t flags;
#endif /* CONFIG_BFQ_CGROUP_DEBUG */
};
#ifdef CONFIG_BFQ_GROUP_IOSCHED
/*
* struct bfq_group_data - per-blkcg storage for the blkio subsystem.
*
* @ps: @blkcg_policy_storage that this structure inherits
* @weight: weight of the bfq_group
*/
struct bfq_group_data {
/* must be the first member */
struct blkcg_policy_data pd;
unsigned int weight;
};
/**
* struct bfq_group - per (device, cgroup) data structure.
* @entity: schedulable entity to insert into the parent group sched_data.
* @sched_data: own sched_data, to contain child entities (they may be
* both bfq_queues and bfq_groups).
* @bfqd: the bfq_data for the device this group acts upon.
* @async_bfqq: array of async queues for all the tasks belonging to
* the group, one queue per ioprio value per ioprio_class,
* except for the idle class that has only one queue.
* @async_idle_bfqq: async queue for the idle class (ioprio is ignored).
* @my_entity: pointer to @entity, %NULL for the toplevel group; used
* to avoid too many special cases during group creation/
* migration.
* @stats: stats for this bfqg.
* @active_entities: number of active entities belonging to the group;
* unused for the root group. Used to know whether there
* are groups with more than one active @bfq_entity
* (see the comments to the function
* bfq_bfqq_may_idle()).
* @rq_pos_tree: rbtree sorted by next_request position, used when
* determining if two or more queues have interleaving
* requests (see bfq_find_close_cooperator()).
*
* Each (device, cgroup) pair has its own bfq_group, i.e., for each cgroup
* there is a set of bfq_groups, each one collecting the lower-level
* entities belonging to the group that are acting on the same device.
*
* Locking works as follows:
* o @bfqd is protected by the queue lock, RCU is used to access it
* from the readers.
* o All the other fields are protected by the @bfqd queue lock.
*/
struct bfq_group {
/* must be the first member */
struct blkg_policy_data pd;
block, bfq: access and cache blkg data only when safe In blk-cgroup, operations on blkg objects are protected with the request_queue lock. This is no more the lock that protects I/O-scheduler operations in blk-mq. In fact, the latter are now protected with a finer-grained per-scheduler-instance lock. As a consequence, although blkg lookups are also rcu-protected, blk-mq I/O schedulers may see inconsistent data when they access blkg and blkg-related objects. BFQ does access these objects, and does incur this problem, in the following case. The blkg_lookup performed in bfq_get_queue, being protected (only) through rcu, may happen to return the address of a copy of the original blkg. If this is the case, then the blkg_get performed in bfq_get_queue, to pin down the blkg, is useless: it does not prevent blk-cgroup code from destroying both the original blkg and all objects directly or indirectly referred by the copy of the blkg. BFQ accesses these objects, which typically causes a crash for NULL-pointer dereference of memory-protection violation. Some additional protection mechanism should be added to blk-cgroup to address this issue. In the meantime, this commit provides a quick temporary fix for BFQ: cache (when safe) blkg data that might disappear right after a blkg_lookup. In particular, this commit exploits the following facts to achieve its goal without introducing further locks. Destroy operations on a blkg invoke, as a first step, hooks of the scheduler associated with the blkg. And these hooks are executed with bfqd->lock held for BFQ. As a consequence, for any blkg associated with the request queue an instance of BFQ is attached to, we are guaranteed that such a blkg is not destroyed, and that all the pointers it contains are consistent, while that instance is holding its bfqd->lock. A blkg_lookup performed with bfqd->lock held then returns a fully consistent blkg, which remains consistent until this lock is held. In more detail, this holds even if the returned blkg is a copy of the original one. Finally, also the object describing a group inside BFQ needs to be protected from destruction on the blkg_free of the original blkg (which invokes bfq_pd_free). This commit adds private refcounting for this object, to let it disappear only after no bfq_queue refers to it any longer. This commit also removes or updates some stale comments on locking issues related to blk-cgroup operations. Reported-by: Tomas Konir <tomas.konir@gmail.com> Reported-by: Lee Tibbert <lee.tibbert@gmail.com> Reported-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Tomas Konir <tomas.konir@gmail.com> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@fb.com>
2017-06-05 08:11:15 +00:00
/* cached path for this blkg (see comments in bfq_bic_update_cgroup) */
char blkg_path[128];
/* reference counter (see comments in bfq_bic_update_cgroup) */
int ref;
struct bfq_entity entity;
struct bfq_sched_data sched_data;
void *bfqd;
struct bfq_queue *async_bfqq[2][IOPRIO_NR_LEVELS];
struct bfq_queue *async_idle_bfqq;
struct bfq_entity *my_entity;
int active_entities;
struct rb_root rq_pos_tree;
struct bfqg_stats stats;
};
#else
struct bfq_group {
struct bfq_entity entity;
struct bfq_sched_data sched_data;
struct bfq_queue *async_bfqq[2][IOPRIO_NR_LEVELS];
struct bfq_queue *async_idle_bfqq;
struct rb_root rq_pos_tree;
};
#endif
/* --------------- main algorithm interface ----------------- */
#define BFQ_SERVICE_TREE_INIT ((struct bfq_service_tree) \
{ RB_ROOT, RB_ROOT, NULL, NULL, 0, 0 })
extern const int bfq_timeout;
struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync);
void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync);
struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic);
void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq);
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
block, bfq: do not idle for lowest-weight queues In most cases, it is detrimental for throughput to plug I/O dispatch when the in-service bfq_queue becomes temporarily empty (plugging is performed to wait for the possible arrival, soon, of new I/O from the in-service queue). There is however a case where plugging is needed for service guarantees. If a bfq_queue, say Q, has a higher weight than some other active bfq_queue, and is sync, i.e., contains sync I/O, then, to guarantee that Q does receive a higher share of the throughput than other lower-weight queues, it is necessary to plug I/O dispatch when Q remains temporarily empty while being served. For this reason, BFQ performs I/O plugging when some active bfq_queue has a higher weight than some other active bfq_queue. But this is unnecessarily overkill. In fact, if the in-service bfq_queue actually has a weight lower than or equal to the other queues, then the queue *must not* be guaranteed a higher share of the throughput than the other queues. So, not plugging I/O cannot cause any harm to the queue. And can boost throughput. Taking advantage of this fact, this commit does not plug I/O for sync bfq_queues with a weight lower than or equal to the weights of the other queues. Here is an example of the resulting throughput boost with the dbench workload, which is particularly nasty for BFQ. With the dbench test in the Phoronix suite, BFQ reaches its lowest total throughput with 6 clients on a filesystem with journaling, in case the journaling daemon has a higher weight than normal processes. Before this commit, the total throughput was ~80 MB/sec on a PLEXTOR PX-256M5, after this commit it is ~100 MB/sec. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:28 +00:00
struct rb_root_cached *root);
block, bfq: add/remove entity weights correctly To keep I/O throughput high as often as possible, BFQ performs I/O-dispatch plugging (aka device idling) only when beneficial exactly for throughput, or when needed for service guarantees (low latency, fairness). An important case where the latter condition holds is when the scenario is 'asymmetric' in terms of weights: i.e., when some bfq_queue or whole group of queues has a higher weight, and thus has to receive more service, than other queues or groups. Without dispatch plugging, lower-weight queues/groups may unjustly steal bandwidth to higher-weight queues/groups. To detect asymmetric scenarios, BFQ checks some sufficient conditions. One of these conditions is that active groups have different weights. BFQ controls this condition by maintaining a special set of unique weights of active groups (group_weights_tree). To this purpose, in the function bfq_active_insert/bfq_active_extract BFQ adds/removes the weight of a group to/from this set. Unfortunately, the function bfq_active_extract may happen to be invoked also for a group that is still active (to preserve the correct update of the next queue to serve, see comments in function bfq_no_longer_next_in_service() for details). In this case, removing the weight of the group makes the set group_weights_tree inconsistent. Service-guarantee violations follow. This commit addresses this issue by moving group_weights_tree insertions from their previous location (in bfq_active_insert) into the function __bfq_activate_entity, and by moving group_weights_tree extractions from bfq_active_extract to when the entity that represents a group remains throughly idle, i.e., with no request either enqueued or dispatched. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-06-25 19:55:34 +00:00
void __bfq_weights_tree_remove(struct bfq_data *bfqd,
block, bfq: improve asymmetric scenarios detection bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-10-12 09:55:57 +00:00
struct bfq_queue *bfqq,
block, bfq: do not idle for lowest-weight queues In most cases, it is detrimental for throughput to plug I/O dispatch when the in-service bfq_queue becomes temporarily empty (plugging is performed to wait for the possible arrival, soon, of new I/O from the in-service queue). There is however a case where plugging is needed for service guarantees. If a bfq_queue, say Q, has a higher weight than some other active bfq_queue, and is sync, i.e., contains sync I/O, then, to guarantee that Q does receive a higher share of the throughput than other lower-weight queues, it is necessary to plug I/O dispatch when Q remains temporarily empty while being served. For this reason, BFQ performs I/O plugging when some active bfq_queue has a higher weight than some other active bfq_queue. But this is unnecessarily overkill. In fact, if the in-service bfq_queue actually has a weight lower than or equal to the other queues, then the queue *must not* be guaranteed a higher share of the throughput than the other queues. So, not plugging I/O cannot cause any harm to the queue. And can boost throughput. Taking advantage of this fact, this commit does not plug I/O for sync bfq_queues with a weight lower than or equal to the weights of the other queues. Here is an example of the resulting throughput boost with the dbench workload, which is particularly nasty for BFQ. With the dbench test in the Phoronix suite, BFQ reaches its lowest total throughput with 6 clients on a filesystem with journaling, in case the journaling daemon has a higher weight than normal processes. Before this commit, the total throughput was ~80 MB/sec on a PLEXTOR PX-256M5, after this commit it is ~100 MB/sec. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2019-03-12 08:59:28 +00:00
struct rb_root_cached *root);
block, bfq: add/remove entity weights correctly To keep I/O throughput high as often as possible, BFQ performs I/O-dispatch plugging (aka device idling) only when beneficial exactly for throughput, or when needed for service guarantees (low latency, fairness). An important case where the latter condition holds is when the scenario is 'asymmetric' in terms of weights: i.e., when some bfq_queue or whole group of queues has a higher weight, and thus has to receive more service, than other queues or groups. Without dispatch plugging, lower-weight queues/groups may unjustly steal bandwidth to higher-weight queues/groups. To detect asymmetric scenarios, BFQ checks some sufficient conditions. One of these conditions is that active groups have different weights. BFQ controls this condition by maintaining a special set of unique weights of active groups (group_weights_tree). To this purpose, in the function bfq_active_insert/bfq_active_extract BFQ adds/removes the weight of a group to/from this set. Unfortunately, the function bfq_active_extract may happen to be invoked also for a group that is still active (to preserve the correct update of the next queue to serve, see comments in function bfq_no_longer_next_in_service() for details). In this case, removing the weight of the group makes the set group_weights_tree inconsistent. Service-guarantee violations follow. This commit addresses this issue by moving group_weights_tree insertions from their previous location (in bfq_active_insert) into the function __bfq_activate_entity, and by moving group_weights_tree extractions from bfq_active_extract to when the entity that represents a group remains throughly idle, i.e., with no request either enqueued or dispatched. Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2018-06-25 19:55:34 +00:00
void bfq_weights_tree_remove(struct bfq_data *bfqd,
struct bfq_queue *bfqq);
void bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bool compensate, enum bfqq_expiration reason);
void bfq_put_queue(struct bfq_queue *bfqq);
void bfq_end_wr_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg);
void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq);
void bfq_schedule_dispatch(struct bfq_data *bfqd);
void bfq_put_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg);
/* ------------ end of main algorithm interface -------------- */
/* ---------------- cgroups-support interface ---------------- */
void bfqg_stats_update_legacy_io(struct request_queue *q, struct request *rq);
void bfqg_stats_update_io_add(struct bfq_group *bfqg, struct bfq_queue *bfqq,
unsigned int op);
void bfqg_stats_update_io_remove(struct bfq_group *bfqg, unsigned int op);
void bfqg_stats_update_io_merged(struct bfq_group *bfqg, unsigned int op);
void bfqg_stats_update_completion(struct bfq_group *bfqg, u64 start_time_ns,
u64 io_start_time_ns, unsigned int op);
void bfqg_stats_update_dequeue(struct bfq_group *bfqg);
void bfqg_stats_set_start_empty_time(struct bfq_group *bfqg);
void bfqg_stats_update_idle_time(struct bfq_group *bfqg);
void bfqg_stats_set_start_idle_time(struct bfq_group *bfqg);
void bfqg_stats_update_avg_queue_size(struct bfq_group *bfqg);
void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
struct bfq_group *bfqg);
void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg);
void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio);
void bfq_end_wr_async(struct bfq_data *bfqd);
struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
struct blkcg *blkcg);
struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node);
block, bfq: access and cache blkg data only when safe In blk-cgroup, operations on blkg objects are protected with the request_queue lock. This is no more the lock that protects I/O-scheduler operations in blk-mq. In fact, the latter are now protected with a finer-grained per-scheduler-instance lock. As a consequence, although blkg lookups are also rcu-protected, blk-mq I/O schedulers may see inconsistent data when they access blkg and blkg-related objects. BFQ does access these objects, and does incur this problem, in the following case. The blkg_lookup performed in bfq_get_queue, being protected (only) through rcu, may happen to return the address of a copy of the original blkg. If this is the case, then the blkg_get performed in bfq_get_queue, to pin down the blkg, is useless: it does not prevent blk-cgroup code from destroying both the original blkg and all objects directly or indirectly referred by the copy of the blkg. BFQ accesses these objects, which typically causes a crash for NULL-pointer dereference of memory-protection violation. Some additional protection mechanism should be added to blk-cgroup to address this issue. In the meantime, this commit provides a quick temporary fix for BFQ: cache (when safe) blkg data that might disappear right after a blkg_lookup. In particular, this commit exploits the following facts to achieve its goal without introducing further locks. Destroy operations on a blkg invoke, as a first step, hooks of the scheduler associated with the blkg. And these hooks are executed with bfqd->lock held for BFQ. As a consequence, for any blkg associated with the request queue an instance of BFQ is attached to, we are guaranteed that such a blkg is not destroyed, and that all the pointers it contains are consistent, while that instance is holding its bfqd->lock. A blkg_lookup performed with bfqd->lock held then returns a fully consistent blkg, which remains consistent until this lock is held. In more detail, this holds even if the returned blkg is a copy of the original one. Finally, also the object describing a group inside BFQ needs to be protected from destruction on the blkg_free of the original blkg (which invokes bfq_pd_free). This commit adds private refcounting for this object, to let it disappear only after no bfq_queue refers to it any longer. This commit also removes or updates some stale comments on locking issues related to blk-cgroup operations. Reported-by: Tomas Konir <tomas.konir@gmail.com> Reported-by: Lee Tibbert <lee.tibbert@gmail.com> Reported-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Tomas Konir <tomas.konir@gmail.com> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@fb.com>
2017-06-05 08:11:15 +00:00
void bfqg_and_blkg_put(struct bfq_group *bfqg);
#ifdef CONFIG_BFQ_GROUP_IOSCHED
extern struct cftype bfq_blkcg_legacy_files[];
extern struct cftype bfq_blkg_files[];
extern struct blkcg_policy blkcg_policy_bfq;
#endif
/* ------------- end of cgroups-support interface ------------- */
/* - interface of the internal hierarchical B-WF2Q+ scheduler - */
#ifdef CONFIG_BFQ_GROUP_IOSCHED
/* both next loops stop at one of the child entities of the root group */
#define for_each_entity(entity) \
for (; entity ; entity = entity->parent)
/*
* For each iteration, compute parent in advance, so as to be safe if
* entity is deallocated during the iteration. Such a deallocation may
* happen as a consequence of a bfq_put_queue that frees the bfq_queue
* containing entity.
*/
#define for_each_entity_safe(entity, parent) \
for (; entity && ({ parent = entity->parent; 1; }); entity = parent)
#else /* CONFIG_BFQ_GROUP_IOSCHED */
/*
* Next two macros are fake loops when cgroups support is not
* enabled. I fact, in such a case, there is only one level to go up
* (to reach the root group).
*/
#define for_each_entity(entity) \
for (; entity ; entity = NULL)
#define for_each_entity_safe(entity, parent) \
for (parent = NULL; entity ; entity = parent)
#endif /* CONFIG_BFQ_GROUP_IOSCHED */
struct bfq_queue *bfq_entity_to_bfqq(struct bfq_entity *entity);
unsigned int bfq_tot_busy_queues(struct bfq_data *bfqd);
struct bfq_service_tree *bfq_entity_service_tree(struct bfq_entity *entity);
struct bfq_entity *bfq_entity_of(struct rb_node *node);
unsigned short bfq_ioprio_to_weight(int ioprio);
void bfq_put_idle_entity(struct bfq_service_tree *st,
struct bfq_entity *entity);
struct bfq_service_tree *
__bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
block, bfq: don't change ioprio class for a bfq_queue on a service tree On each deactivation or re-scheduling (after being served) of a bfq_queue, BFQ invokes the function __bfq_entity_update_weight_prio(), to perform pending updates of ioprio, weight and ioprio class for the bfq_queue. BFQ also invokes this function on I/O-request dispatches, to raise or lower weights more quickly when needed, thereby improving latency. However, the entity representing the bfq_queue may be on the active (sub)tree of a service tree when this happens, and, although with a very low probability, the bfq_queue may happen to also have a pending change of its ioprio class. If both conditions hold when __bfq_entity_update_weight_prio() is invoked, then the entity moves to a sort of hybrid state: the new service tree for the entity, as returned by bfq_entity_service_tree(), differs from service tree on which the entity still is. The functions that handle activations and deactivations of entities do not cope with such a hybrid state (and would need to become more complex to cope). This commit addresses this issue by just making __bfq_entity_update_weight_prio() not perform also a possible pending change of ioprio class, when invoked on an I/O-request dispatch for a bfq_queue. Such a change is thus postponed to when __bfq_entity_update_weight_prio() is invoked on deactivation or re-scheduling of the bfq_queue. Reported-by: Marco Piazza <mpiazza@gmail.com> Reported-by: Laurentiu Nicola <lnicola@dend.ro> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-03 08:00:10 +00:00
struct bfq_entity *entity,
bool update_class_too);
void bfq_bfqq_served(struct bfq_queue *bfqq, int served);
void bfq_bfqq_charge_time(struct bfq_data *bfqd, struct bfq_queue *bfqq,
unsigned long time_ms);
bool __bfq_deactivate_entity(struct bfq_entity *entity,
bool ins_into_idle_tree);
bool next_queue_may_preempt(struct bfq_data *bfqd);
struct bfq_queue *bfq_get_next_queue(struct bfq_data *bfqd);
2019-04-10 08:38:33 +00:00
bool __bfq_bfqd_reset_in_service(struct bfq_data *bfqd);
void bfq_deactivate_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bool ins_into_idle_tree, bool expiration);
void bfq_activate_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq);
block, bfq: make lookup_next_entity push up vtime on expirations To provide a very smooth service, bfq starts to serve a bfq_queue only if the queue is 'eligible', i.e., if the same queue would have started to be served in the ideal, perfectly fair system that bfq simulates internally. This is obtained by associating each queue with a virtual start time, and by computing a special system virtual time quantity: a queue is eligible only if the system virtual time has reached the virtual start time of the queue. Finally, bfq guarantees that, when a new queue must be set in service, there is always at least one eligible entity for each active parent entity in the scheduler. To provide this guarantee, the function __bfq_lookup_next_entity pushes up, for each parent entity on which it is invoked, the system virtual time to the minimum among the virtual start times of the entities in the active tree for the parent entity (more precisely, the push up occurs if the system virtual time happens to be lower than all such virtual start times). There is however a circumstance in which __bfq_lookup_next_entity cannot push up the system virtual time for a parent entity, even if the system virtual time is lower than the virtual start times of all the child entities in the active tree. It happens if one of the child entities is in service. In fact, in such a case, there is already an eligible entity, the in-service one, even if it may not be not present in the active tree (because in-service entities may be removed from the active tree). Unfortunately, in the last re-design of the hierarchical-scheduling engine, the reset of the pointer to the in-service entity for a given parent entity--reset to be done as a consequence of the expiration of the in-service entity--always happens after the function __bfq_lookup_next_entity has been invoked. This causes the function to think that there is still an entity in service for the parent entity, and then that the system virtual time cannot be pushed up, even if actually such a no-more-in-service entity has already been properly reinserted into the active tree (or in some other tree if no more active). Yet, the system virtual time *had* to be pushed up, to be ready to correctly choose the next queue to serve. Because of the lack of this push up, bfq may wrongly set in service a queue that had been speculatively pre-computed as the possible next-in-service queue, but that would no more be the one to serve after the expiration and the reinsertion into the active trees of the previously in-service entities. This commit addresses this issue by making __bfq_lookup_next_entity properly push up the system virtual time if an expiration is occurring. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-08-31 06:46:29 +00:00
void bfq_requeue_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bool expiration);
void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bool expiration);
void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq);
/* --------------- end of interface of B-WF2Q+ ---------------- */
/* Logging facilities. */
static inline void bfq_bfqq_name(struct bfq_queue *bfqq, char *str, int len)
{
char type = bfq_bfqq_sync(bfqq) ? 'S' : 'A';
if (bfqq->pid != -1)
snprintf(str, len, "bfq%d%c", bfqq->pid, type);
else
snprintf(str, len, "bfqSHARED-%c", type);
}
#ifdef CONFIG_BFQ_GROUP_IOSCHED
struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
#define bfq_log_bfqq(bfqd, bfqq, fmt, args...) do { \
char pid_str[MAX_BFQQ_NAME_LENGTH]; \
if (likely(!blk_trace_note_message_enabled((bfqd)->queue))) \
break; \
bfq_bfqq_name((bfqq), pid_str, MAX_BFQQ_NAME_LENGTH); \
blk_add_cgroup_trace_msg((bfqd)->queue, \
bfqg_to_blkg(bfqq_group(bfqq))->blkcg, \
"%s " fmt, pid_str, ##args); \
} while (0)
#define bfq_log_bfqg(bfqd, bfqg, fmt, args...) do { \
blk_add_cgroup_trace_msg((bfqd)->queue, \
bfqg_to_blkg(bfqg)->blkcg, fmt, ##args); \
} while (0)
#else /* CONFIG_BFQ_GROUP_IOSCHED */
#define bfq_log_bfqq(bfqd, bfqq, fmt, args...) do { \
char pid_str[MAX_BFQQ_NAME_LENGTH]; \
if (likely(!blk_trace_note_message_enabled((bfqd)->queue))) \
break; \
bfq_bfqq_name((bfqq), pid_str, MAX_BFQQ_NAME_LENGTH); \
blk_add_trace_msg((bfqd)->queue, "%s " fmt, pid_str, ##args); \
} while (0)
#define bfq_log_bfqg(bfqd, bfqg, fmt, args...) do {} while (0)
#endif /* CONFIG_BFQ_GROUP_IOSCHED */
#define bfq_log(bfqd, fmt, args...) \
blk_add_trace_msg((bfqd)->queue, "bfq " fmt, ##args)
#endif /* _BFQ_H */