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![Khazhismel Kumykov](/assets/img/avatar_default.png)
Move the last used path to the end of the list (least preferred) so that ties are more evenly distributed. For example, in case with three paths with one that is slower than others, the remaining two would be unevenly used if they tie. This is due to the rotation not being a truely fair distribution. Illustrated: paths a, b, c, 'c' has 1 outstanding IO, a and b are 'tied' Three possible rotations: (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' ... So 'a' is used 2x more than 'b', although they should be used evenly. With this change, the most recently used path is always the least preferred, removing this bias resulting in even distribution. (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' (c, b, a) -> best path 'b' ... Signed-off-by: Khazhismel Kumykov <khazhy@google.com> Reviewed-by: Martin Wilck <mwilck@suse.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
362 lines
8.8 KiB
C
362 lines
8.8 KiB
C
/*
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* Copyright (C) 2007-2009 NEC Corporation. All Rights Reserved.
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*
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* Module Author: Kiyoshi Ueda
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*
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* This file is released under the GPL.
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*
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* Throughput oriented path selector.
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*/
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#include "dm.h"
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#include "dm-path-selector.h"
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#include <linux/slab.h>
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#include <linux/module.h>
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#define DM_MSG_PREFIX "multipath service-time"
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#define ST_MIN_IO 1
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#define ST_MAX_RELATIVE_THROUGHPUT 100
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#define ST_MAX_RELATIVE_THROUGHPUT_SHIFT 7
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#define ST_MAX_INFLIGHT_SIZE ((size_t)-1 >> ST_MAX_RELATIVE_THROUGHPUT_SHIFT)
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#define ST_VERSION "0.3.0"
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struct selector {
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struct list_head valid_paths;
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struct list_head failed_paths;
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spinlock_t lock;
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};
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struct path_info {
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struct list_head list;
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struct dm_path *path;
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unsigned repeat_count;
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unsigned relative_throughput;
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atomic_t in_flight_size; /* Total size of in-flight I/Os */
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};
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static struct selector *alloc_selector(void)
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{
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struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
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if (s) {
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INIT_LIST_HEAD(&s->valid_paths);
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INIT_LIST_HEAD(&s->failed_paths);
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spin_lock_init(&s->lock);
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}
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return s;
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}
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static int st_create(struct path_selector *ps, unsigned argc, char **argv)
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{
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struct selector *s = alloc_selector();
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if (!s)
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return -ENOMEM;
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ps->context = s;
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return 0;
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}
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static void free_paths(struct list_head *paths)
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{
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struct path_info *pi, *next;
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list_for_each_entry_safe(pi, next, paths, list) {
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list_del(&pi->list);
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kfree(pi);
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}
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}
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static void st_destroy(struct path_selector *ps)
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{
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struct selector *s = ps->context;
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free_paths(&s->valid_paths);
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free_paths(&s->failed_paths);
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kfree(s);
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ps->context = NULL;
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}
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static int st_status(struct path_selector *ps, struct dm_path *path,
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status_type_t type, char *result, unsigned maxlen)
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{
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unsigned sz = 0;
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struct path_info *pi;
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if (!path)
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DMEMIT("0 ");
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else {
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pi = path->pscontext;
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switch (type) {
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case STATUSTYPE_INFO:
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DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
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pi->relative_throughput);
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break;
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case STATUSTYPE_TABLE:
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DMEMIT("%u %u ", pi->repeat_count,
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pi->relative_throughput);
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break;
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}
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}
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return sz;
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}
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static int st_add_path(struct path_selector *ps, struct dm_path *path,
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int argc, char **argv, char **error)
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{
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struct selector *s = ps->context;
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struct path_info *pi;
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unsigned repeat_count = ST_MIN_IO;
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unsigned relative_throughput = 1;
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char dummy;
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unsigned long flags;
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/*
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* Arguments: [<repeat_count> [<relative_throughput>]]
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* <repeat_count>: The number of I/Os before switching path.
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* If not given, default (ST_MIN_IO) is used.
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* <relative_throughput>: The relative throughput value of
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* the path among all paths in the path-group.
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* The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
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* If not given, minimum value '1' is used.
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* If '0' is given, the path isn't selected while
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* other paths having a positive value are
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* available.
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*/
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if (argc > 2) {
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*error = "service-time ps: incorrect number of arguments";
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return -EINVAL;
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}
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if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
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*error = "service-time ps: invalid repeat count";
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return -EINVAL;
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}
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if (repeat_count > 1) {
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DMWARN_LIMIT("repeat_count > 1 is deprecated, using 1 instead");
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repeat_count = 1;
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}
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if ((argc == 2) &&
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(sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
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relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
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*error = "service-time ps: invalid relative_throughput value";
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return -EINVAL;
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}
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/* allocate the path */
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pi = kmalloc(sizeof(*pi), GFP_KERNEL);
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if (!pi) {
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*error = "service-time ps: Error allocating path context";
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return -ENOMEM;
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}
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pi->path = path;
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pi->repeat_count = repeat_count;
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pi->relative_throughput = relative_throughput;
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atomic_set(&pi->in_flight_size, 0);
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path->pscontext = pi;
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spin_lock_irqsave(&s->lock, flags);
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list_add_tail(&pi->list, &s->valid_paths);
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spin_unlock_irqrestore(&s->lock, flags);
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return 0;
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}
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static void st_fail_path(struct path_selector *ps, struct dm_path *path)
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{
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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list_move(&pi->list, &s->failed_paths);
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spin_unlock_irqrestore(&s->lock, flags);
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}
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static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
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{
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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list_move_tail(&pi->list, &s->valid_paths);
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spin_unlock_irqrestore(&s->lock, flags);
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return 0;
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}
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/*
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* Compare the estimated service time of 2 paths, pi1 and pi2,
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* for the incoming I/O.
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*
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* Returns:
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* < 0 : pi1 is better
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* 0 : no difference between pi1 and pi2
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* > 0 : pi2 is better
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*
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* Description:
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* Basically, the service time is estimated by:
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* ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
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* To reduce the calculation, some optimizations are made.
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* (See comments inline)
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*/
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static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
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size_t incoming)
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{
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size_t sz1, sz2, st1, st2;
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sz1 = atomic_read(&pi1->in_flight_size);
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sz2 = atomic_read(&pi2->in_flight_size);
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/*
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* Case 1: Both have same throughput value. Choose less loaded path.
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*/
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if (pi1->relative_throughput == pi2->relative_throughput)
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return sz1 - sz2;
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/*
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* Case 2a: Both have same load. Choose higher throughput path.
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* Case 2b: One path has no throughput value. Choose the other one.
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*/
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if (sz1 == sz2 ||
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!pi1->relative_throughput || !pi2->relative_throughput)
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return pi2->relative_throughput - pi1->relative_throughput;
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/*
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* Case 3: Calculate service time. Choose faster path.
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* Service time using pi1:
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* st1 = (sz1 + incoming) / pi1->relative_throughput
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* Service time using pi2:
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* st2 = (sz2 + incoming) / pi2->relative_throughput
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*
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* To avoid the division, transform the expression to use
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* multiplication.
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* Because ->relative_throughput > 0 here, if st1 < st2,
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* the expressions below are the same meaning:
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* (sz1 + incoming) / pi1->relative_throughput <
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* (sz2 + incoming) / pi2->relative_throughput
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* (sz1 + incoming) * pi2->relative_throughput <
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* (sz2 + incoming) * pi1->relative_throughput
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* So use the later one.
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*/
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sz1 += incoming;
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sz2 += incoming;
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if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
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sz2 >= ST_MAX_INFLIGHT_SIZE)) {
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/*
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* Size may be too big for multiplying pi->relative_throughput
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* and overflow.
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* To avoid the overflow and mis-selection, shift down both.
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*/
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sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
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sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
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}
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st1 = sz1 * pi2->relative_throughput;
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st2 = sz2 * pi1->relative_throughput;
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if (st1 != st2)
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return st1 - st2;
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/*
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* Case 4: Service time is equal. Choose higher throughput path.
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*/
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return pi2->relative_throughput - pi1->relative_throughput;
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}
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static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes)
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{
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struct selector *s = ps->context;
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struct path_info *pi = NULL, *best = NULL;
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struct dm_path *ret = NULL;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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if (list_empty(&s->valid_paths))
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goto out;
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list_for_each_entry(pi, &s->valid_paths, list)
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if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
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best = pi;
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if (!best)
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goto out;
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/* Move most recently used to least preferred to evenly balance. */
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list_move_tail(&best->list, &s->valid_paths);
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ret = best->path;
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out:
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spin_unlock_irqrestore(&s->lock, flags);
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return ret;
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}
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static int st_start_io(struct path_selector *ps, struct dm_path *path,
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size_t nr_bytes)
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{
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struct path_info *pi = path->pscontext;
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atomic_add(nr_bytes, &pi->in_flight_size);
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return 0;
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}
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static int st_end_io(struct path_selector *ps, struct dm_path *path,
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size_t nr_bytes)
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{
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struct path_info *pi = path->pscontext;
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atomic_sub(nr_bytes, &pi->in_flight_size);
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return 0;
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}
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static struct path_selector_type st_ps = {
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.name = "service-time",
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.module = THIS_MODULE,
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.table_args = 2,
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.info_args = 2,
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.create = st_create,
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.destroy = st_destroy,
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.status = st_status,
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.add_path = st_add_path,
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.fail_path = st_fail_path,
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.reinstate_path = st_reinstate_path,
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.select_path = st_select_path,
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.start_io = st_start_io,
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.end_io = st_end_io,
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};
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static int __init dm_st_init(void)
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{
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int r = dm_register_path_selector(&st_ps);
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if (r < 0)
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DMERR("register failed %d", r);
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DMINFO("version " ST_VERSION " loaded");
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return r;
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}
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static void __exit dm_st_exit(void)
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{
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int r = dm_unregister_path_selector(&st_ps);
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if (r < 0)
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DMERR("unregister failed %d", r);
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}
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module_init(dm_st_init);
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module_exit(dm_st_exit);
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MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
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MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
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MODULE_LICENSE("GPL");
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