mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-28 20:02:17 +00:00
![Boqun Feng](/assets/img/avatar_default.png)
The current doc of workqueue API suggests that work items are non-reentrant: any work item is guaranteed to be executed by at most one worker system-wide at any given time. However this is not true, the following case can cause a work item W executed by two workers at the same time: queue_work_on(0, WQ1, W); // after a worker picks up W and clear the pending bit queue_work_on(1, WQ2, W); // workers on CPU0 and CPU1 will execute W in the same time. , which means the non-reentrance of a work item is conditional, and Lai Jiangshan provided a nice summary[1] of the conditions, therefore use it to describe a work item instance and improve the doc. [1]: https://lore.kernel.org/lkml/CAJhGHyDudet_xyNk=8xnuO2==o-u06s0E0GZVP4Q67nmQ84Ceg@mail.gmail.com/ Suggested-by: Matthew Wilcox <willy@infradead.org> Suggested-by: Tejun Heo <tj@kernel.org> Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org>
413 lines
16 KiB
ReStructuredText
413 lines
16 KiB
ReStructuredText
====================================
|
|
Concurrency Managed Workqueue (cmwq)
|
|
====================================
|
|
|
|
:Date: September, 2010
|
|
:Author: Tejun Heo <tj@kernel.org>
|
|
:Author: Florian Mickler <florian@mickler.org>
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
There are many cases where an asynchronous process execution context
|
|
is needed and the workqueue (wq) API is the most commonly used
|
|
mechanism for such cases.
|
|
|
|
When such an asynchronous execution context is needed, a work item
|
|
describing which function to execute is put on a queue. An
|
|
independent thread serves as the asynchronous execution context. The
|
|
queue is called workqueue and the thread is called worker.
|
|
|
|
While there are work items on the workqueue the worker executes the
|
|
functions associated with the work items one after the other. When
|
|
there is no work item left on the workqueue the worker becomes idle.
|
|
When a new work item gets queued, the worker begins executing again.
|
|
|
|
|
|
Why cmwq?
|
|
=========
|
|
|
|
In the original wq implementation, a multi threaded (MT) wq had one
|
|
worker thread per CPU and a single threaded (ST) wq had one worker
|
|
thread system-wide. A single MT wq needed to keep around the same
|
|
number of workers as the number of CPUs. The kernel grew a lot of MT
|
|
wq users over the years and with the number of CPU cores continuously
|
|
rising, some systems saturated the default 32k PID space just booting
|
|
up.
|
|
|
|
Although MT wq wasted a lot of resource, the level of concurrency
|
|
provided was unsatisfactory. The limitation was common to both ST and
|
|
MT wq albeit less severe on MT. Each wq maintained its own separate
|
|
worker pool. An MT wq could provide only one execution context per CPU
|
|
while an ST wq one for the whole system. Work items had to compete for
|
|
those very limited execution contexts leading to various problems
|
|
including proneness to deadlocks around the single execution context.
|
|
|
|
The tension between the provided level of concurrency and resource
|
|
usage also forced its users to make unnecessary tradeoffs like libata
|
|
choosing to use ST wq for polling PIOs and accepting an unnecessary
|
|
limitation that no two polling PIOs can progress at the same time. As
|
|
MT wq don't provide much better concurrency, users which require
|
|
higher level of concurrency, like async or fscache, had to implement
|
|
their own thread pool.
|
|
|
|
Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
|
|
focus on the following goals.
|
|
|
|
* Maintain compatibility with the original workqueue API.
|
|
|
|
* Use per-CPU unified worker pools shared by all wq to provide
|
|
flexible level of concurrency on demand without wasting a lot of
|
|
resource.
|
|
|
|
* Automatically regulate worker pool and level of concurrency so that
|
|
the API users don't need to worry about such details.
|
|
|
|
|
|
The Design
|
|
==========
|
|
|
|
In order to ease the asynchronous execution of functions a new
|
|
abstraction, the work item, is introduced.
|
|
|
|
A work item is a simple struct that holds a pointer to the function
|
|
that is to be executed asynchronously. Whenever a driver or subsystem
|
|
wants a function to be executed asynchronously it has to set up a work
|
|
item pointing to that function and queue that work item on a
|
|
workqueue.
|
|
|
|
Special purpose threads, called worker threads, execute the functions
|
|
off of the queue, one after the other. If no work is queued, the
|
|
worker threads become idle. These worker threads are managed in so
|
|
called worker-pools.
|
|
|
|
The cmwq design differentiates between the user-facing workqueues that
|
|
subsystems and drivers queue work items on and the backend mechanism
|
|
which manages worker-pools and processes the queued work items.
|
|
|
|
There are two worker-pools, one for normal work items and the other
|
|
for high priority ones, for each possible CPU and some extra
|
|
worker-pools to serve work items queued on unbound workqueues - the
|
|
number of these backing pools is dynamic.
|
|
|
|
Subsystems and drivers can create and queue work items through special
|
|
workqueue API functions as they see fit. They can influence some
|
|
aspects of the way the work items are executed by setting flags on the
|
|
workqueue they are putting the work item on. These flags include
|
|
things like CPU locality, concurrency limits, priority and more. To
|
|
get a detailed overview refer to the API description of
|
|
``alloc_workqueue()`` below.
|
|
|
|
When a work item is queued to a workqueue, the target worker-pool is
|
|
determined according to the queue parameters and workqueue attributes
|
|
and appended on the shared worklist of the worker-pool. For example,
|
|
unless specifically overridden, a work item of a bound workqueue will
|
|
be queued on the worklist of either normal or highpri worker-pool that
|
|
is associated to the CPU the issuer is running on.
|
|
|
|
For any worker pool implementation, managing the concurrency level
|
|
(how many execution contexts are active) is an important issue. cmwq
|
|
tries to keep the concurrency at a minimal but sufficient level.
|
|
Minimal to save resources and sufficient in that the system is used at
|
|
its full capacity.
|
|
|
|
Each worker-pool bound to an actual CPU implements concurrency
|
|
management by hooking into the scheduler. The worker-pool is notified
|
|
whenever an active worker wakes up or sleeps and keeps track of the
|
|
number of the currently runnable workers. Generally, work items are
|
|
not expected to hog a CPU and consume many cycles. That means
|
|
maintaining just enough concurrency to prevent work processing from
|
|
stalling should be optimal. As long as there are one or more runnable
|
|
workers on the CPU, the worker-pool doesn't start execution of a new
|
|
work, but, when the last running worker goes to sleep, it immediately
|
|
schedules a new worker so that the CPU doesn't sit idle while there
|
|
are pending work items. This allows using a minimal number of workers
|
|
without losing execution bandwidth.
|
|
|
|
Keeping idle workers around doesn't cost other than the memory space
|
|
for kthreads, so cmwq holds onto idle ones for a while before killing
|
|
them.
|
|
|
|
For unbound workqueues, the number of backing pools is dynamic.
|
|
Unbound workqueue can be assigned custom attributes using
|
|
``apply_workqueue_attrs()`` and workqueue will automatically create
|
|
backing worker pools matching the attributes. The responsibility of
|
|
regulating concurrency level is on the users. There is also a flag to
|
|
mark a bound wq to ignore the concurrency management. Please refer to
|
|
the API section for details.
|
|
|
|
Forward progress guarantee relies on that workers can be created when
|
|
more execution contexts are necessary, which in turn is guaranteed
|
|
through the use of rescue workers. All work items which might be used
|
|
on code paths that handle memory reclaim are required to be queued on
|
|
wq's that have a rescue-worker reserved for execution under memory
|
|
pressure. Else it is possible that the worker-pool deadlocks waiting
|
|
for execution contexts to free up.
|
|
|
|
|
|
Application Programming Interface (API)
|
|
=======================================
|
|
|
|
``alloc_workqueue()`` allocates a wq. The original
|
|
``create_*workqueue()`` functions are deprecated and scheduled for
|
|
removal. ``alloc_workqueue()`` takes three arguments - ``@name``,
|
|
``@flags`` and ``@max_active``. ``@name`` is the name of the wq and
|
|
also used as the name of the rescuer thread if there is one.
|
|
|
|
A wq no longer manages execution resources but serves as a domain for
|
|
forward progress guarantee, flush and work item attributes. ``@flags``
|
|
and ``@max_active`` control how work items are assigned execution
|
|
resources, scheduled and executed.
|
|
|
|
|
|
``flags``
|
|
---------
|
|
|
|
``WQ_UNBOUND``
|
|
Work items queued to an unbound wq are served by the special
|
|
worker-pools which host workers which are not bound to any
|
|
specific CPU. This makes the wq behave as a simple execution
|
|
context provider without concurrency management. The unbound
|
|
worker-pools try to start execution of work items as soon as
|
|
possible. Unbound wq sacrifices locality but is useful for
|
|
the following cases.
|
|
|
|
* Wide fluctuation in the concurrency level requirement is
|
|
expected and using bound wq may end up creating large number
|
|
of mostly unused workers across different CPUs as the issuer
|
|
hops through different CPUs.
|
|
|
|
* Long running CPU intensive workloads which can be better
|
|
managed by the system scheduler.
|
|
|
|
``WQ_FREEZABLE``
|
|
A freezable wq participates in the freeze phase of the system
|
|
suspend operations. Work items on the wq are drained and no
|
|
new work item starts execution until thawed.
|
|
|
|
``WQ_MEM_RECLAIM``
|
|
All wq which might be used in the memory reclaim paths **MUST**
|
|
have this flag set. The wq is guaranteed to have at least one
|
|
execution context regardless of memory pressure.
|
|
|
|
``WQ_HIGHPRI``
|
|
Work items of a highpri wq are queued to the highpri
|
|
worker-pool of the target cpu. Highpri worker-pools are
|
|
served by worker threads with elevated nice level.
|
|
|
|
Note that normal and highpri worker-pools don't interact with
|
|
each other. Each maintains its separate pool of workers and
|
|
implements concurrency management among its workers.
|
|
|
|
``WQ_CPU_INTENSIVE``
|
|
Work items of a CPU intensive wq do not contribute to the
|
|
concurrency level. In other words, runnable CPU intensive
|
|
work items will not prevent other work items in the same
|
|
worker-pool from starting execution. This is useful for bound
|
|
work items which are expected to hog CPU cycles so that their
|
|
execution is regulated by the system scheduler.
|
|
|
|
Although CPU intensive work items don't contribute to the
|
|
concurrency level, start of their executions is still
|
|
regulated by the concurrency management and runnable
|
|
non-CPU-intensive work items can delay execution of CPU
|
|
intensive work items.
|
|
|
|
This flag is meaningless for unbound wq.
|
|
|
|
|
|
``max_active``
|
|
--------------
|
|
|
|
``@max_active`` determines the maximum number of execution contexts
|
|
per CPU which can be assigned to the work items of a wq. For example,
|
|
with ``@max_active`` of 16, at most 16 work items of the wq can be
|
|
executing at the same time per CPU.
|
|
|
|
Currently, for a bound wq, the maximum limit for ``@max_active`` is
|
|
512 and the default value used when 0 is specified is 256. For an
|
|
unbound wq, the limit is higher of 512 and 4 *
|
|
``num_possible_cpus()``. These values are chosen sufficiently high
|
|
such that they are not the limiting factor while providing protection
|
|
in runaway cases.
|
|
|
|
The number of active work items of a wq is usually regulated by the
|
|
users of the wq, more specifically, by how many work items the users
|
|
may queue at the same time. Unless there is a specific need for
|
|
throttling the number of active work items, specifying '0' is
|
|
recommended.
|
|
|
|
Some users depend on the strict execution ordering of ST wq. The
|
|
combination of ``@max_active`` of 1 and ``WQ_UNBOUND`` used to
|
|
achieve this behavior. Work items on such wq were always queued to the
|
|
unbound worker-pools and only one work item could be active at any given
|
|
time thus achieving the same ordering property as ST wq.
|
|
|
|
In the current implementation the above configuration only guarantees
|
|
ST behavior within a given NUMA node. Instead ``alloc_ordered_queue()`` should
|
|
be used to achieve system-wide ST behavior.
|
|
|
|
|
|
Example Execution Scenarios
|
|
===========================
|
|
|
|
The following example execution scenarios try to illustrate how cmwq
|
|
behave under different configurations.
|
|
|
|
Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
|
|
w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
|
|
again before finishing. w1 and w2 burn CPU for 5ms then sleep for
|
|
10ms.
|
|
|
|
Ignoring all other tasks, works and processing overhead, and assuming
|
|
simple FIFO scheduling, the following is one highly simplified version
|
|
of possible sequences of events with the original wq. ::
|
|
|
|
TIME IN MSECS EVENT
|
|
0 w0 starts and burns CPU
|
|
5 w0 sleeps
|
|
15 w0 wakes up and burns CPU
|
|
20 w0 finishes
|
|
20 w1 starts and burns CPU
|
|
25 w1 sleeps
|
|
35 w1 wakes up and finishes
|
|
35 w2 starts and burns CPU
|
|
40 w2 sleeps
|
|
50 w2 wakes up and finishes
|
|
|
|
And with cmwq with ``@max_active`` >= 3, ::
|
|
|
|
TIME IN MSECS EVENT
|
|
0 w0 starts and burns CPU
|
|
5 w0 sleeps
|
|
5 w1 starts and burns CPU
|
|
10 w1 sleeps
|
|
10 w2 starts and burns CPU
|
|
15 w2 sleeps
|
|
15 w0 wakes up and burns CPU
|
|
20 w0 finishes
|
|
20 w1 wakes up and finishes
|
|
25 w2 wakes up and finishes
|
|
|
|
If ``@max_active`` == 2, ::
|
|
|
|
TIME IN MSECS EVENT
|
|
0 w0 starts and burns CPU
|
|
5 w0 sleeps
|
|
5 w1 starts and burns CPU
|
|
10 w1 sleeps
|
|
15 w0 wakes up and burns CPU
|
|
20 w0 finishes
|
|
20 w1 wakes up and finishes
|
|
20 w2 starts and burns CPU
|
|
25 w2 sleeps
|
|
35 w2 wakes up and finishes
|
|
|
|
Now, let's assume w1 and w2 are queued to a different wq q1 which has
|
|
``WQ_CPU_INTENSIVE`` set, ::
|
|
|
|
TIME IN MSECS EVENT
|
|
0 w0 starts and burns CPU
|
|
5 w0 sleeps
|
|
5 w1 and w2 start and burn CPU
|
|
10 w1 sleeps
|
|
15 w2 sleeps
|
|
15 w0 wakes up and burns CPU
|
|
20 w0 finishes
|
|
20 w1 wakes up and finishes
|
|
25 w2 wakes up and finishes
|
|
|
|
|
|
Guidelines
|
|
==========
|
|
|
|
* Do not forget to use ``WQ_MEM_RECLAIM`` if a wq may process work
|
|
items which are used during memory reclaim. Each wq with
|
|
``WQ_MEM_RECLAIM`` set has an execution context reserved for it. If
|
|
there is dependency among multiple work items used during memory
|
|
reclaim, they should be queued to separate wq each with
|
|
``WQ_MEM_RECLAIM``.
|
|
|
|
* Unless strict ordering is required, there is no need to use ST wq.
|
|
|
|
* Unless there is a specific need, using 0 for @max_active is
|
|
recommended. In most use cases, concurrency level usually stays
|
|
well under the default limit.
|
|
|
|
* A wq serves as a domain for forward progress guarantee
|
|
(``WQ_MEM_RECLAIM``, flush and work item attributes. Work items
|
|
which are not involved in memory reclaim and don't need to be
|
|
flushed as a part of a group of work items, and don't require any
|
|
special attribute, can use one of the system wq. There is no
|
|
difference in execution characteristics between using a dedicated wq
|
|
and a system wq.
|
|
|
|
* Unless work items are expected to consume a huge amount of CPU
|
|
cycles, using a bound wq is usually beneficial due to the increased
|
|
level of locality in wq operations and work item execution.
|
|
|
|
|
|
Debugging
|
|
=========
|
|
|
|
Because the work functions are executed by generic worker threads
|
|
there are a few tricks needed to shed some light on misbehaving
|
|
workqueue users.
|
|
|
|
Worker threads show up in the process list as: ::
|
|
|
|
root 5671 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/0:1]
|
|
root 5672 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/1:2]
|
|
root 5673 0.0 0.0 0 0 ? S 12:12 0:00 [kworker/0:0]
|
|
root 5674 0.0 0.0 0 0 ? S 12:13 0:00 [kworker/1:0]
|
|
|
|
If kworkers are going crazy (using too much cpu), there are two types
|
|
of possible problems:
|
|
|
|
1. Something being scheduled in rapid succession
|
|
2. A single work item that consumes lots of cpu cycles
|
|
|
|
The first one can be tracked using tracing: ::
|
|
|
|
$ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
|
|
$ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
|
|
(wait a few secs)
|
|
^C
|
|
|
|
If something is busy looping on work queueing, it would be dominating
|
|
the output and the offender can be determined with the work item
|
|
function.
|
|
|
|
For the second type of problems it should be possible to just check
|
|
the stack trace of the offending worker thread. ::
|
|
|
|
$ cat /proc/THE_OFFENDING_KWORKER/stack
|
|
|
|
The work item's function should be trivially visible in the stack
|
|
trace.
|
|
|
|
Non-reentrance Conditions
|
|
=========================
|
|
|
|
Workqueue guarantees that a work item cannot be re-entrant if the following
|
|
conditions hold after a work item gets queued:
|
|
|
|
1. The work function hasn't been changed.
|
|
2. No one queues the work item to another workqueue.
|
|
3. The work item hasn't been reinitiated.
|
|
|
|
In other words, if the above conditions hold, the work item is guaranteed to be
|
|
executed by at most one worker system-wide at any given time.
|
|
|
|
Note that requeuing the work item (to the same queue) in the self function
|
|
doesn't break these conditions, so it's safe to do. Otherwise, caution is
|
|
required when breaking the conditions inside a work function.
|
|
|
|
|
|
Kernel Inline Documentations Reference
|
|
======================================
|
|
|
|
.. kernel-doc:: include/linux/workqueue.h
|
|
|
|
.. kernel-doc:: kernel/workqueue.c
|