xref: /linux/Documentation/core-api/workqueue.rst (revision 40ccd6aa3e2e05be93394e3cd560c718dedfcc77)
1=========
2Workqueue
3=========
4
5:Date: September, 2010
6:Author: Tejun Heo <tj@kernel.org>
7:Author: Florian Mickler <florian@mickler.org>
8
9
10Introduction
11============
12
13There are many cases where an asynchronous process execution context
14is needed and the workqueue (wq) API is the most commonly used
15mechanism for such cases.
16
17When such an asynchronous execution context is needed, a work item
18describing which function to execute is put on a queue.  An
19independent thread serves as the asynchronous execution context.  The
20queue is called workqueue and the thread is called worker.
21
22While there are work items on the workqueue the worker executes the
23functions associated with the work items one after the other.  When
24there is no work item left on the workqueue the worker becomes idle.
25When a new work item gets queued, the worker begins executing again.
26
27
28Why Concurrency Managed Workqueue?
29==================================
30
31In the original wq implementation, a multi threaded (MT) wq had one
32worker thread per CPU and a single threaded (ST) wq had one worker
33thread system-wide.  A single MT wq needed to keep around the same
34number of workers as the number of CPUs.  The kernel grew a lot of MT
35wq users over the years and with the number of CPU cores continuously
36rising, some systems saturated the default 32k PID space just booting
37up.
38
39Although MT wq wasted a lot of resource, the level of concurrency
40provided was unsatisfactory.  The limitation was common to both ST and
41MT wq albeit less severe on MT.  Each wq maintained its own separate
42worker pool.  An MT wq could provide only one execution context per CPU
43while an ST wq one for the whole system.  Work items had to compete for
44those very limited execution contexts leading to various problems
45including proneness to deadlocks around the single execution context.
46
47The tension between the provided level of concurrency and resource
48usage also forced its users to make unnecessary tradeoffs like libata
49choosing to use ST wq for polling PIOs and accepting an unnecessary
50limitation that no two polling PIOs can progress at the same time.  As
51MT wq don't provide much better concurrency, users which require
52higher level of concurrency, like async or fscache, had to implement
53their own thread pool.
54
55Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
56focus on the following goals.
57
58* Maintain compatibility with the original workqueue API.
59
60* Use per-CPU unified worker pools shared by all wq to provide
61  flexible level of concurrency on demand without wasting a lot of
62  resource.
63
64* Automatically regulate worker pool and level of concurrency so that
65  the API users don't need to worry about such details.
66
67
68The Design
69==========
70
71In order to ease the asynchronous execution of functions a new
72abstraction, the work item, is introduced.
73
74A work item is a simple struct that holds a pointer to the function
75that is to be executed asynchronously.  Whenever a driver or subsystem
76wants a function to be executed asynchronously it has to set up a work
77item pointing to that function and queue that work item on a
78workqueue.
79
80A work item can be executed in either a thread or the BH (softirq) context.
81
82For threaded workqueues, special purpose threads, called [k]workers, execute
83the functions off of the queue, one after the other. If no work is queued,
84the worker threads become idle. These worker threads are managed in
85worker-pools.
86
87The cmwq design differentiates between the user-facing workqueues that
88subsystems and drivers queue work items on and the backend mechanism
89which manages worker-pools and processes the queued work items.
90
91There are two worker-pools, one for normal work items and the other
92for high priority ones, for each possible CPU and some extra
93worker-pools to serve work items queued on unbound workqueues - the
94number of these backing pools is dynamic.
95
96BH workqueues use the same framework. However, as there can only be one
97concurrent execution context, there's no need to worry about concurrency.
98Each per-CPU BH worker pool contains only one pseudo worker which represents
99the BH execution context. A BH workqueue can be considered a convenience
100interface to softirq.
101
102Subsystems and drivers can create and queue work items through special
103workqueue API functions as they see fit. They can influence some
104aspects of the way the work items are executed by setting flags on the
105workqueue they are putting the work item on. These flags include
106things like CPU locality, concurrency limits, priority and more.  To
107get a detailed overview refer to the API description of
108``alloc_workqueue()`` below.
109
110When a work item is queued to a workqueue, the target worker-pool is
111determined according to the queue parameters and workqueue attributes
112and appended on the shared worklist of the worker-pool.  For example,
113unless specifically overridden, a work item of a bound workqueue will
114be queued on the worklist of either normal or highpri worker-pool that
115is associated to the CPU the issuer is running on.
116
117For any thread pool implementation, managing the concurrency level
118(how many execution contexts are active) is an important issue.  cmwq
119tries to keep the concurrency at a minimal but sufficient level.
120Minimal to save resources and sufficient in that the system is used at
121its full capacity.
122
123Each worker-pool bound to an actual CPU implements concurrency
124management by hooking into the scheduler.  The worker-pool is notified
125whenever an active worker wakes up or sleeps and keeps track of the
126number of the currently runnable workers.  Generally, work items are
127not expected to hog a CPU and consume many cycles.  That means
128maintaining just enough concurrency to prevent work processing from
129stalling should be optimal.  As long as there are one or more runnable
130workers on the CPU, the worker-pool doesn't start execution of a new
131work, but, when the last running worker goes to sleep, it immediately
132schedules a new worker so that the CPU doesn't sit idle while there
133are pending work items.  This allows using a minimal number of workers
134without losing execution bandwidth.
135
136Keeping idle workers around doesn't cost other than the memory space
137for kthreads, so cmwq holds onto idle ones for a while before killing
138them.
139
140For unbound workqueues, the number of backing pools is dynamic.
141Unbound workqueue can be assigned custom attributes using
142``apply_workqueue_attrs()`` and workqueue will automatically create
143backing worker pools matching the attributes.  The responsibility of
144regulating concurrency level is on the users.  There is also a flag to
145mark a bound wq to ignore the concurrency management.  Please refer to
146the API section for details.
147
148Forward progress guarantee relies on that workers can be created when
149more execution contexts are necessary, which in turn is guaranteed
150through the use of rescue workers.  All work items which might be used
151on code paths that handle memory reclaim are required to be queued on
152wq's that have a rescue-worker reserved for execution under memory
153pressure.  Else it is possible that the worker-pool deadlocks waiting
154for execution contexts to free up.
155
156
157Application Programming Interface (API)
158=======================================
159
160``alloc_workqueue()`` allocates a wq.  The original
161``create_*workqueue()`` functions are deprecated and scheduled for
162removal.  ``alloc_workqueue()`` takes three arguments - ``@name``,
163``@flags`` and ``@max_active``.  ``@name`` is the name of the wq and
164also used as the name of the rescuer thread if there is one.
165
166A wq no longer manages execution resources but serves as a domain for
167forward progress guarantee, flush and work item attributes. ``@flags``
168and ``@max_active`` control how work items are assigned execution
169resources, scheduled and executed.
170
171
172``flags``
173---------
174
175``WQ_BH``
176  BH workqueues can be considered a convenience interface to softirq. BH
177  workqueues are always per-CPU and all BH work items are executed in the
178  queueing CPU's softirq context in the queueing order.
179
180  All BH workqueues must have 0 ``max_active`` and ``WQ_HIGHPRI`` is the
181  only allowed additional flag.
182
183  BH work items cannot sleep. All other features such as delayed queueing,
184  flushing and canceling are supported.
185
186``WQ_UNBOUND``
187  Work items queued to an unbound wq are served by the special
188  worker-pools which host workers which are not bound to any
189  specific CPU.  This makes the wq behave as a simple execution
190  context provider without concurrency management.  The unbound
191  worker-pools try to start execution of work items as soon as
192  possible.  Unbound wq sacrifices locality but is useful for
193  the following cases.
194
195  * Wide fluctuation in the concurrency level requirement is
196    expected and using bound wq may end up creating large number
197    of mostly unused workers across different CPUs as the issuer
198    hops through different CPUs.
199
200  * Long running CPU intensive workloads which can be better
201    managed by the system scheduler.
202
203``WQ_FREEZABLE``
204  A freezable wq participates in the freeze phase of the system
205  suspend operations.  Work items on the wq are drained and no
206  new work item starts execution until thawed.
207
208``WQ_MEM_RECLAIM``
209  All wq which might be used in the memory reclaim paths **MUST**
210  have this flag set.  The wq is guaranteed to have at least one
211  execution context regardless of memory pressure.
212
213``WQ_HIGHPRI``
214  Work items of a highpri wq are queued to the highpri
215  worker-pool of the target cpu.  Highpri worker-pools are
216  served by worker threads with elevated nice level.
217
218  Note that normal and highpri worker-pools don't interact with
219  each other.  Each maintains its separate pool of workers and
220  implements concurrency management among its workers.
221
222``WQ_CPU_INTENSIVE``
223  Work items of a CPU intensive wq do not contribute to the
224  concurrency level.  In other words, runnable CPU intensive
225  work items will not prevent other work items in the same
226  worker-pool from starting execution.  This is useful for bound
227  work items which are expected to hog CPU cycles so that their
228  execution is regulated by the system scheduler.
229
230  Although CPU intensive work items don't contribute to the
231  concurrency level, start of their executions is still
232  regulated by the concurrency management and runnable
233  non-CPU-intensive work items can delay execution of CPU
234  intensive work items.
235
236  This flag is meaningless for unbound wq.
237
238
239``max_active``
240--------------
241
242``@max_active`` determines the maximum number of execution contexts per
243CPU which can be assigned to the work items of a wq. For example, with
244``@max_active`` of 16, at most 16 work items of the wq can be executing
245at the same time per CPU. This is always a per-CPU attribute, even for
246unbound workqueues.
247
248The maximum limit for ``@max_active`` is 512 and the default value used
249when 0 is specified is 256. These values are chosen sufficiently high
250such that they are not the limiting factor while providing protection in
251runaway cases.
252
253The number of active work items of a wq is usually regulated by the
254users of the wq, more specifically, by how many work items the users
255may queue at the same time.  Unless there is a specific need for
256throttling the number of active work items, specifying '0' is
257recommended.
258
259Some users depend on strict execution ordering where only one work item
260is in flight at any given time and the work items are processed in
261queueing order. While the combination of ``@max_active`` of 1 and
262``WQ_UNBOUND`` used to achieve this behavior, this is no longer the
263case. Use ``alloc_ordered_queue()`` instead.
264
265
266Example Execution Scenarios
267===========================
268
269The following example execution scenarios try to illustrate how cmwq
270behave under different configurations.
271
272 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
273 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
274 again before finishing.  w1 and w2 burn CPU for 5ms then sleep for
275 10ms.
276
277Ignoring all other tasks, works and processing overhead, and assuming
278simple FIFO scheduling, the following is one highly simplified version
279of possible sequences of events with the original wq. ::
280
281 TIME IN MSECS	EVENT
282 0		w0 starts and burns CPU
283 5		w0 sleeps
284 15		w0 wakes up and burns CPU
285 20		w0 finishes
286 20		w1 starts and burns CPU
287 25		w1 sleeps
288 35		w1 wakes up and finishes
289 35		w2 starts and burns CPU
290 40		w2 sleeps
291 50		w2 wakes up and finishes
292
293And with cmwq with ``@max_active`` >= 3, ::
294
295 TIME IN MSECS	EVENT
296 0		w0 starts and burns CPU
297 5		w0 sleeps
298 5		w1 starts and burns CPU
299 10		w1 sleeps
300 10		w2 starts and burns CPU
301 15		w2 sleeps
302 15		w0 wakes up and burns CPU
303 20		w0 finishes
304 20		w1 wakes up and finishes
305 25		w2 wakes up and finishes
306
307If ``@max_active`` == 2, ::
308
309 TIME IN MSECS	EVENT
310 0		w0 starts and burns CPU
311 5		w0 sleeps
312 5		w1 starts and burns CPU
313 10		w1 sleeps
314 15		w0 wakes up and burns CPU
315 20		w0 finishes
316 20		w1 wakes up and finishes
317 20		w2 starts and burns CPU
318 25		w2 sleeps
319 35		w2 wakes up and finishes
320
321Now, let's assume w1 and w2 are queued to a different wq q1 which has
322``WQ_CPU_INTENSIVE`` set, ::
323
324 TIME IN MSECS	EVENT
325 0		w0 starts and burns CPU
326 5		w0 sleeps
327 5		w1 and w2 start and burn CPU
328 10		w1 sleeps
329 15		w2 sleeps
330 15		w0 wakes up and burns CPU
331 20		w0 finishes
332 20		w1 wakes up and finishes
333 25		w2 wakes up and finishes
334
335
336Guidelines
337==========
338
339* Do not forget to use ``WQ_MEM_RECLAIM`` if a wq may process work
340  items which are used during memory reclaim.  Each wq with
341  ``WQ_MEM_RECLAIM`` set has an execution context reserved for it.  If
342  there is dependency among multiple work items used during memory
343  reclaim, they should be queued to separate wq each with
344  ``WQ_MEM_RECLAIM``.
345
346* Unless strict ordering is required, there is no need to use ST wq.
347
348* Unless there is a specific need, using 0 for @max_active is
349  recommended.  In most use cases, concurrency level usually stays
350  well under the default limit.
351
352* A wq serves as a domain for forward progress guarantee
353  (``WQ_MEM_RECLAIM``, flush and work item attributes.  Work items
354  which are not involved in memory reclaim and don't need to be
355  flushed as a part of a group of work items, and don't require any
356  special attribute, can use one of the system wq.  There is no
357  difference in execution characteristics between using a dedicated wq
358  and a system wq.
359
360* Unless work items are expected to consume a huge amount of CPU
361  cycles, using a bound wq is usually beneficial due to the increased
362  level of locality in wq operations and work item execution.
363
364
365Affinity Scopes
366===============
367
368An unbound workqueue groups CPUs according to its affinity scope to improve
369cache locality. For example, if a workqueue is using the default affinity
370scope of "cache", it will group CPUs according to last level cache
371boundaries. A work item queued on the workqueue will be assigned to a worker
372on one of the CPUs which share the last level cache with the issuing CPU.
373Once started, the worker may or may not be allowed to move outside the scope
374depending on the ``affinity_strict`` setting of the scope.
375
376Workqueue currently supports the following affinity scopes.
377
378``default``
379  Use the scope in module parameter ``workqueue.default_affinity_scope``
380  which is always set to one of the scopes below.
381
382``cpu``
383  CPUs are not grouped. A work item issued on one CPU is processed by a
384  worker on the same CPU. This makes unbound workqueues behave as per-cpu
385  workqueues without concurrency management.
386
387``smt``
388  CPUs are grouped according to SMT boundaries. This usually means that the
389  logical threads of each physical CPU core are grouped together.
390
391``cache``
392  CPUs are grouped according to cache boundaries. Which specific cache
393  boundary is used is determined by the arch code. L3 is used in a lot of
394  cases. This is the default affinity scope.
395
396``numa``
397  CPUs are grouped according to NUMA boundaries.
398
399``system``
400  All CPUs are put in the same group. Workqueue makes no effort to process a
401  work item on a CPU close to the issuing CPU.
402
403The default affinity scope can be changed with the module parameter
404``workqueue.default_affinity_scope`` and a specific workqueue's affinity
405scope can be changed using ``apply_workqueue_attrs()``.
406
407If ``WQ_SYSFS`` is set, the workqueue will have the following affinity scope
408related interface files under its ``/sys/devices/virtual/workqueue/WQ_NAME/``
409directory.
410
411``affinity_scope``
412  Read to see the current affinity scope. Write to change.
413
414  When default is the current scope, reading this file will also show the
415  current effective scope in parentheses, for example, ``default (cache)``.
416
417``affinity_strict``
418  0 by default indicating that affinity scopes are not strict. When a work
419  item starts execution, workqueue makes a best-effort attempt to ensure
420  that the worker is inside its affinity scope, which is called
421  repatriation. Once started, the scheduler is free to move the worker
422  anywhere in the system as it sees fit. This enables benefiting from scope
423  locality while still being able to utilize other CPUs if necessary and
424  available.
425
426  If set to 1, all workers of the scope are guaranteed always to be in the
427  scope. This may be useful when crossing affinity scopes has other
428  implications, for example, in terms of power consumption or workload
429  isolation. Strict NUMA scope can also be used to match the workqueue
430  behavior of older kernels.
431
432
433Affinity Scopes and Performance
434===============================
435
436It'd be ideal if an unbound workqueue's behavior is optimal for vast
437majority of use cases without further tuning. Unfortunately, in the current
438kernel, there exists a pronounced trade-off between locality and utilization
439necessitating explicit configurations when workqueues are heavily used.
440
441Higher locality leads to higher efficiency where more work is performed for
442the same number of consumed CPU cycles. However, higher locality may also
443cause lower overall system utilization if the work items are not spread
444enough across the affinity scopes by the issuers. The following performance
445testing with dm-crypt clearly illustrates this trade-off.
446
447The tests are run on a CPU with 12-cores/24-threads split across four L3
448caches (AMD Ryzen 9 3900x). CPU clock boost is turned off for consistency.
449``/dev/dm-0`` is a dm-crypt device created on NVME SSD (Samsung 990 PRO) and
450opened with ``cryptsetup`` with default settings.
451
452
453Scenario 1: Enough issuers and work spread across the machine
454-------------------------------------------------------------
455
456The command used: ::
457
458  $ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k --ioengine=libaio \
459    --iodepth=64 --runtime=60 --numjobs=24 --time_based --group_reporting \
460    --name=iops-test-job --verify=sha512
461
462There are 24 issuers, each issuing 64 IOs concurrently. ``--verify=sha512``
463makes ``fio`` generate and read back the content each time which makes
464execution locality matter between the issuer and ``kcryptd``. The following
465are the read bandwidths and CPU utilizations depending on different affinity
466scope settings on ``kcryptd`` measured over five runs. Bandwidths are in
467MiBps, and CPU util in percents.
468
469.. list-table::
470   :widths: 16 20 20
471   :header-rows: 1
472
473   * - Affinity
474     - Bandwidth (MiBps)
475     - CPU util (%)
476
477   * - system
478     - 1159.40 ±1.34
479     - 99.31 ±0.02
480
481   * - cache
482     - 1166.40 ±0.89
483     - 99.34 ±0.01
484
485   * - cache (strict)
486     - 1166.00 ±0.71
487     - 99.35 ±0.01
488
489With enough issuers spread across the system, there is no downside to
490"cache", strict or otherwise. All three configurations saturate the whole
491machine but the cache-affine ones outperform by 0.6% thanks to improved
492locality.
493
494
495Scenario 2: Fewer issuers, enough work for saturation
496-----------------------------------------------------
497
498The command used: ::
499
500  $ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k \
501    --ioengine=libaio --iodepth=64 --runtime=60 --numjobs=8 \
502    --time_based --group_reporting --name=iops-test-job --verify=sha512
503
504The only difference from the previous scenario is ``--numjobs=8``. There are
505a third of the issuers but is still enough total work to saturate the
506system.
507
508.. list-table::
509   :widths: 16 20 20
510   :header-rows: 1
511
512   * - Affinity
513     - Bandwidth (MiBps)
514     - CPU util (%)
515
516   * - system
517     - 1155.40 ±0.89
518     - 97.41 ±0.05
519
520   * - cache
521     - 1154.40 ±1.14
522     - 96.15 ±0.09
523
524   * - cache (strict)
525     - 1112.00 ±4.64
526     - 93.26 ±0.35
527
528This is more than enough work to saturate the system. Both "system" and
529"cache" are nearly saturating the machine but not fully. "cache" is using
530less CPU but the better efficiency puts it at the same bandwidth as
531"system".
532
533Eight issuers moving around over four L3 cache scope still allow "cache
534(strict)" to mostly saturate the machine but the loss of work conservation
535is now starting to hurt with 3.7% bandwidth loss.
536
537
538Scenario 3: Even fewer issuers, not enough work to saturate
539-----------------------------------------------------------
540
541The command used: ::
542
543  $ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k \
544    --ioengine=libaio --iodepth=64 --runtime=60 --numjobs=4 \
545    --time_based --group_reporting --name=iops-test-job --verify=sha512
546
547Again, the only difference is ``--numjobs=4``. With the number of issuers
548reduced to four, there now isn't enough work to saturate the whole system
549and the bandwidth becomes dependent on completion latencies.
550
551.. list-table::
552   :widths: 16 20 20
553   :header-rows: 1
554
555   * - Affinity
556     - Bandwidth (MiBps)
557     - CPU util (%)
558
559   * - system
560     - 993.60 ±1.82
561     - 75.49 ±0.06
562
563   * - cache
564     - 973.40 ±1.52
565     - 74.90 ±0.07
566
567   * - cache (strict)
568     - 828.20 ±4.49
569     - 66.84 ±0.29
570
571Now, the tradeoff between locality and utilization is clearer. "cache" shows
5722% bandwidth loss compared to "system" and "cache (struct)" whopping 20%.
573
574
575Conclusion and Recommendations
576------------------------------
577
578In the above experiments, the efficiency advantage of the "cache" affinity
579scope over "system" is, while consistent and noticeable, small. However, the
580impact is dependent on the distances between the scopes and may be more
581pronounced in processors with more complex topologies.
582
583While the loss of work-conservation in certain scenarios hurts, it is a lot
584better than "cache (strict)" and maximizing workqueue utilization is
585unlikely to be the common case anyway. As such, "cache" is the default
586affinity scope for unbound pools.
587
588* As there is no one option which is great for most cases, workqueue usages
589  that may consume a significant amount of CPU are recommended to configure
590  the workqueues using ``apply_workqueue_attrs()`` and/or enable
591  ``WQ_SYSFS``.
592
593* An unbound workqueue with strict "cpu" affinity scope behaves the same as
594  ``WQ_CPU_INTENSIVE`` per-cpu workqueue. There is no real advanage to the
595  latter and an unbound workqueue provides a lot more flexibility.
596
597* Affinity scopes are introduced in Linux v6.5. To emulate the previous
598  behavior, use strict "numa" affinity scope.
599
600* The loss of work-conservation in non-strict affinity scopes is likely
601  originating from the scheduler. There is no theoretical reason why the
602  kernel wouldn't be able to do the right thing and maintain
603  work-conservation in most cases. As such, it is possible that future
604  scheduler improvements may make most of these tunables unnecessary.
605
606
607Examining Configuration
608=======================
609
610Use tools/workqueue/wq_dump.py to examine unbound CPU affinity
611configuration, worker pools and how workqueues map to the pools: ::
612
613  $ tools/workqueue/wq_dump.py
614  Affinity Scopes
615  ===============
616  wq_unbound_cpumask=0000000f
617
618  CPU
619    nr_pods  4
620    pod_cpus [0]=00000001 [1]=00000002 [2]=00000004 [3]=00000008
621    pod_node [0]=0 [1]=0 [2]=1 [3]=1
622    cpu_pod  [0]=0 [1]=1 [2]=2 [3]=3
623
624  SMT
625    nr_pods  4
626    pod_cpus [0]=00000001 [1]=00000002 [2]=00000004 [3]=00000008
627    pod_node [0]=0 [1]=0 [2]=1 [3]=1
628    cpu_pod  [0]=0 [1]=1 [2]=2 [3]=3
629
630  CACHE (default)
631    nr_pods  2
632    pod_cpus [0]=00000003 [1]=0000000c
633    pod_node [0]=0 [1]=1
634    cpu_pod  [0]=0 [1]=0 [2]=1 [3]=1
635
636  NUMA
637    nr_pods  2
638    pod_cpus [0]=00000003 [1]=0000000c
639    pod_node [0]=0 [1]=1
640    cpu_pod  [0]=0 [1]=0 [2]=1 [3]=1
641
642  SYSTEM
643    nr_pods  1
644    pod_cpus [0]=0000000f
645    pod_node [0]=-1
646    cpu_pod  [0]=0 [1]=0 [2]=0 [3]=0
647
648  Worker Pools
649  ============
650  pool[00] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  0
651  pool[01] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  0
652  pool[02] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  1
653  pool[03] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  1
654  pool[04] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  2
655  pool[05] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  2
656  pool[06] ref= 1 nice=  0 idle/workers=  3/  3 cpu=  3
657  pool[07] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  3
658  pool[08] ref=42 nice=  0 idle/workers=  6/  6 cpus=0000000f
659  pool[09] ref=28 nice=  0 idle/workers=  3/  3 cpus=00000003
660  pool[10] ref=28 nice=  0 idle/workers= 17/ 17 cpus=0000000c
661  pool[11] ref= 1 nice=-20 idle/workers=  1/  1 cpus=0000000f
662  pool[12] ref= 2 nice=-20 idle/workers=  1/  1 cpus=00000003
663  pool[13] ref= 2 nice=-20 idle/workers=  1/  1 cpus=0000000c
664
665  Workqueue CPU -> pool
666  =====================
667  [    workqueue \ CPU              0  1  2  3 dfl]
668  events                   percpu   0  2  4  6
669  events_highpri           percpu   1  3  5  7
670  events_long              percpu   0  2  4  6
671  events_unbound           unbound  9  9 10 10  8
672  events_freezable         percpu   0  2  4  6
673  events_power_efficient   percpu   0  2  4  6
674  events_freezable_pwr_ef  percpu   0  2  4  6
675  rcu_gp                   percpu   0  2  4  6
676  rcu_par_gp               percpu   0  2  4  6
677  slub_flushwq             percpu   0  2  4  6
678  netns                    ordered  8  8  8  8  8
679  ...
680
681See the command's help message for more info.
682
683
684Monitoring
685==========
686
687Use tools/workqueue/wq_monitor.py to monitor workqueue operations: ::
688
689  $ tools/workqueue/wq_monitor.py events
690                              total  infl  CPUtime  CPUhog CMW/RPR  mayday rescued
691  events                      18545     0      6.1       0       5       -       -
692  events_highpri                  8     0      0.0       0       0       -       -
693  events_long                     3     0      0.0       0       0       -       -
694  events_unbound              38306     0      0.1       -       7       -       -
695  events_freezable                0     0      0.0       0       0       -       -
696  events_power_efficient      29598     0      0.2       0       0       -       -
697  events_freezable_pwr_ef        10     0      0.0       0       0       -       -
698  sock_diag_events                0     0      0.0       0       0       -       -
699
700                              total  infl  CPUtime  CPUhog CMW/RPR  mayday rescued
701  events                      18548     0      6.1       0       5       -       -
702  events_highpri                  8     0      0.0       0       0       -       -
703  events_long                     3     0      0.0       0       0       -       -
704  events_unbound              38322     0      0.1       -       7       -       -
705  events_freezable                0     0      0.0       0       0       -       -
706  events_power_efficient      29603     0      0.2       0       0       -       -
707  events_freezable_pwr_ef        10     0      0.0       0       0       -       -
708  sock_diag_events                0     0      0.0       0       0       -       -
709
710  ...
711
712See the command's help message for more info.
713
714
715Debugging
716=========
717
718Because the work functions are executed by generic worker threads
719there are a few tricks needed to shed some light on misbehaving
720workqueue users.
721
722Worker threads show up in the process list as: ::
723
724  root      5671  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/0:1]
725  root      5672  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/1:2]
726  root      5673  0.0  0.0      0     0 ?        S    12:12   0:00 [kworker/0:0]
727  root      5674  0.0  0.0      0     0 ?        S    12:13   0:00 [kworker/1:0]
728
729If kworkers are going crazy (using too much cpu), there are two types
730of possible problems:
731
732	1. Something being scheduled in rapid succession
733	2. A single work item that consumes lots of cpu cycles
734
735The first one can be tracked using tracing: ::
736
737	$ echo workqueue:workqueue_queue_work > /sys/kernel/tracing/set_event
738	$ cat /sys/kernel/tracing/trace_pipe > out.txt
739	(wait a few secs)
740	^C
741
742If something is busy looping on work queueing, it would be dominating
743the output and the offender can be determined with the work item
744function.
745
746For the second type of problems it should be possible to just check
747the stack trace of the offending worker thread. ::
748
749	$ cat /proc/THE_OFFENDING_KWORKER/stack
750
751The work item's function should be trivially visible in the stack
752trace.
753
754
755Non-reentrance Conditions
756=========================
757
758Workqueue guarantees that a work item cannot be re-entrant if the following
759conditions hold after a work item gets queued:
760
761        1. The work function hasn't been changed.
762        2. No one queues the work item to another workqueue.
763        3. The work item hasn't been reinitiated.
764
765In other words, if the above conditions hold, the work item is guaranteed to be
766executed by at most one worker system-wide at any given time.
767
768Note that requeuing the work item (to the same queue) in the self function
769doesn't break these conditions, so it's safe to do. Otherwise, caution is
770required when breaking the conditions inside a work function.
771
772
773Kernel Inline Documentations Reference
774======================================
775
776.. kernel-doc:: include/linux/workqueue.h
777
778.. kernel-doc:: kernel/workqueue.c
779