Root/Documentation/workqueue.txt

1
2Concurrency Managed Workqueue (cmwq)
3
4September, 2010 Tejun Heo <tj@kernel.org>
5            Florian Mickler <florian@mickler.org>
6
7CONTENTS
8
91. Introduction
102. Why cmwq?
113. The Design
124. Application Programming Interface (API)
135. Example Execution Scenarios
146. Guidelines
157. Debugging
16
17
181. Introduction
19
20There are many cases where an asynchronous process execution context
21is needed and the workqueue (wq) API is the most commonly used
22mechanism for such cases.
23
24When such an asynchronous execution context is needed, a work item
25describing which function to execute is put on a queue. An
26independent thread serves as the asynchronous execution context. The
27queue is called workqueue and the thread is called worker.
28
29While there are work items on the workqueue the worker executes the
30functions associated with the work items one after the other. When
31there is no work item left on the workqueue the worker becomes idle.
32When a new work item gets queued, the worker begins executing again.
33
34
352. Why cmwq?
36
37In the original wq implementation, a multi threaded (MT) wq had one
38worker thread per CPU and a single threaded (ST) wq had one worker
39thread system-wide. A single MT wq needed to keep around the same
40number of workers as the number of CPUs. The kernel grew a lot of MT
41wq users over the years and with the number of CPU cores continuously
42rising, some systems saturated the default 32k PID space just booting
43up.
44
45Although MT wq wasted a lot of resource, the level of concurrency
46provided was unsatisfactory. The limitation was common to both ST and
47MT wq albeit less severe on MT. Each wq maintained its own separate
48worker pool. A MT wq could provide only one execution context per CPU
49while a ST wq one for the whole system. Work items had to compete for
50those very limited execution contexts leading to various problems
51including proneness to deadlocks around the single execution context.
52
53The tension between the provided level of concurrency and resource
54usage also forced its users to make unnecessary tradeoffs like libata
55choosing to use ST wq for polling PIOs and accepting an unnecessary
56limitation that no two polling PIOs can progress at the same time. As
57MT wq don't provide much better concurrency, users which require
58higher level of concurrency, like async or fscache, had to implement
59their own thread pool.
60
61Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
62focus on the following goals.
63
64* Maintain compatibility with the original workqueue API.
65
66* Use per-CPU unified worker pools shared by all wq to provide
67  flexible level of concurrency on demand without wasting a lot of
68  resource.
69
70* Automatically regulate worker pool and level of concurrency so that
71  the API users don't need to worry about such details.
72
73
743. The Design
75
76In order to ease the asynchronous execution of functions a new
77abstraction, the work item, is introduced.
78
79A work item is a simple struct that holds a pointer to the function
80that is to be executed asynchronously. Whenever a driver or subsystem
81wants a function to be executed asynchronously it has to set up a work
82item pointing to that function and queue that work item on a
83workqueue.
84
85Special purpose threads, called worker threads, execute the functions
86off of the queue, one after the other. If no work is queued, the
87worker threads become idle. These worker threads are managed in so
88called thread-pools.
89
90The cmwq design differentiates between the user-facing workqueues that
91subsystems and drivers queue work items on and the backend mechanism
92which manages thread-pool and processes the queued work items.
93
94The backend is called gcwq. There is one gcwq for each possible CPU
95and one gcwq to serve work items queued on unbound workqueues.
96
97Subsystems and drivers can create and queue work items through special
98workqueue API functions as they see fit. They can influence some
99aspects of the way the work items are executed by setting flags on the
100workqueue they are putting the work item on. These flags include
101things like CPU locality, reentrancy, concurrency limits and more. To
102get a detailed overview refer to the API description of
103alloc_workqueue() below.
104
105When a work item is queued to a workqueue, the target gcwq is
106determined according to the queue parameters and workqueue attributes
107and appended on the shared worklist of the gcwq. For example, unless
108specifically overridden, a work item of a bound workqueue will be
109queued on the worklist of exactly that gcwq that is associated to the
110CPU the issuer is running on.
111
112For any worker pool implementation, managing the concurrency level
113(how many execution contexts are active) is an important issue. cmwq
114tries to keep the concurrency at a minimal but sufficient level.
115Minimal to save resources and sufficient in that the system is used at
116its full capacity.
117
118Each gcwq bound to an actual CPU implements concurrency management by
119hooking into the scheduler. The gcwq is notified whenever an active
120worker wakes up or sleeps and keeps track of the number of the
121currently runnable workers. Generally, work items are not expected to
122hog a CPU and consume many cycles. That means maintaining just enough
123concurrency to prevent work processing from stalling should be
124optimal. As long as there are one or more runnable workers on the
125CPU, the gcwq doesn't start execution of a new work, but, when the
126last running worker goes to sleep, it immediately schedules a new
127worker so that the CPU doesn't sit idle while there are pending work
128items. This allows using a minimal number of workers without losing
129execution bandwidth.
130
131Keeping idle workers around doesn't cost other than the memory space
132for kthreads, so cmwq holds onto idle ones for a while before killing
133them.
134
135For an unbound wq, the above concurrency management doesn't apply and
136the gcwq for the pseudo unbound CPU tries to start executing all work
137items as soon as possible. The responsibility of regulating
138concurrency level is on the users. There is also a flag to mark a
139bound wq to ignore the concurrency management. Please refer to the
140API section for details.
141
142Forward progress guarantee relies on that workers can be created when
143more execution contexts are necessary, which in turn is guaranteed
144through the use of rescue workers. All work items which might be used
145on code paths that handle memory reclaim are required to be queued on
146wq's that have a rescue-worker reserved for execution under memory
147pressure. Else it is possible that the thread-pool deadlocks waiting
148for execution contexts to free up.
149
150
1514. Application Programming Interface (API)
152
153alloc_workqueue() allocates a wq. The original create_*workqueue()
154functions are deprecated and scheduled for removal. alloc_workqueue()
155takes three arguments - @name, @flags and @max_active. @name is the
156name of the wq and also used as the name of the rescuer thread if
157there is one.
158
159A wq no longer manages execution resources but serves as a domain for
160forward progress guarantee, flush and work item attributes. @flags
161and @max_active control how work items are assigned execution
162resources, scheduled and executed.
163
164@flags:
165
166  WQ_NON_REENTRANT
167
168    By default, a wq guarantees non-reentrance only on the same
169    CPU. A work item may not be executed concurrently on the same
170    CPU by multiple workers but is allowed to be executed
171    concurrently on multiple CPUs. This flag makes sure
172    non-reentrance is enforced across all CPUs. Work items queued
173    to a non-reentrant wq are guaranteed to be executed by at most
174    one worker system-wide at any given time.
175
176  WQ_UNBOUND
177
178    Work items queued to an unbound wq are served by a special
179    gcwq which hosts workers which are not bound to any specific
180    CPU. This makes the wq behave as a simple execution context
181    provider without concurrency management. The unbound gcwq
182    tries to start execution of work items as soon as possible.
183    Unbound wq sacrifices locality but is useful for the following
184    cases.
185
186    * Wide fluctuation in the concurrency level requirement is
187      expected and using bound wq may end up creating large number
188      of mostly unused workers across different CPUs as the issuer
189      hops through different CPUs.
190
191    * Long running CPU intensive workloads which can be better
192      managed by the system scheduler.
193
194  WQ_FREEZABLE
195
196    A freezable wq participates in the freeze phase of the system
197    suspend operations. Work items on the wq are drained and no
198    new work item starts execution until thawed.
199
200  WQ_MEM_RECLAIM
201
202    All wq which might be used in the memory reclaim paths _MUST_
203    have this flag set. The wq is guaranteed to have at least one
204    execution context regardless of memory pressure.
205
206  WQ_HIGHPRI
207
208    Work items of a highpri wq are queued at the head of the
209    worklist of the target gcwq and start execution regardless of
210    the current concurrency level. In other words, highpri work
211    items will always start execution as soon as execution
212    resource is available.
213
214    Ordering among highpri work items is preserved - a highpri
215    work item queued after another highpri work item will start
216    execution after the earlier highpri work item starts.
217
218    Although highpri work items are not held back by other
219    runnable work items, they still contribute to the concurrency
220    level. Highpri work items in runnable state will prevent
221    non-highpri work items from starting execution.
222
223    This flag is meaningless for unbound wq.
224
225  WQ_CPU_INTENSIVE
226
227    Work items of a CPU intensive wq do not contribute to the
228    concurrency level. In other words, runnable CPU intensive
229    work items will not prevent other work items from starting
230    execution. This is useful for bound work items which are
231    expected to hog CPU cycles so that their execution is
232    regulated by the system scheduler.
233
234    Although CPU intensive work items don't contribute to the
235    concurrency level, start of their executions is still
236    regulated by the concurrency management and runnable
237    non-CPU-intensive work items can delay execution of CPU
238    intensive work items.
239
240    This flag is meaningless for unbound wq.
241
242  WQ_HIGHPRI | WQ_CPU_INTENSIVE
243
244    This combination makes the wq avoid interaction with
245    concurrency management completely and behave as a simple
246    per-CPU execution context provider. Work items queued on a
247    highpri CPU-intensive wq start execution as soon as resources
248    are available and don't affect execution of other work items.
249
250@max_active:
251
252@max_active determines the maximum number of execution contexts per
253CPU which can be assigned to the work items of a wq. For example,
254with @max_active of 16, at most 16 work items of the wq can be
255executing at the same time per CPU.
256
257Currently, for a bound wq, the maximum limit for @max_active is 512
258and the default value used when 0 is specified is 256. For an unbound
259wq, the limit is higher of 512 and 4 * num_possible_cpus(). These
260values are chosen sufficiently high such that they are not the
261limiting factor while providing protection in runaway cases.
262
263The number of active work items of a wq is usually regulated by the
264users of the wq, more specifically, by how many work items the users
265may queue at the same time. Unless there is a specific need for
266throttling the number of active work items, specifying '0' is
267recommended.
268
269Some users depend on the strict execution ordering of ST wq. The
270combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
271behavior. Work items on such wq are always queued to the unbound gcwq
272and only one work item can be active at any given time thus achieving
273the same ordering property as ST wq.
274
275
2765. Example Execution Scenarios
277
278The following example execution scenarios try to illustrate how cmwq
279behave under different configurations.
280
281 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
282 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
283 again before finishing. w1 and w2 burn CPU for 5ms then sleep for
284 10ms.
285
286Ignoring all other tasks, works and processing overhead, and assuming
287simple FIFO scheduling, the following is one highly simplified version
288of possible sequences of events with the original wq.
289
290 TIME IN MSECS EVENT
291 0 w0 starts and burns CPU
292 5 w0 sleeps
293 15 w0 wakes up and burns CPU
294 20 w0 finishes
295 20 w1 starts and burns CPU
296 25 w1 sleeps
297 35 w1 wakes up and finishes
298 35 w2 starts and burns CPU
299 40 w2 sleeps
300 50 w2 wakes up and finishes
301
302And with cmwq with @max_active >= 3,
303
304 TIME IN MSECS EVENT
305 0 w0 starts and burns CPU
306 5 w0 sleeps
307 5 w1 starts and burns CPU
308 10 w1 sleeps
309 10 w2 starts and burns CPU
310 15 w2 sleeps
311 15 w0 wakes up and burns CPU
312 20 w0 finishes
313 20 w1 wakes up and finishes
314 25 w2 wakes up and finishes
315
316If @max_active == 2,
317
318 TIME IN MSECS EVENT
319 0 w0 starts and burns CPU
320 5 w0 sleeps
321 5 w1 starts and burns CPU
322 10 w1 sleeps
323 15 w0 wakes up and burns CPU
324 20 w0 finishes
325 20 w1 wakes up and finishes
326 20 w2 starts and burns CPU
327 25 w2 sleeps
328 35 w2 wakes up and finishes
329
330Now, let's assume w1 and w2 are queued to a different wq q1 which has
331WQ_HIGHPRI set,
332
333 TIME IN MSECS EVENT
334 0 w1 and w2 start and burn CPU
335 5 w1 sleeps
336 10 w2 sleeps
337 10 w0 starts and burns CPU
338 15 w0 sleeps
339 15 w1 wakes up and finishes
340 20 w2 wakes up and finishes
341 25 w0 wakes up and burns CPU
342 30 w0 finishes
343
344If q1 has WQ_CPU_INTENSIVE set,
345
346 TIME IN MSECS EVENT
347 0 w0 starts and burns CPU
348 5 w0 sleeps
349 5 w1 and w2 start and burn CPU
350 10 w1 sleeps
351 15 w2 sleeps
352 15 w0 wakes up and burns CPU
353 20 w0 finishes
354 20 w1 wakes up and finishes
355 25 w2 wakes up and finishes
356
357
3586. Guidelines
359
360* Do not forget to use WQ_MEM_RECLAIM if a wq may process work items
361  which are used during memory reclaim. Each wq with WQ_MEM_RECLAIM
362  set has an execution context reserved for it. If there is
363  dependency among multiple work items used during memory reclaim,
364  they should be queued to separate wq each with WQ_MEM_RECLAIM.
365
366* Unless strict ordering is required, there is no need to use ST wq.
367
368* Unless there is a specific need, using 0 for @max_active is
369  recommended. In most use cases, concurrency level usually stays
370  well under the default limit.
371
372* A wq serves as a domain for forward progress guarantee
373  (WQ_MEM_RECLAIM, flush and work item attributes. Work items which
374  are not involved in memory reclaim and don't need to be flushed as a
375  part of a group of work items, and don't require any special
376  attribute, can use one of the system wq. There is no difference in
377  execution characteristics between using a dedicated wq and a system
378  wq.
379
380* Unless work items are expected to consume a huge amount of CPU
381  cycles, using a bound wq is usually beneficial due to the increased
382  level of locality in wq operations and work item execution.
383
384
3857. Debugging
386
387Because the work functions are executed by generic worker threads
388there are a few tricks needed to shed some light on misbehaving
389workqueue users.
390
391Worker threads show up in the process list as:
392
393root 5671 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/0:1]
394root 5672 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/1:2]
395root 5673 0.0 0.0 0 0 ? S 12:12 0:00 [kworker/0:0]
396root 5674 0.0 0.0 0 0 ? S 12:13 0:00 [kworker/1:0]
397
398If kworkers are going crazy (using too much cpu), there are two types
399of possible problems:
400
401    1. Something beeing scheduled in rapid succession
402    2. A single work item that consumes lots of cpu cycles
403
404The first one can be tracked using tracing:
405
406    $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
407    $ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
408    (wait a few secs)
409    ^C
410
411If something is busy looping on work queueing, it would be dominating
412the output and the offender can be determined with the work item
413function.
414
415For the second type of problems it should be possible to just check
416the stack trace of the offending worker thread.
417
418    $ cat /proc/THE_OFFENDING_KWORKER/stack
419
420The work item's function should be trivially visible in the stack
421trace.
422

Archive Download this file



interactive