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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	====================================
2	Concurrency Managed Workqueue (cmwq)
3	====================================
5	:Date: September, 2010
6	:Author: Tejun Heo <tj@kernel.org>
7	:Author: Florian Mickler <florian@mickler.org>
10	Introduction
11	============
13	There are many cases where an asynchronous process execution context
14	is needed and the workqueue (wq) API is the most commonly used
15	mechanism for such cases.
17	When such an asynchronous execution context is needed, a work item
18	describing which function to execute is put on a queue.  An
19	independent thread serves as the asynchronous execution context.  The
20	queue is called workqueue and the thread is called worker.
22	While there are work items on the workqueue the worker executes the
23	functions associated with the work items one after the other.  When
24	there is no work item left on the workqueue the worker becomes idle.
25	When a new work item gets queued, the worker begins executing again.
28	Why cmwq?
29	=========
31	In the original wq implementation, a multi threaded (MT) wq had one
32	worker thread per CPU and a single threaded (ST) wq had one worker
33	thread system-wide.  A single MT wq needed to keep around the same
34	number of workers as the number of CPUs.  The kernel grew a lot of MT
35	wq users over the years and with the number of CPU cores continuously
36	rising, some systems saturated the default 32k PID space just booting
37	up.
39	Although MT wq wasted a lot of resource, the level of concurrency
40	provided was unsatisfactory.  The limitation was common to both ST and
41	MT wq albeit less severe on MT.  Each wq maintained its own separate
42	worker pool.  An MT wq could provide only one execution context per CPU
43	while an ST wq one for the whole system.  Work items had to compete for
44	those very limited execution contexts leading to various problems
45	including proneness to deadlocks around the single execution context.
47	The tension between the provided level of concurrency and resource
48	usage also forced its users to make unnecessary tradeoffs like libata
49	choosing to use ST wq for polling PIOs and accepting an unnecessary
50	limitation that no two polling PIOs can progress at the same time.  As
51	MT wq don't provide much better concurrency, users which require
52	higher level of concurrency, like async or fscache, had to implement
53	their own thread pool.
55	Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
56	focus on the following goals.
58	* Maintain compatibility with the original workqueue API.
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.
64	* Automatically regulate worker pool and level of concurrency so that
65	  the API users don't need to worry about such details.
68	The Design
69	==========
71	In order to ease the asynchronous execution of functions a new
72	abstraction, the work item, is introduced.
74	A work item is a simple struct that holds a pointer to the function
75	that is to be executed asynchronously.  Whenever a driver or subsystem
76	wants a function to be executed asynchronously it has to set up a work
77	item pointing to that function and queue that work item on a
78	workqueue.
80	Special purpose threads, called worker threads, execute the functions
81	off of the queue, one after the other.  If no work is queued, the
82	worker threads become idle.  These worker threads are managed in so
83	called worker-pools.
85	The cmwq design differentiates between the user-facing workqueues that
86	subsystems and drivers queue work items on and the backend mechanism
87	which manages worker-pools and processes the queued work items.
89	There are two worker-pools, one for normal work items and the other
90	for high priority ones, for each possible CPU and some extra
91	worker-pools to serve work items queued on unbound workqueues - the
92	number of these backing pools is dynamic.
94	Subsystems and drivers can create and queue work items through special
95	workqueue API functions as they see fit. They can influence some
96	aspects of the way the work items are executed by setting flags on the
97	workqueue they are putting the work item on. These flags include
98	things like CPU locality, concurrency limits, priority and more.  To
99	get a detailed overview refer to the API description of
100	``alloc_workqueue()`` below.
102	When a work item is queued to a workqueue, the target worker-pool is
103	determined according to the queue parameters and workqueue attributes
104	and appended on the shared worklist of the worker-pool.  For example,
105	unless specifically overridden, a work item of a bound workqueue will
106	be queued on the worklist of either normal or highpri worker-pool that
107	is associated to the CPU the issuer is running on.
109	For any worker pool implementation, managing the concurrency level
110	(how many execution contexts are active) is an important issue.  cmwq
111	tries to keep the concurrency at a minimal but sufficient level.
112	Minimal to save resources and sufficient in that the system is used at
113	its full capacity.
115	Each worker-pool bound to an actual CPU implements concurrency
116	management by hooking into the scheduler.  The worker-pool is notified
117	whenever an active worker wakes up or sleeps and keeps track of the
118	number of the currently runnable workers.  Generally, work items are
119	not expected to hog a CPU and consume many cycles.  That means
120	maintaining just enough concurrency to prevent work processing from
121	stalling should be optimal.  As long as there are one or more runnable
122	workers on the CPU, the worker-pool doesn't start execution of a new
123	work, but, when the last running worker goes to sleep, it immediately
124	schedules a new worker so that the CPU doesn't sit idle while there
125	are pending work items.  This allows using a minimal number of workers
126	without losing execution bandwidth.
128	Keeping idle workers around doesn't cost other than the memory space
129	for kthreads, so cmwq holds onto idle ones for a while before killing
130	them.
132	For unbound workqueues, the number of backing pools is dynamic.
133	Unbound workqueue can be assigned custom attributes using
134	``apply_workqueue_attrs()`` and workqueue will automatically create
135	backing worker pools matching the attributes.  The responsibility of
136	regulating concurrency level is on the users.  There is also a flag to
137	mark a bound wq to ignore the concurrency management.  Please refer to
138	the API section for details.
140	Forward progress guarantee relies on that workers can be created when
141	more execution contexts are necessary, which in turn is guaranteed
142	through the use of rescue workers.  All work items which might be used
143	on code paths that handle memory reclaim are required to be queued on
144	wq's that have a rescue-worker reserved for execution under memory
145	pressure.  Else it is possible that the worker-pool deadlocks waiting
146	for execution contexts to free up.
149	Application Programming Interface (API)
150	=======================================
152	``alloc_workqueue()`` allocates a wq.  The original
153	``create_*workqueue()`` functions are deprecated and scheduled for
154	removal.  ``alloc_workqueue()`` takes three arguments - ``@name``,
155	``@flags`` and ``@max_active``.  ``@name`` is the name of the wq and
156	also used as the name of the rescuer thread if there is one.
158	A wq no longer manages execution resources but serves as a domain for
159	forward progress guarantee, flush and work item attributes. ``@flags``
160	and ``@max_active`` control how work items are assigned execution
161	resources, scheduled and executed.
164	``flags``
165	---------
167	``WQ_UNBOUND``
168	  Work items queued to an unbound wq are served by the special
169	  worker-pools which host workers which are not bound to any
170	  specific CPU.  This makes the wq behave as a simple execution
171	  context provider without concurrency management.  The unbound
172	  worker-pools try to start execution of work items as soon as
173	  possible.  Unbound wq sacrifices locality but is useful for
174	  the following cases.
176	  * Wide fluctuation in the concurrency level requirement is
177	    expected and using bound wq may end up creating large number
178	    of mostly unused workers across different CPUs as the issuer
179	    hops through different CPUs.
181	  * Long running CPU intensive workloads which can be better
182	    managed by the system scheduler.
185	  A freezable wq participates in the freeze phase of the system
186	  suspend operations.  Work items on the wq are drained and no
187	  new work item starts execution until thawed.
190	  All wq which might be used in the memory reclaim paths **MUST**
191	  have this flag set.  The wq is guaranteed to have at least one
192	  execution context regardless of memory pressure.
194	``WQ_HIGHPRI``
195	  Work items of a highpri wq are queued to the highpri
196	  worker-pool of the target cpu.  Highpri worker-pools are
197	  served by worker threads with elevated nice level.
199	  Note that normal and highpri worker-pools don't interact with
200	  each other.  Each maintains its separate pool of workers and
201	  implements concurrency management among its workers.
204	  Work items of a CPU intensive wq do not contribute to the
205	  concurrency level.  In other words, runnable CPU intensive
206	  work items will not prevent other work items in the same
207	  worker-pool from starting execution.  This is useful for bound
208	  work items which are expected to hog CPU cycles so that their
209	  execution is regulated by the system scheduler.
211	  Although CPU intensive work items don't contribute to the
212	  concurrency level, start of their executions is still
213	  regulated by the concurrency management and runnable
214	  non-CPU-intensive work items can delay execution of CPU
215	  intensive work items.
217	  This flag is meaningless for unbound wq.
219	Note that the flag ``WQ_NON_REENTRANT`` no longer exists as all
220	workqueues are now non-reentrant - any work item is guaranteed to be
221	executed by at most one worker system-wide at any given time.
224	``max_active``
225	--------------
227	``@max_active`` determines the maximum number of execution contexts
228	per CPU which can be assigned to the work items of a wq.  For example,
229	with ``@max_active`` of 16, at most 16 work items of the wq can be
230	executing at the same time per CPU.
232	Currently, for a bound wq, the maximum limit for ``@max_active`` is
233	512 and the default value used when 0 is specified is 256.  For an
234	unbound wq, the limit is higher of 512 and 4 *
235	``num_possible_cpus()``.  These values are chosen sufficiently high
236	such that they are not the limiting factor while providing protection
237	in runaway cases.
239	The number of active work items of a wq is usually regulated by the
240	users of the wq, more specifically, by how many work items the users
241	may queue at the same time.  Unless there is a specific need for
242	throttling the number of active work items, specifying '0' is
243	recommended.
245	Some users depend on the strict execution ordering of ST wq.  The
246	combination of ``@max_active`` of 1 and ``WQ_UNBOUND`` used to
247	achieve this behavior.  Work items on such wq were always queued to the
248	unbound worker-pools and only one work item could be active at any given
249	time thus achieving the same ordering property as ST wq.
251	In the current implementation the above configuration only guarantees
252	ST behavior within a given NUMA node. Instead ``alloc_ordered_queue()`` should
253	be used to achieve system-wide ST behavior.
256	Example Execution Scenarios
257	===========================
259	The following example execution scenarios try to illustrate how cmwq
260	behave under different configurations.
262	 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
263	 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
264	 again before finishing.  w1 and w2 burn CPU for 5ms then sleep for
265	 10ms.
267	Ignoring all other tasks, works and processing overhead, and assuming
268	simple FIFO scheduling, the following is one highly simplified version
269	of possible sequences of events with the original wq. ::
272	 0		w0 starts and burns CPU
273	 5		w0 sleeps
274	 15		w0 wakes up and burns CPU
275	 20		w0 finishes
276	 20		w1 starts and burns CPU
277	 25		w1 sleeps
278	 35		w1 wakes up and finishes
279	 35		w2 starts and burns CPU
280	 40		w2 sleeps
281	 50		w2 wakes up and finishes
283	And with cmwq with ``@max_active`` >= 3, ::
286	 0		w0 starts and burns CPU
287	 5		w0 sleeps
288	 5		w1 starts and burns CPU
289	 10		w1 sleeps
290	 10		w2 starts and burns CPU
291	 15		w2 sleeps
292	 15		w0 wakes up and burns CPU
293	 20		w0 finishes
294	 20		w1 wakes up and finishes
295	 25		w2 wakes up and finishes
297	If ``@max_active`` == 2, ::
300	 0		w0 starts and burns CPU
301	 5		w0 sleeps
302	 5		w1 starts and burns CPU
303	 10		w1 sleeps
304	 15		w0 wakes up and burns CPU
305	 20		w0 finishes
306	 20		w1 wakes up and finishes
307	 20		w2 starts and burns CPU
308	 25		w2 sleeps
309	 35		w2 wakes up and finishes
311	Now, let's assume w1 and w2 are queued to a different wq q1 which has
312	``WQ_CPU_INTENSIVE`` set, ::
315	 0		w0 starts and burns CPU
316	 5		w0 sleeps
317	 5		w1 and w2 start and burn CPU
318	 10		w1 sleeps
319	 15		w2 sleeps
320	 15		w0 wakes up and burns CPU
321	 20		w0 finishes
322	 20		w1 wakes up and finishes
323	 25		w2 wakes up and finishes
326	Guidelines
327	==========
329	* Do not forget to use ``WQ_MEM_RECLAIM`` if a wq may process work
330	  items which are used during memory reclaim.  Each wq with
331	  ``WQ_MEM_RECLAIM`` set has an execution context reserved for it.  If
332	  there is dependency among multiple work items used during memory
333	  reclaim, they should be queued to separate wq each with
334	  ``WQ_MEM_RECLAIM``.
336	* Unless strict ordering is required, there is no need to use ST wq.
338	* Unless there is a specific need, using 0 for @max_active is
339	  recommended.  In most use cases, concurrency level usually stays
340	  well under the default limit.
342	* A wq serves as a domain for forward progress guarantee
343	  (``WQ_MEM_RECLAIM``, flush and work item attributes.  Work items
344	  which are not involved in memory reclaim and don't need to be
345	  flushed as a part of a group of work items, and don't require any
346	  special attribute, can use one of the system wq.  There is no
347	  difference in execution characteristics between using a dedicated wq
348	  and a system wq.
350	* Unless work items are expected to consume a huge amount of CPU
351	  cycles, using a bound wq is usually beneficial due to the increased
352	  level of locality in wq operations and work item execution.
355	Debugging
356	=========
358	Because the work functions are executed by generic worker threads
359	there are a few tricks needed to shed some light on misbehaving
360	workqueue users.
362	Worker threads show up in the process list as: ::
364	  root      5671  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/0:1]
365	  root      5672  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/1:2]
366	  root      5673  0.0  0.0      0     0 ?        S    12:12   0:00 [kworker/0:0]
367	  root      5674  0.0  0.0      0     0 ?        S    12:13   0:00 [kworker/1:0]
369	If kworkers are going crazy (using too much cpu), there are two types
370	of possible problems:
372		1. Something being scheduled in rapid succession
373		2. A single work item that consumes lots of cpu cycles
375	The first one can be tracked using tracing: ::
377		$ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
378		$ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
379		(wait a few secs)
380		^C
382	If something is busy looping on work queueing, it would be dominating
383	the output and the offender can be determined with the work item
384	function.
386	For the second type of problems it should be possible to just check
387	the stack trace of the offending worker thread. ::
389		$ cat /proc/THE_OFFENDING_KWORKER/stack
391	The work item's function should be trivially visible in the stack
392	trace.
395	Kernel Inline Documentations Reference
396	======================================
398	.. kernel-doc:: include/linux/workqueue.h
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