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Merge branch 'for-3.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq 

Pull workqueue updates from Tejun Heo:
 "Nothing interesting.  All are doc / comment updates"

* 'for-3.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
  workqueue: Correct/Drop references to gcwq in Documentation
  workqueue: Fix manage_workers() RETURNS description
  workqueue: Comment correction in file header
  workqueue: mark WQ_NON_REENTRANT deprecated
This commit is contained in:
Linus Torvalds 2013-09-03 18:19:21 -07:00
parent 96d4e231d2 546d30c4a2
commit 9ee52a1633
3 changed files with 57 additions and 54 deletions
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90
Documentation/workqueue.txt
View File

@ -85,32 +85,31 @@ 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 thread-pools.
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 thread-pools and processes the queued work items.
which manages worker-pools and processes the queued work items.
The backend is called gcwq. There is one gcwq for each possible CPU
and one gcwq to serve work items queued on unbound workqueues. Each
gcwq has two thread-pools - one for normal work items and the other
for high priority ones.
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, reentrancy, concurrency limits, priority and
more. To get a detailed overview refer to the API description of
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 gcwq and
thread-pool is determined according to the queue parameters and
workqueue attributes and appended on the shared worklist of the
thread-pool. For example, unless specifically overridden, a work item
of a bound workqueue will be queued on the worklist of either normal
or highpri thread-pool of the gcwq that is associated to the CPU the
issuer is running on.
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
@ -118,14 +117,14 @@ 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 thread-pool bound to an actual CPU implements concurrency
management by hooking into the scheduler. The thread-pool is notified
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 thread-pool doesn't start execution of a new
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
@ -135,19 +134,20 @@ 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 an unbound wq, the above concurrency management doesn't apply and
the thread-pools for the pseudo unbound CPU try to start executing all
work items as soon as possible. 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.
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 thread-pool deadlocks waiting
pressure. Else it is possible that the worker-pool deadlocks waiting
for execution contexts to free up.
@ -166,25 +166,15 @@ resources, scheduled and executed.
@flags:
WQ_NON_REENTRANT
By default, a wq guarantees non-reentrance only on the same
CPU. A work item may not be executed concurrently on the same
CPU by multiple workers but is allowed to be executed
concurrently on multiple CPUs. This flag makes sure
non-reentrance is enforced across all CPUs. Work items queued
to a non-reentrant wq are guaranteed to be executed by at most
one worker system-wide at any given time.
WQ_UNBOUND
Work items queued to an unbound wq are served by a special
gcwq which hosts 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 gcwq
tries to start execution of work items as soon as possible.
Unbound wq sacrifices locality but is useful for the following
cases.
Work items queued to an unbound wq are served by the special
woker-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
@ -209,10 +199,10 @@ resources, scheduled and executed.
WQ_HIGHPRI
Work items of a highpri wq are queued to the highpri
thread-pool of the target gcwq. Highpri thread-pools are
worker-pool of the target cpu. Highpri worker-pools are
served by worker threads with elevated nice level.
Note that normal and highpri thread-pools don't interact with
Note that normal and highpri worker-pools don't interact with
each other. Each maintain its separate pool of workers and
implements concurrency management among its workers.
@ -221,7 +211,7 @@ resources, scheduled and executed.
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
thread-pool from starting execution. This is useful for bound
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.
@ -233,6 +223,10 @@ resources, scheduled and executed.
This flag is meaningless for unbound wq.
Note that the flag WQ_NON_REENTRANT no longer exists as all workqueues
are now non-reentrant - any work item is guaranteed to be executed by
at most one worker system-wide at any given time.
@max_active:
@max_active determines the maximum number of execution contexts per
@ -254,9 +248,9 @@ recommended.
Some users depend on the strict execution ordering of ST wq. The
combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
behavior. Work items on such wq are always queued to the unbound gcwq
and only one work item can be active at any given time thus achieving
the same ordering property as ST wq.
behavior. Work items on such wq are always queued to the unbound
worker-pools and only one work item can be active at any given time thus
achieving the same ordering property as ST wq.
5. Example Execution Scenarios

7
include/linux/workqueue.h
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@ -295,7 +295,12 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
* Documentation/workqueue.txt.
*/
enum {
WQ_NON_REENTRANT = 1 << 0, /* guarantee non-reentrance */
/*
* All wqs are now non-reentrant making the following flag
* meaningless. Will be removed.
*/
WQ_NON_REENTRANT = 1 << 0, /* DEPRECATED */
WQ_UNBOUND = 1 << 1, /* not bound to any cpu */
WQ_FREEZABLE = 1 << 2, /* freeze during suspend */
WQ_MEM_RECLAIM = 1 << 3, /* may be used for memory reclaim */

14
kernel/workqueue.c
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@ -16,9 +16,10 @@
*
* This is the generic async execution mechanism. Work items as are
* executed in process context. The worker pool is shared and
* automatically managed. There is one worker pool for each CPU and
* one extra for works which are better served by workers which are
* not bound to any specific CPU.
* automatically managed. There are two worker pools for each CPU (one for
* normal work items and the other for high priority ones) and some extra
* pools for workqueues which are not bound to any specific CPU - the
* number of these backing pools is dynamic.
*
* Please read Documentation/workqueue.txt for details.
*/
@ -2033,8 +2034,11 @@ static bool maybe_destroy_workers(struct worker_pool *pool)
* multiple times. Does GFP_KERNEL allocations.
*
* RETURNS:
* spin_lock_irq(pool->lock) which may be released and regrabbed
* multiple times. Does GFP_KERNEL allocations.
* %false if the pool don't need management and the caller can safely start
* processing works, %true indicates that the function released pool->lock
* and reacquired it to perform some management function and that the
* conditions that the caller verified while holding the lock before
* calling the function might no longer be true.
*/
static bool manage_workers(struct worker *worker)
{