--- /dev/null
+====================================
+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. A MT wq could provide only one execution context per CPU
+while a 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 maintain 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.
+
+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 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`` is used to
+achieve this 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.
+
+
+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.
+
+
+Kernel Inline Documentations Reference
+======================================
+
+.. kernel-doc:: include/linux/workqueue.h
+++ /dev/null
-
-Concurrency Managed Workqueue (cmwq)
-
-September, 2010 Tejun Heo <tj@kernel.org>
- Florian Mickler <florian@mickler.org>
-
-CONTENTS
-
-1. Introduction
-2. Why cmwq?
-3. The Design
-4. Application Programming Interface (API)
-5. Example Execution Scenarios
-6. Guidelines
-7. Debugging
-
-
-1. 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.
-
-
-2. 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. A MT wq could provide only one execution context per CPU
-while a 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.
-
-
-3. 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.
-
-
-4. 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 maintain 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.
-
-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
-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 is used to achieve this
-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
-
-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
-
-
-6. 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.
-
-
-7. 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.
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff