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Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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-
-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.