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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
+
+
+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 thread-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-pool 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.
+
+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 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 is
+determined according to the queue parameters and workqueue attributes
+and appended on the shared worklist of the gcwq. For example, unless
+specifically overridden, a work item of a bound workqueue will be
+queued on the worklist of exactly that gcwq 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 gcwq bound to an actual CPU implements concurrency management by
+hooking into the scheduler. The gcwq 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 gcwq 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 an unbound wq, the above concurrency management doesn't apply and
+the gcwq for the pseudo unbound CPU tries 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.
+
+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
+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_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.
+
+ * 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_FREEZEABLE
+
+ A freezeable 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_RESCUER
+
+ All wq which might be used in the memory reclaim paths _MUST_
+ have this flag set. This reserves one worker exclusively for
+ the execution of this wq under memory pressure.
+
+ WQ_HIGHPRI
+
+ Work items of a highpri wq are queued at the head of the
+ worklist of the target gcwq and start execution regardless of
+ the current concurrency level. In other words, highpri work
+ items will always start execution as soon as execution
+ resource is available.
+
+ Ordering among highpri work items is preserved - a highpri
+ work item queued after another highpri work item will start
+ execution after the earlier highpri work item starts.
+
+ Although highpri work items are not held back by other
+ runnable work items, they still contribute to the concurrency
+ level. Highpri work items in runnable state will prevent
+ non-highpri work items from starting execution.
+
+ This flag is meaningless for unbound wq.
+
+ 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 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.
+
+ WQ_HIGHPRI | WQ_CPU_INTENSIVE
+
+ This combination makes the wq avoid interaction with
+ concurrency management completely and behave as a simple
+ per-CPU execution context provider. Work items queued on a
+ highpri CPU-intensive wq start execution as soon as resources
+ are available and don't affect execution of other work items.
+
+@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 gcwq
+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_HIGHPRI set,
+
+ TIME IN MSECS EVENT
+ 0 w1 and w2 start and burn CPU
+ 5 w1 sleeps
+ 10 w2 sleeps
+ 10 w0 starts and burns CPU
+ 15 w0 sleeps
+ 15 w1 wakes up and finishes
+ 20 w2 wakes up and finishes
+ 25 w0 wakes up and burns CPU
+ 30 w0 finishes
+
+If q1 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_RESCUER if a wq may process work items which
+ are used during memory reclaim. Each wq with WQ_RESCUER set has one
+ rescuer thread 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_RESCUER.
+
+* 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_RESCUER),
+ 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.
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