382 lines
14 KiB
Plaintext
382 lines
14 KiB
Plaintext
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Concurrency Managed Workqueue (cmwq)
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September, 2010 Tejun Heo <tj@kernel.org>
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Florian Mickler <florian@mickler.org>
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CONTENTS
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1. Introduction
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2. Why cmwq?
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3. The Design
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4. Application Programming Interface (API)
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5. Example Execution Scenarios
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6. Guidelines
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1. Introduction
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There are many cases where an asynchronous process execution context
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is needed and the workqueue (wq) API is the most commonly used
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mechanism for such cases.
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When such an asynchronous execution context is needed, a work item
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describing which function to execute is put on a queue. An
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independent thread serves as the asynchronous execution context. The
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queue is called workqueue and the thread is called worker.
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While there are work items on the workqueue the worker executes the
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functions associated with the work items one after the other. When
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there is no work item left on the workqueue the worker becomes idle.
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When a new work item gets queued, the worker begins executing again.
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2. Why cmwq?
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In the original wq implementation, a multi threaded (MT) wq had one
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worker thread per CPU and a single threaded (ST) wq had one worker
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thread system-wide. A single MT wq needed to keep around the same
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number of workers as the number of CPUs. The kernel grew a lot of MT
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wq users over the years and with the number of CPU cores continuously
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rising, some systems saturated the default 32k PID space just booting
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up.
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Although MT wq wasted a lot of resource, the level of concurrency
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provided was unsatisfactory. The limitation was common to both ST and
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MT wq albeit less severe on MT. Each wq maintained its own separate
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worker pool. A MT wq could provide only one execution context per CPU
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while a ST wq one for the whole system. Work items had to compete for
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those very limited execution contexts leading to various problems
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including proneness to deadlocks around the single execution context.
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The tension between the provided level of concurrency and resource
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usage also forced its users to make unnecessary tradeoffs like libata
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choosing to use ST wq for polling PIOs and accepting an unnecessary
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limitation that no two polling PIOs can progress at the same time. As
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MT wq don't provide much better concurrency, users which require
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higher level of concurrency, like async or fscache, had to implement
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their own thread pool.
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Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
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focus on the following goals.
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* Maintain compatibility with the original workqueue API.
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* Use per-CPU unified worker pools shared by all wq to provide
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flexible level of concurrency on demand without wasting a lot of
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resource.
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* Automatically regulate worker pool and level of concurrency so that
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the API users don't need to worry about such details.
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3. The Design
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In order to ease the asynchronous execution of functions a new
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abstraction, the work item, is introduced.
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A work item is a simple struct that holds a pointer to the function
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that is to be executed asynchronously. Whenever a driver or subsystem
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wants a function to be executed asynchronously it has to set up a work
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item pointing to that function and queue that work item on a
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workqueue.
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Special purpose threads, called worker threads, execute the functions
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off of the queue, one after the other. If no work is queued, the
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worker threads become idle. These worker threads are managed in so
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called thread-pools.
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The cmwq design differentiates between the user-facing workqueues that
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subsystems and drivers queue work items on and the backend mechanism
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which manages thread-pool and processes the queued work items.
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The backend is called gcwq. There is one gcwq for each possible CPU
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and one gcwq to serve work items queued on unbound workqueues.
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Subsystems and drivers can create and queue work items through special
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workqueue API functions as they see fit. They can influence some
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aspects of the way the work items are executed by setting flags on the
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workqueue they are putting the work item on. These flags include
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things like CPU locality, reentrancy, concurrency limits and more. To
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get a detailed overview refer to the API description of
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alloc_workqueue() below.
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When a work item is queued to a workqueue, the target gcwq is
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determined according to the queue parameters and workqueue attributes
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and appended on the shared worklist of the gcwq. For example, unless
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specifically overridden, a work item of a bound workqueue will be
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queued on the worklist of exactly that gcwq that is associated to the
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CPU the issuer is running on.
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For any worker pool implementation, managing the concurrency level
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(how many execution contexts are active) is an important issue. cmwq
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tries to keep the concurrency at a minimal but sufficient level.
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Minimal to save resources and sufficient in that the system is used at
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its full capacity.
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Each gcwq bound to an actual CPU implements concurrency management by
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hooking into the scheduler. The gcwq is notified whenever an active
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worker wakes up or sleeps and keeps track of the number of the
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currently runnable workers. Generally, work items are not expected to
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hog a CPU and consume many cycles. That means maintaining just enough
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concurrency to prevent work processing from stalling should be
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optimal. As long as there are one or more runnable workers on the
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CPU, the gcwq doesn't start execution of a new work, but, when the
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last running worker goes to sleep, it immediately schedules a new
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worker so that the CPU doesn't sit idle while there are pending work
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items. This allows using a minimal number of workers without losing
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execution bandwidth.
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Keeping idle workers around doesn't cost other than the memory space
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for kthreads, so cmwq holds onto idle ones for a while before killing
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them.
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For an unbound wq, the above concurrency management doesn't apply and
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the gcwq for the pseudo unbound CPU tries to start executing all work
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items as soon as possible. The responsibility of regulating
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concurrency level is on the users. There is also a flag to mark a
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bound wq to ignore the concurrency management. Please refer to the
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API section for details.
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Forward progress guarantee relies on that workers can be created when
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more execution contexts are necessary, which in turn is guaranteed
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through the use of rescue workers. All work items which might be used
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on code paths that handle memory reclaim are required to be queued on
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wq's that have a rescue-worker reserved for execution under memory
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pressure. Else it is possible that the thread-pool deadlocks waiting
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for execution contexts to free up.
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4. Application Programming Interface (API)
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alloc_workqueue() allocates a wq. The original create_*workqueue()
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functions are deprecated and scheduled for removal. alloc_workqueue()
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takes three arguments - @name, @flags and @max_active. @name is the
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name of the wq and also used as the name of the rescuer thread if
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there is one.
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A wq no longer manages execution resources but serves as a domain for
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forward progress guarantee, flush and work item attributes. @flags
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and @max_active control how work items are assigned execution
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resources, scheduled and executed.
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@flags:
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WQ_NON_REENTRANT
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By default, a wq guarantees non-reentrance only on the same
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CPU. A work item may not be executed concurrently on the same
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CPU by multiple workers but is allowed to be executed
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concurrently on multiple CPUs. This flag makes sure
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non-reentrance is enforced across all CPUs. Work items queued
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to a non-reentrant wq are guaranteed to be executed by at most
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one worker system-wide at any given time.
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WQ_UNBOUND
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Work items queued to an unbound wq are served by a special
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gcwq which hosts workers which are not bound to any specific
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CPU. This makes the wq behave as a simple execution context
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provider without concurrency management. The unbound gcwq
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tries to start execution of work items as soon as possible.
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Unbound wq sacrifices locality but is useful for the following
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cases.
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* Wide fluctuation in the concurrency level requirement is
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expected and using bound wq may end up creating large number
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of mostly unused workers across different CPUs as the issuer
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hops through different CPUs.
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* Long running CPU intensive workloads which can be better
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managed by the system scheduler.
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WQ_FREEZABLE
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A freezable wq participates in the freeze phase of the system
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suspend operations. Work items on the wq are drained and no
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new work item starts execution until thawed.
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WQ_MEM_RECLAIM
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All wq which might be used in the memory reclaim paths _MUST_
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have this flag set. The wq is guaranteed to have at least one
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execution context regardless of memory pressure.
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WQ_HIGHPRI
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Work items of a highpri wq are queued at the head of the
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worklist of the target gcwq and start execution regardless of
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the current concurrency level. In other words, highpri work
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items will always start execution as soon as execution
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resource is available.
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Ordering among highpri work items is preserved - a highpri
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work item queued after another highpri work item will start
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execution after the earlier highpri work item starts.
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Although highpri work items are not held back by other
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runnable work items, they still contribute to the concurrency
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level. Highpri work items in runnable state will prevent
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non-highpri work items from starting execution.
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This flag is meaningless for unbound wq.
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WQ_CPU_INTENSIVE
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Work items of a CPU intensive wq do not contribute to the
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concurrency level. In other words, runnable CPU intensive
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work items will not prevent other work items from starting
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execution. This is useful for bound work items which are
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expected to hog CPU cycles so that their execution is
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regulated by the system scheduler.
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Although CPU intensive work items don't contribute to the
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concurrency level, start of their executions is still
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regulated by the concurrency management and runnable
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non-CPU-intensive work items can delay execution of CPU
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intensive work items.
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This flag is meaningless for unbound wq.
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WQ_HIGHPRI | WQ_CPU_INTENSIVE
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This combination makes the wq avoid interaction with
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concurrency management completely and behave as a simple
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per-CPU execution context provider. Work items queued on a
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highpri CPU-intensive wq start execution as soon as resources
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are available and don't affect execution of other work items.
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@max_active:
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@max_active determines the maximum number of execution contexts per
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CPU which can be assigned to the work items of a wq. For example,
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with @max_active of 16, at most 16 work items of the wq can be
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executing at the same time per CPU.
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Currently, for a bound wq, the maximum limit for @max_active is 512
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and the default value used when 0 is specified is 256. For an unbound
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wq, the limit is higher of 512 and 4 * num_possible_cpus(). These
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values are chosen sufficiently high such that they are not the
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limiting factor while providing protection in runaway cases.
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The number of active work items of a wq is usually regulated by the
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users of the wq, more specifically, by how many work items the users
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may queue at the same time. Unless there is a specific need for
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throttling the number of active work items, specifying '0' is
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recommended.
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Some users depend on the strict execution ordering of ST wq. The
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combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
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behavior. Work items on such wq are always queued to the unbound gcwq
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and only one work item can be active at any given time thus achieving
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the same ordering property as ST wq.
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5. Example Execution Scenarios
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The following example execution scenarios try to illustrate how cmwq
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behave under different configurations.
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Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
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w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
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again before finishing. w1 and w2 burn CPU for 5ms then sleep for
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10ms.
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Ignoring all other tasks, works and processing overhead, and assuming
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simple FIFO scheduling, the following is one highly simplified version
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of possible sequences of events with the original wq.
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TIME IN MSECS EVENT
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0 w0 starts and burns CPU
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5 w0 sleeps
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15 w0 wakes up and burns CPU
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20 w0 finishes
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20 w1 starts and burns CPU
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25 w1 sleeps
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35 w1 wakes up and finishes
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35 w2 starts and burns CPU
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40 w2 sleeps
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50 w2 wakes up and finishes
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And with cmwq with @max_active >= 3,
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TIME IN MSECS EVENT
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0 w0 starts and burns CPU
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5 w0 sleeps
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5 w1 starts and burns CPU
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10 w1 sleeps
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10 w2 starts and burns CPU
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15 w2 sleeps
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15 w0 wakes up and burns CPU
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20 w0 finishes
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20 w1 wakes up and finishes
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25 w2 wakes up and finishes
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If @max_active == 2,
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TIME IN MSECS EVENT
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0 w0 starts and burns CPU
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5 w0 sleeps
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5 w1 starts and burns CPU
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10 w1 sleeps
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15 w0 wakes up and burns CPU
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20 w0 finishes
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20 w1 wakes up and finishes
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20 w2 starts and burns CPU
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25 w2 sleeps
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35 w2 wakes up and finishes
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Now, let's assume w1 and w2 are queued to a different wq q1 which has
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WQ_HIGHPRI set,
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TIME IN MSECS EVENT
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0 w1 and w2 start and burn CPU
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5 w1 sleeps
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10 w2 sleeps
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10 w0 starts and burns CPU
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15 w0 sleeps
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15 w1 wakes up and finishes
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20 w2 wakes up and finishes
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25 w0 wakes up and burns CPU
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30 w0 finishes
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If q1 has WQ_CPU_INTENSIVE set,
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TIME IN MSECS EVENT
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0 w0 starts and burns CPU
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5 w0 sleeps
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5 w1 and w2 start and burn CPU
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10 w1 sleeps
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15 w2 sleeps
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15 w0 wakes up and burns CPU
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20 w0 finishes
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20 w1 wakes up and finishes
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25 w2 wakes up and finishes
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6. Guidelines
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* Do not forget to use WQ_MEM_RECLAIM if a wq may process work items
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which are used during memory reclaim. Each wq with WQ_MEM_RECLAIM
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set has an execution context reserved for it. If there is
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dependency among multiple work items used during memory reclaim,
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they should be queued to separate wq each with WQ_MEM_RECLAIM.
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* Unless strict ordering is required, there is no need to use ST wq.
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* Unless there is a specific need, using 0 for @max_active is
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recommended. In most use cases, concurrency level usually stays
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well under the default limit.
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* A wq serves as a domain for forward progress guarantee
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(WQ_MEM_RECLAIM, flush and work item attributes. Work items which
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are not involved in memory reclaim and don't need to be flushed as a
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part of a group of work items, and don't require any special
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attribute, can use one of the system wq. There is no difference in
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execution characteristics between using a dedicated wq and a system
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wq.
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* Unless work items are expected to consume a huge amount of CPU
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cycles, using a bound wq is usually beneficial due to the increased
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level of locality in wq operations and work item execution.
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