2Concurrency Managed Workqueue (cmwq)
   4September, 2010         Tejun Heo <>
   5                        Florian Mickler <>
   91. Introduction
  102. Why cmwq?
  113. The Design
  124. Application Programming Interface (API)
  135. Example Execution Scenarios
  146. Guidelines
  157. Debugging
  181. Introduction
  20There are many cases where an asynchronous process execution context
  21is needed and the workqueue (wq) API is the most commonly used
  22mechanism for such cases.
  24When such an asynchronous execution context is needed, a work item
  25describing which function to execute is put on a queue.  An
  26independent thread serves as the asynchronous execution context.  The
  27queue is called workqueue and the thread is called worker.
  29While there are work items on the workqueue the worker executes the
  30functions associated with the work items one after the other.  When
  31there is no work item left on the workqueue the worker becomes idle.
  32When a new work item gets queued, the worker begins executing again.
  352. Why cmwq?
  37In the original wq implementation, a multi threaded (MT) wq had one
  38worker thread per CPU and a single threaded (ST) wq had one worker
  39thread system-wide.  A single MT wq needed to keep around the same
  40number of workers as the number of CPUs.  The kernel grew a lot of MT
  41wq users over the years and with the number of CPU cores continuously
  42rising, some systems saturated the default 32k PID space just booting
  45Although MT wq wasted a lot of resource, the level of concurrency
  46provided was unsatisfactory.  The limitation was common to both ST and
  47MT wq albeit less severe on MT.  Each wq maintained its own separate
  48worker pool.  A MT wq could provide only one execution context per CPU
  49while a ST wq one for the whole system.  Work items had to compete for
  50those very limited execution contexts leading to various problems
  51including proneness to deadlocks around the single execution context.
  53The tension between the provided level of concurrency and resource
  54usage also forced its users to make unnecessary tradeoffs like libata
  55choosing to use ST wq for polling PIOs and accepting an unnecessary
  56limitation that no two polling PIOs can progress at the same time.  As
  57MT wq don't provide much better concurrency, users which require
  58higher level of concurrency, like async or fscache, had to implement
  59their own thread pool.
  61Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
  62focus on the following goals.
  64* Maintain compatibility with the original workqueue API.
  66* Use per-CPU unified worker pools shared by all wq to provide
  67  flexible level of concurrency on demand without wasting a lot of
  68  resource.
  70* Automatically regulate worker pool and level of concurrency so that
  71  the API users don't need to worry about such details.
  743. The Design
  76In order to ease the asynchronous execution of functions a new
  77abstraction, the work item, is introduced.
  79A work item is a simple struct that holds a pointer to the function
  80that is to be executed asynchronously.  Whenever a driver or subsystem
  81wants a function to be executed asynchronously it has to set up a work
  82item pointing to that function and queue that work item on a
  85Special purpose threads, called worker threads, execute the functions
  86off of the queue, one after the other.  If no work is queued, the
  87worker threads become idle.  These worker threads are managed in so
  88called thread-pools.
  90The cmwq design differentiates between the user-facing workqueues that
  91subsystems and drivers queue work items on and the backend mechanism
  92which manages thread-pools and processes the queued work items.
  94The backend is called gcwq.  There is one gcwq for each possible CPU
  95and one gcwq to serve work items queued on unbound workqueues.  Each
  96gcwq has two thread-pools - one for normal work items and the other
  97for high priority ones.
  99Subsystems and drivers can create and queue work items through special
 100workqueue API functions as they see fit. They can influence some
 101aspects of the way the work items are executed by setting flags on the
 102workqueue they are putting the work item on. These flags include
 103things like CPU locality, reentrancy, concurrency limits, priority and
 104more.  To get a detailed overview refer to the API description of
 105alloc_workqueue() below.
 107When a work item is queued to a workqueue, the target gcwq and
 108thread-pool is determined according to the queue parameters and
 109workqueue attributes and appended on the shared worklist of the
 110thread-pool.  For example, unless specifically overridden, a work item
 111of a bound workqueue will be queued on the worklist of either normal
 112or highpri thread-pool of the gcwq that is associated to the CPU the
 113issuer is running on.
 115For any worker pool implementation, managing the concurrency level
 116(how many execution contexts are active) is an important issue.  cmwq
 117tries to keep the concurrency at a minimal but sufficient level.
 118Minimal to save resources and sufficient in that the system is used at
 119its full capacity.
 121Each thread-pool bound to an actual CPU implements concurrency
 122management by hooking into the scheduler.  The thread-pool is notified
 123whenever an active worker wakes up or sleeps and keeps track of the
 124number of the currently runnable workers.  Generally, work items are
 125not expected to hog a CPU and consume many cycles.  That means
 126maintaining just enough concurrency to prevent work processing from
 127stalling should be optimal.  As long as there are one or more runnable
 128workers on the CPU, the thread-pool doesn't start execution of a new
 129work, but, when the last running worker goes to sleep, it immediately
 130schedules a new worker so that the CPU doesn't sit idle while there
 131are pending work items.  This allows using a minimal number of workers
 132without losing execution bandwidth.
 134Keeping idle workers around doesn't cost other than the memory space
 135for kthreads, so cmwq holds onto idle ones for a while before killing
 138For an unbound wq, the above concurrency management doesn't apply and
 139the thread-pools for the pseudo unbound CPU try to start executing all
 140work items as soon as possible.  The responsibility of regulating
 141concurrency level is on the users.  There is also a flag to mark a
 142bound wq to ignore the concurrency management.  Please refer to the
 143API section for details.
 145Forward progress guarantee relies on that workers can be created when
 146more execution contexts are necessary, which in turn is guaranteed
 147through the use of rescue workers.  All work items which might be used
 148on code paths that handle memory reclaim are required to be queued on
 149wq's that have a rescue-worker reserved for execution under memory
 150pressure.  Else it is possible that the thread-pool deadlocks waiting
 151for execution contexts to free up.
 1544. Application Programming Interface (API)
 156alloc_workqueue() allocates a wq.  The original create_*workqueue()
 157functions are deprecated and scheduled for removal.  alloc_workqueue()
 158takes three arguments - @name, @flags and @max_active.  @name is the
 159name of the wq and also used as the name of the rescuer thread if
 160there is one.
 162A wq no longer manages execution resources but serves as a domain for
 163forward progress guarantee, flush and work item attributes.  @flags
 164and @max_active control how work items are assigned execution
 165resources, scheduled and executed.
 171        By default, a wq guarantees non-reentrance only on the same
 172        CPU.  A work item may not be executed concurrently on the same
 173        CPU by multiple workers but is allowed to be executed
 174        concurrently on multiple CPUs.  This flag makes sure
 175        non-reentrance is enforced across all CPUs.  Work items queued
 176        to a non-reentrant wq are guaranteed to be executed by at most
 177        one worker system-wide at any given time.
 181        Work items queued to an unbound wq are served by a special
 182        gcwq which hosts workers which are not bound to any specific
 183        CPU.  This makes the wq behave as a simple execution context
 184        provider without concurrency management.  The unbound gcwq
 185        tries to start execution of work items as soon as possible.
 186        Unbound wq sacrifices locality but is useful for the following
 187        cases.
 189        * Wide fluctuation in the concurrency level requirement is
 190          expected and using bound wq may end up creating large number
 191          of mostly unused workers across different CPUs as the issuer
 192          hops through different CPUs.
 194        * Long running CPU intensive workloads which can be better
 195          managed by the system scheduler.
 199        A freezable wq participates in the freeze phase of the system
 200        suspend operations.  Work items on the wq are drained and no
 201        new work item starts execution until thawed.
 205        All wq which might be used in the memory reclaim paths _MUST_
 206        have this flag set.  The wq is guaranteed to have at least one
 207        execution context regardless of memory pressure.
 211        Work items of a highpri wq are queued to the highpri
 212        thread-pool of the target gcwq.  Highpri thread-pools are
 213        served by worker threads with elevated nice level.
 215        Note that normal and highpri thread-pools don't interact with
 216        each other.  Each maintain its separate pool of workers and
 217        implements concurrency management among its workers.
 221        Work items of a CPU intensive wq do not contribute to the
 222        concurrency level.  In other words, runnable CPU intensive
 223        work items will not prevent other work items in the same
 224        thread-pool from starting execution.  This is useful for bound
 225        work items which are expected to hog CPU cycles so that their
 226        execution is regulated by the system scheduler.
 228        Although CPU intensive work items don't contribute to the
 229        concurrency level, start of their executions is still
 230        regulated by the concurrency management and runnable
 231        non-CPU-intensive work items can delay execution of CPU
 232        intensive work items.
 234        This flag is meaningless for unbound wq.
 238@max_active determines the maximum number of execution contexts per
 239CPU which can be assigned to the work items of a wq.  For example,
 240with @max_active of 16, at most 16 work items of the wq can be
 241executing at the same time per CPU.
 243Currently, for a bound wq, the maximum limit for @max_active is 512
 244and the default value used when 0 is specified is 256.  For an unbound
 245wq, the limit is higher of 512 and 4 * num_possible_cpus().  These
 246values are chosen sufficiently high such that they are not the
 247limiting factor while providing protection in runaway cases.
 249The number of active work items of a wq is usually regulated by the
 250users of the wq, more specifically, by how many work items the users
 251may queue at the same time.  Unless there is a specific need for
 252throttling the number of active work items, specifying '0' is
 255Some users depend on the strict execution ordering of ST wq.  The
 256combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
 257behavior.  Work items on such wq are always queued to the unbound gcwq
 258and only one work item can be active at any given time thus achieving
 259the same ordering property as ST wq.
 2625. Example Execution Scenarios
 264The following example execution scenarios try to illustrate how cmwq
 265behave under different configurations.
 267 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
 268 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
 269 again before finishing.  w1 and w2 burn CPU for 5ms then sleep for
 270 10ms.
 272Ignoring all other tasks, works and processing overhead, and assuming
 273simple FIFO scheduling, the following is one highly simplified version
 274of possible sequences of events with the original wq.
 277 0              w0 starts and burns CPU
 278 5              w0 sleeps
 279 15             w0 wakes up and burns CPU
 280 20             w0 finishes
 281 20             w1 starts and burns CPU
 282 25             w1 sleeps
 283 35             w1 wakes up and finishes
 284 35             w2 starts and burns CPU
 285 40             w2 sleeps
 286 50             w2 wakes up and finishes
 288And with cmwq with @max_active >= 3,
 291 0              w0 starts and burns CPU
 292 5              w0 sleeps
 293 5              w1 starts and burns CPU
 294 10             w1 sleeps
 295 10             w2 starts and burns CPU
 296 15             w2 sleeps
 297 15             w0 wakes up and burns CPU
 298 20             w0 finishes
 299 20             w1 wakes up and finishes
 300 25             w2 wakes up and finishes
 302If @max_active == 2,
 305 0              w0 starts and burns CPU
 306 5              w0 sleeps
 307 5              w1 starts and burns CPU
 308 10             w1 sleeps
 309 15             w0 wakes up and burns CPU
 20             w1 stars up and finishes
                w1 starts and burns CPU
                w2 wakeps
 35             w2 stars up and finishes

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