linux/Documentation/workqueue.txt
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   1
   2Concurrency Managed Workqueue (cmwq)
   3
   4September, 2010         Tejun Heo <tj@kernel.org>
   5                        Florian Mickler <florian@mickler.org>
   6
   7CONTENTS
   8
   91. Introduction
  102. Why cmwq?
  113. The Design
  124. Application Programming Interface (API)
  135. Example Execution Scenarios
  146. Guidelines
  157. Debugging
  16
  17
  181. Introduction
  19
  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.
  23
  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.
  28
  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.
  33
  34
  352. Why cmwq?
  36
  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
  43up.
  44
  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.
  52
  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.
  60
  61Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
  62focus on the following goals.
  63
  64* Maintain compatibility with the original workqueue API.
  65
  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.
  69
  70* Automatically regulate worker pool and level of concurrency so that
  71  the API users don't need to worry about such details.
  72
  73
  743. The Design
  75
  76In order to ease the asynchronous execution of functions a new
  77abstraction, the work item, is introduced.
  78
  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
  83workqueue.
  84
  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 worker-pools.
  89
  90The cmwq design differentiates between the user-facing workqueues that
  91subsystems and drivers queue work items on and the backend mechanism
  92which manages worker-pools and processes the queued work items.
  93
  94There are two worker-pools, one for normal work items and the other
  95for high priority ones, for each possible CPU and some extra
  96worker-pools to serve work items queued on unbound workqueues - the
  97number of these backing pools is dynamic.
  98
  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, concurrency limits, priority and more.  To
 104get a detailed overview refer to the API description of
 105alloc_workqueue() below.
 106
 107When a work item is queued to a workqueue, the target worker-pool is
 108determined according to the queue parameters and workqueue attributes
 109and appended on the shared worklist of the worker-pool.  For example,
 110unless specifically overridden, a work item of a bound workqueue will
 111be queued on the worklist of either normal or highpri worker-pool that
 112is associated to the CPU the issuer is running on.
 113
 114For any worker pool implementation, managing the concurrency level
 115(how many execution contexts are active) is an important issue.  cmwq
 116tries to keep the concurrency at a minimal but sufficient level.
 117Minimal to save resources and sufficient in that the system is used at
 118its full capacity.
 119
 120Each worker-pool bound to an actual CPU implements concurrency
 121management by hooking into the scheduler.  The worker-pool is notified
 122whenever an active worker wakes up or sleeps and keeps track of the
 123number of the currently runnable workers.  Generally, work items are
 124not expected to hog a CPU and consume many cycles.  That means
 125maintaining just enough concurrency to prevent work processing from
 126stalling should be optimal.  As long as there are one or more runnable
 127workers on the CPU, the worker-pool doesn't start execution of a new
 128work, but, when the last running worker goes to sleep, it immediately
 129schedules a new worker so that the CPU doesn't sit idle while there
 130are pending work items.  This allows using a minimal number of workers
 131without losing execution bandwidth.
 132
 133Keeping idle workers around doesn't cost other than the memory space
 134for kthreads, so cmwq holds onto idle ones for a while before killing
 135them.
 136
 137For unbound workqueues, the number of backing pools is dynamic.
 138Unbound workqueue can be assigned custom attributes using
 139apply_workqueue_attrs() and workqueue will automatically create
 140backing worker pools matching the attributes.  The responsibility of
 141regulating concurrency level is on the users.  There is also a flag to
 142mark a bound wq to ignore the concurrency management.  Please refer to
 143the API section for details.
 144
 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 worker-pool deadlocks waiting
 151for execution contexts to free up.
 152
 153
 1544. Application Programming Interface (API)
 155
 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.
 161
 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.
 166
 167@flags:
 168
 169  WQ_UNBOUND
 170
 171        Work items queued to an unbound wq are served by the special
 172        woker-pools which host workers which are not bound to any
 173        specific CPU.  This makes the wq behave as a simple execution
 174        context provider without concurrency management.  The unbound
 175        worker-pools try to start execution of work items as soon as
 176        possible.  Unbound wq sacrifices locality but is useful for
 177        the following cases.
 178
 179        * Wide fluctuation in the concurrency level requirement is
 180          expected and using bound wq may end up creating large number
 181          of mostly unused workers across different CPUs as the issuer
 182          hops through different CPUs.
 183
 184        * Long running CPU intensive workloads which can be better
 185          managed by the system scheduler.
 186
 187  WQ_FREEZABLE
 188
 189        A freezable wq participates in the freeze phase of the system
 190        suspend operations.  Work items on the wq are drained and no
 191        new work item starts execution until thawed.
 192
 193  WQ_MEM_RECLAIM
 194
 195        All wq which might be used in the memory reclaim paths _MUST_
 196        have this flag set.  The wq is guaranteed to have at least one
 197        execution context regardless of memory pressure.
 198
 199  WQ_HIGHPRI
 200
 201        Work items of a highpri wq are queued to the highpri
 202        worker-pool of the target cpu.  Highpri worker-pools are
 203        served by worker threads with elevated nice level.
 204
 205        Note that normal and highpri worker-pools don't interact with
 206        each other.  Each maintain its separate pool of workers and
 207        implements concurrency management among its workers.
 208
 209  WQ_CPU_INTENSIVE
 210
 211        Work items of a CPU intensive wq do not contribute to the
 212        concurrency level.  In other words, runnable CPU intensive
 213        work items will not prevent other work items in the same
 214        worker-pool from starting execution.  This is useful for bound
 215        work items which are expected to hog CPU cycles so that their
 216        execution is regulated by the system scheduler.
 217
 218        Although CPU intensive work items don't contribute to the
 219        concurrency level, start of their executions is still
 220        regulated by the concurrency management and runnable
 221        non-CPU-intensive work items can delay execution of CPU
 222        intensive work items.
 223
 224        This flag is meaningless for unbound wq.
 225
 226Note that the flag WQ_NON_REENTRANT no longer exists as all workqueues
 227are now non-reentrant - any work item is guaranteed to be executed by
 228at most one worker system-wide at any given time.
 229
 230@max_active:
 231
 232@max_active determines the maximum number of execution contexts per
 233CPU which can be assigned to the work items of a wq.  For example,
 234with @max_active of 16, at most 16 work items of the wq can be
 235executing at the same time per CPU.
 236
 237Currently, for a bound wq, the maximum limit for @max_active is 512
 238and the default value used when 0 is specified is 256.  For an unbound
 239wq, the limit is higher of 512 and 4 * num_possible_cpus().  These
 240values are chosen sufficiently high such that they are not the
 241limiting factor while providing protection in runaway cases.
 242
 243The number of active work items of a wq is usually regulated by the
 244users of the wq, more specifically, by how many work items the users
 245may queue at the same time.  Unless there is a specific need for
 246throttling the number of active work items, specifying '0' is
 247recommended.
 248
 249Some users depend on the strict execution ordering of ST wq.  The
 250combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
 251behavior.  Work items on such wq are always queued to the unbound
 252worker-pools and only one work item can be active at any given time thus
 253achieving the same ordering property as ST wq.
 254
 255
 2565. Example Execution Scenarios
 257
 258The following example execution scenarios try to illustrate how cmwq
 259behave under different configurations.
 260
 261 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
 262 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
 263 again before finishing.  w1 and w2 burn CPU for 5ms then sleep for
 264 10ms.
 265
 266Ignoring all other tasks, works and processing overhead, and assuming
 267simple FIFO scheduling, the following is one highly simplified version
 268of possible sequences of events with the original wq.
 269
 270 TIME IN MSECS  EVENT
 271 0              w0 starts and burns CPU
 272 5              w0 sleeps
 273 15             w0 wakes up and burns CPU
 274 20             w0 finishes
 275 20             w1 starts and burns CPU
 276 25             w1 sleeps
 277 35             w1 wakes up and finishes
 278 35             w2 starts and burns CPU
 279 40             w2 sleeps
 280 50             w2 wakes up and finishes
 281
 282And with cmwq with @max_active >= 3,
 283
 284 TIME IN MSECS  EVENT
 285 0              w0 starts and burns CPU
 286 5              w0 sleeps
 287 5              w1 starts and burns CPU
 288 10             w1 sleeps
 289 10             w2 starts and burns CPU
 290 15             w2 sleeps
 291 15             w0 wakes up and burns CPU
 292 20             w0 finishes
 293 20             w1 wakes up and finishes
 294 25             w2 wakes up and finishes
 295
 296If @max_active == 2,
 297
 298 TIME IN MSECS  EVENT
 299 0              w0 starts and burns CPU
 300 5              w0 sleeps
 301 5              w1 starts and burns CPU
 302 10             w1 sleeps
 303 15             w0 wakes up and burns CPU
 304 20             w0 finishes
 305 20             w1 wakes up and finishes
 306 20             w2 starts and burns CPU
 307 25             w2 sleeps
 308 35             w2 wakes up and finishes
 309
 310Now, let's assume w1 and w2 are queued to a different wq q1 which has
 311WQ_CPU_INTENSIVE set,
 312
 313 TIME IN MSECS  EVENT
 314 0              w0 starts and burns CPU
 315 5              w0 sleeps
 316 5              w1 and w2 start and burn CPU
 317 10             w1 sleeps
 318 15             w2 sleeps
 319 15             w0 wakes up and burns CPU
 320 20             w0 finishes
 321 20             w1 wakes up and finishes
 322 25             w2 wakes up and finishes
 323
 324
 3256. Guidelines
 326
 327* Do not forget to use WQ_MEM_RECLAIM if a wq may process work items
 328  which are used during memory reclaim.  Each wq with WQ_MEM_RECLAIM
 329  set has an execution context reserved for it.  If there is
 330  dependency among multiple work items used during memory reclaim,
 331  they should be queued to separate wq each with WQ_MEM_RECLAIM.
 332
 333* Unless strict ordering is required, there is no need to use ST wq.
 334
 335* Unless there is a specific need, using 0 for @max_active is
 336  recommended.  In most use cases, concurrency level usually stays
 337  well under the default limit.
 338
 339* A wq serves as a domain for forward progress guarantee
 340  (WQ_MEM_RECLAIM, flush and work item attributes.  Work items which
 341  are not involved in memory reclaim and don't need to be flushed as a
 342  part of a group of work items, and don't require any special
 343  attribute, can use one of the system wq.  There is no difference in
 344  execution characteristics between using a dedicated wq and a system
 345  wq.
 346
 347* Unless work items are expected to consume a huge amount of CPU
 348  cycles, using a bound wq is usually beneficial due to the increased
 349  level of locality in wq operations and work item execution.
 350
 351
 3527. Debugging
 353
 354Because the work functions are executed by generic worker threads
 355there are a few tricks needed to shed some light on misbehaving
 356workqueue users.
 357
 358Worker threads show up in the process list as:
 359
 360root      5671  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/0:1]
 361root      5672  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/1:2]
 362root      5673  0.0  0.0      0     0 ?        S    12:12   0:00 [kworker/0:0]
 363root      5674  0.0  0.0      0     0 ?        S    12:13   0:00 [kworker/1:0]
 364
 365If kworkers are going crazy (using too much cpu), there are two types
 366of possible problems:
 367
 368        1. Something beeing scheduled in rapid succession
 369        2. A single work item that consumes lots of cpu cycles
 370
 371The first one can be tracked using tracing:
 372
 373        $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
 374        $ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
 375        (wait a few secs)
 376        ^C
 377
 378If something is busy looping on work queueing, it would be dominating
 379the output and the offender can be determined with the work item
 380function.
 381
 382For the second type of problems it should be possible to just check
 383the stack trace of the offending worker thread.
 384
 385        $ cat /proc/THE_OFFENDING_KWORKER/stack
 386
 387The work item's function should be trivially visible in the stack
 388trace.
 389
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