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1 ==================================== 2 Concurrency Managed Workqueue (cmwq) 3 ==================================== 4 5 :Date: September, 2010 6 :Author: Tejun Heo <firstname.lastname@example.org> 7 :Author: Florian Mickler <email@example.com> 8 9 10 Introduction 11 ============ 12 13 There are many cases where an asynchronous process execution context 14 is needed and the workqueue (wq) API is the most commonly used 15 mechanism for such cases. 16 17 When such an asynchronous execution context is needed, a work item 18 describing which function to execute is put on a queue. An 19 independent thread serves as the asynchronous execution context. The 20 queue is called workqueue and the thread is called worker. 21 22 While there are work items on the workqueue the worker executes the 23 functions associated with the work items one after the other. When 24 there is no work item left on the workqueue the worker becomes idle. 25 When a new work item gets queued, the worker begins executing again. 26 27 28 Why cmwq? 29 ========= 30 31 In the original wq implementation, a multi threaded (MT) wq had one 32 worker thread per CPU and a single threaded (ST) wq had one worker 33 thread system-wide. A single MT wq needed to keep around the same 34 number of workers as the number of CPUs. The kernel grew a lot of MT 35 wq users over the years and with the number of CPU cores continuously 36 rising, some systems saturated the default 32k PID space just booting 37 up. 38 39 Although MT wq wasted a lot of resource, the level of concurrency 40 provided was unsatisfactory. The limitation was common to both ST and 41 MT wq albeit less severe on MT. Each wq maintained its own separate 42 worker pool. An MT wq could provide only one execution context per CPU 43 while an ST wq one for the whole system. Work items had to compete for 44 those very limited execution contexts leading to various problems 45 including proneness to deadlocks around the single execution context. 46 47 The tension between the provided level of concurrency and resource 48 usage also forced its users to make unnecessary tradeoffs like libata 49 choosing to use ST wq for polling PIOs and accepting an unnecessary 50 limitation that no two polling PIOs can progress at the same time. As 51 MT wq don't provide much better concurrency, users which require 52 higher level of concurrency, like async or fscache, had to implement 53 their own thread pool. 54 55 Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with 56 focus on the following goals. 57 58 * Maintain compatibility with the original workqueue API. 59 60 * Use per-CPU unified worker pools shared by all wq to provide 61 flexible level of concurrency on demand without wasting a lot of 62 resource. 63 64 * Automatically regulate worker pool and level of concurrency so that 65 the API users don't need to worry about such details. 66 67 68 The Design 69 ========== 70 71 In order to ease the asynchronous execution of functions a new 72 abstraction, the work item, is introduced. 73 74 A work item is a simple struct that holds a pointer to the function 75 that is to be executed asynchronously. Whenever a driver or subsystem 76 wants a function to be executed asynchronously it has to set up a work 77 item pointing to that function and queue that work item on a 78 workqueue. 79 80 Special purpose threads, called worker threads, execute the functions 81 off of the queue, one after the other. If no work is queued, the 82 worker threads become idle. These worker threads are managed in so 83 called worker-pools. 84 85 The cmwq design differentiates between the user-facing workqueues that 86 subsystems and drivers queue work items on and the backend mechanism 87 which manages worker-pools and processes the queued work items. 88 89 There are two worker-pools, one for normal work items and the other 90 for high priority ones, for each possible CPU and some extra 91 worker-pools to serve work items queued on unbound workqueues - the 92 number of these backing pools is dynamic. 93 94 Subsystems and drivers can create and queue work items through special 95 workqueue API functions as they see fit. They can influence some 96 aspects of the way the work items are executed by setting flags on the 97 workqueue they are putting the work item on. These flags include 98 things like CPU locality, concurrency limits, priority and more. To 99 get a detailed overview refer to the API description of 100 ``alloc_workqueue()`` below. 101 102 When a work item is queued to a workqueue, the target worker-pool is 103 determined according to the queue parameters and workqueue attributes 104 and appended on the shared worklist of the worker-pool. For example, 105 unless specifically overridden, a work item of a bound workqueue will 106 be queued on the worklist of either normal or highpri worker-pool that 107 is associated to the CPU the issuer is running on. 108 109 For any worker pool implementation, managing the concurrency level 110 (how many execution contexts are active) is an important issue. cmwq 111 tries to keep the concurrency at a minimal but sufficient level. 112 Minimal to save resources and sufficient in that the system is used at 113 its full capacity. 114 115 Each worker-pool bound to an actual CPU implements concurrency 116 management by hooking into the scheduler. The worker-pool is notified 117 whenever an active worker wakes up or sleeps and keeps track of the 118 number of the currently runnable workers. Generally, work items are 119 not expected to hog a CPU and consume many cycles. That means 120 maintaining just enough concurrency to prevent work processing from 121 stalling should be optimal. As long as there are one or more runnable 122 workers on the CPU, the worker-pool doesn't start execution of a new 123 work, but, when the last running worker goes to sleep, it immediately 124 schedules a new worker so that the CPU doesn't sit idle while there 125 are pending work items. This allows using a minimal number of workers 126 without losing execution bandwidth. 127 128 Keeping idle workers around doesn't cost other than the memory space 129 for kthreads, so cmwq holds onto idle ones for a while before killing 130 them. 131 132 For unbound workqueues, the number of backing pools is dynamic. 133 Unbound workqueue can be assigned custom attributes using 134 ``apply_workqueue_attrs()`` and workqueue will automatically create 135 backing worker pools matching the attributes. The responsibility of 136 regulating concurrency level is on the users. There is also a flag to 137 mark a bound wq to ignore the concurrency management. Please refer to 138 the API section for details. 139 140 Forward progress guarantee relies on that workers can be created when 141 more execution contexts are necessary, which in turn is guaranteed 142 through the use of rescue workers. All work items which might be used 143 on code paths that handle memory reclaim are required to be queued on 144 wq's that have a rescue-worker reserved for execution under memory 145 pressure. Else it is possible that the worker-pool deadlocks waiting 146 for execution contexts to free up. 147 148 149 Application Programming Interface (API) 150 ======================================= 151 152 ``alloc_workqueue()`` allocates a wq. The original 153 ``create_*workqueue()`` functions are deprecated and scheduled for 154 removal. ``alloc_workqueue()`` takes three arguments - ``@name``, 155 ``@flags`` and ``@max_active``. ``@name`` is the name of the wq and 156 also used as the name of the rescuer thread if there is one. 157 158 A wq no longer manages execution resources but serves as a domain for 159 forward progress guarantee, flush and work item attributes. ``@flags`` 160 and ``@max_active`` control how work items are assigned execution 161 resources, scheduled and executed. 162 163 164 ``flags`` 165 --------- 166 167 ``WQ_UNBOUND`` 168 Work items queued to an unbound wq are served by the special 169 worker-pools which host workers which are not bound to any 170 specific CPU. This makes the wq behave as a simple execution 171 context provider without concurrency management. The unbound 172 worker-pools try to start execution of work items as soon as 173 possible. Unbound wq sacrifices locality but is useful for 174 the following cases. 175 176 * Wide fluctuation in the concurrency level requirement is 177 expected and using bound wq may end up creating large number 178 of mostly unused workers across different CPUs as the issuer 179 hops through different CPUs. 180 181 * Long running CPU intensive workloads which can be better 182 managed by the system scheduler. 183 184 ``WQ_FREEZABLE`` 185 A freezable wq participates in the freeze phase of the system 186 suspend operations. Work items on the wq are drained and no 187 new work item starts execution until thawed. 188 189 ``WQ_MEM_RECLAIM`` 190 All wq which might be used in the memory reclaim paths **MUST** 191 have this flag set. The wq is guaranteed to have at least one 192 execution context regardless of memory pressure. 193 194 ``WQ_HIGHPRI`` 195 Work items of a highpri wq are queued to the highpri 196 worker-pool of the target cpu. Highpri worker-pools are 197 served by worker threads with elevated nice level. 198 199 Note that normal and highpri worker-pools don't interact with 200 each other. Each maintains its separate pool of workers and 201 implements concurrency management among its workers. 202 203 ``WQ_CPU_INTENSIVE`` 204 Work items of a CPU intensive wq do not contribute to the 205 concurrency level. In other words, runnable CPU intensive 206 work items will not prevent other work items in the same 207 worker-pool from starting execution. This is useful for bound 208 work items which are expected to hog CPU cycles so that their 209 execution is regulated by the system scheduler. 210 211 Although CPU intensive work items don't contribute to the 212 concurrency level, start of their executions is still 213 regulated by the concurrency management and runnable 214 non-CPU-intensive work items can delay execution of CPU 215 intensive work items. 216 217 This flag is meaningless for unbound wq. 218 219 Note that the flag ``WQ_NON_REENTRANT`` no longer exists as all 220 workqueues are now non-reentrant - any work item is guaranteed to be 221 executed by at most one worker system-wide at any given time. 222 223 224 ``max_active`` 225 -------------- 226 227 ``@max_active`` determines the maximum number of execution contexts 228 per CPU which can be assigned to the work items of a wq. For example, 229 with ``@max_active`` of 16, at most 16 work items of the wq can be 230 executing at the same time per CPU. 231 232 Currently, for a bound wq, the maximum limit for ``@max_active`` is 233 512 and the default value used when 0 is specified is 256. For an 234 unbound wq, the limit is higher of 512 and 4 * 235 ``num_possible_cpus()``. These values are chosen sufficiently high 236 such that they are not the limiting factor while providing protection 237 in runaway cases. 238 239 The number of active work items of a wq is usually regulated by the 240 users of the wq, more specifically, by how many work items the users 241 may queue at the same time. Unless there is a specific need for 242 throttling the number of active work items, specifying '0' is 243 recommended. 244 245 Some users depend on the strict execution ordering of ST wq. The 246 combination of ``@max_active`` of 1 and ``WQ_UNBOUND`` used to 247 achieve this behavior. Work items on such wq were always queued to the 248 unbound worker-pools and only one work item could be active at any given 249 time thus achieving the same ordering property as ST wq. 250 251 In the current implementation the above configuration only guarantees 252 ST behavior within a given NUMA node. Instead ``alloc_ordered_queue()`` should 253 be used to achieve system-wide ST behavior. 254 255 256 Example Execution Scenarios 257 =========================== 258 259 The following example execution scenarios try to illustrate how cmwq 260 behave under different configurations. 261 262 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU. 263 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms 264 again before finishing. w1 and w2 burn CPU for 5ms then sleep for 265 10ms. 266 267 Ignoring all other tasks, works and processing overhead, and assuming 268 simple FIFO scheduling, the following is one highly simplified version 269 of possible sequences of events with the original wq. :: 270 271 TIME IN MSECS EVENT 272 0 w0 starts and burns CPU 273 5 w0 sleeps 274 15 w0 wakes up and burns CPU 275 20 w0 finishes 276 20 w1 starts and burns CPU 277 25 w1 sleeps 278 35 w1 wakes up and finishes 279 35 w2 starts and burns CPU 280 40 w2 sleeps 281 50 w2 wakes up and finishes 282 283 And with cmwq with ``@max_active`` >= 3, :: 284 285 TIME IN MSECS EVENT 286 0 w0 starts and burns CPU 287 5 w0 sleeps 288 5 w1 starts and burns CPU 289 10 w1 sleeps 290 10 w2 starts and burns CPU 291 15 w2 sleeps 292 15 w0 wakes up and burns CPU 293 20 w0 finishes 294 20 w1 wakes up and finishes 295 25 w2 wakes up and finishes 296 297 If ``@max_active`` == 2, :: 298 299 TIME IN MSECS EVENT 300 0 w0 starts and burns CPU 301 5 w0 sleeps 302 5 w1 starts and burns CPU 303 10 w1 sleeps 304 15 w0 wakes up and burns CPU 305 20 w0 finishes 306 20 w1 wakes up and finishes 307 20 w2 starts and burns CPU 308 25 w2 sleeps 309 35 w2 wakes up and finishes 310 311 Now, let's assume w1 and w2 are queued to a different wq q1 which has 312 ``WQ_CPU_INTENSIVE`` set, :: 313 314 TIME IN MSECS EVENT 315 0 w0 starts and burns CPU 316 5 w0 sleeps 317 5 w1 and w2 start and burn CPU 318 10 w1 sleeps 319 15 w2 sleeps 320 15 w0 wakes up and burns CPU 321 20 w0 finishes 322 20 w1 wakes up and finishes 323 25 w2 wakes up and finishes 324 325 326 Guidelines 327 ========== 328 329 * Do not forget to use ``WQ_MEM_RECLAIM`` if a wq may process work 330 items which are used during memory reclaim. Each wq with 331 ``WQ_MEM_RECLAIM`` set has an execution context reserved for it. If 332 there is dependency among multiple work items used during memory 333 reclaim, they should be queued to separate wq each with 334 ``WQ_MEM_RECLAIM``. 335 336 * Unless strict ordering is required, there is no need to use ST wq. 337 338 * Unless there is a specific need, using 0 for @max_active is 339 recommended. In most use cases, concurrency level usually stays 340 well under the default limit. 341 342 * A wq serves as a domain for forward progress guarantee 343 (``WQ_MEM_RECLAIM``, flush and work item attributes. Work items 344 which are not involved in memory reclaim and don't need to be 345 flushed as a part of a group of work items, and don't require any 346 special attribute, can use one of the system wq. There is no 347 difference in execution characteristics between using a dedicated wq 348 and a system wq. 349 350 * Unless work items are expected to consume a huge amount of CPU 351 cycles, using a bound wq is usually beneficial due to the increased 352 level of locality in wq operations and work item execution. 353 354 355 Debugging 356 ========= 357 358 Because the work functions are executed by generic worker threads 359 there are a few tricks needed to shed some light on misbehaving 360 workqueue users. 361 362 Worker threads show up in the process list as: :: 363 364 root 5671 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/0:1] 365 root 5672 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/1:2] 366 root 5673 0.0 0.0 0 0 ? S 12:12 0:00 [kworker/0:0] 367 root 5674 0.0 0.0 0 0 ? S 12:13 0:00 [kworker/1:0] 368 369 If kworkers are going crazy (using too much cpu), there are two types 370 of possible problems: 371 372 1. Something being scheduled in rapid succession 373 2. A single work item that consumes lots of cpu cycles 374 375 The first one can be tracked using tracing: :: 376 377 $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event 378 $ cat /sys/kernel/debug/tracing/trace_pipe > out.txt 379 (wait a few secs) 380 ^C 381 382 If something is busy looping on work queueing, it would be dominating 383 the output and the offender can be determined with the work item 384 function. 385 386 For the second type of problems it should be possible to just check 387 the stack trace of the offending worker thread. :: 388 389 $ cat /proc/THE_OFFENDING_KWORKER/stack 390 391 The work item's function should be trivially visible in the stack 392 trace. 393 394 395 Kernel Inline Documentations Reference 396 ====================================== 397 398 .. kernel-doc:: include/linux/workqueue.h