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 15 16 171. Introduction 18 19There are many cases where an asynchronous process execution context 20is needed and the workqueue (wq) API is the most commonly used 21mechanism for such cases. 22 23When such an asynchronous execution context is needed, a work item 24describing which function to execute is put on a queue. An 25independent thread serves as the asynchronous execution context. The 26queue is called workqueue and the thread is called worker. 27 28While there are work items on the workqueue the worker executes the 29functions associated with the work items one after the other. When 30there is no work item left on the workqueue the worker becomes idle. 31When a new work item gets queued, the worker begins executing again. 32 33 342. Why cmwq? 35 36In the original wq implementation, a multi threaded (MT) wq had one 37worker thread per CPU and a single threaded (ST) wq had one worker 38thread system-wide. A single MT wq needed to keep around the same 39number of workers as the number of CPUs. The kernel grew a lot of MT 40wq users over the years and with the number of CPU cores continuously 41rising, some systems saturated the default 32k PID space just booting 42up. 43 44Although MT wq wasted a lot of resource, the level of concurrency 45provided was unsatisfactory. The limitation was common to both ST and 46MT wq albeit less severe on MT. Each wq maintained its own separate 47worker pool. A MT wq could provide only one execution context per CPU 48while a ST wq one for the whole system. Work items had to compete for 49those very limited execution contexts leading to various problems 50including proneness to deadlocks around the single execution context. 51 52The tension between the provided level of concurrency and resource 53usage also forced its users to make unnecessary tradeoffs like libata 54choosing to use ST wq for polling PIOs and accepting an unnecessary 55limitation that no two polling PIOs can progress at the same time. As 56MT wq don't provide much better concurrency, users which require 57higher level of concurrency, like async or fscache, had to implement 58their own thread pool. 59 60Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with 61focus on the following goals. 62 63* Maintain compatibility with the original workqueue API. 64 65* Use per-CPU unified worker pools shared by all wq to provide 66 flexible level of concurrency on demand without wasting a lot of 67 resource. 68 69* Automatically regulate worker pool and level of concurrency so that 70 the API users don't need to worry about such details. 71 72 733. The Design 74 75In order to ease the asynchronous execution of functions a new 76abstraction, the work item, is introduced. 77 78A work item is a simple struct that holds a pointer to the function 79that is to be executed asynchronously. Whenever a driver or subsystem 80wants a function to be executed asynchronously it has to set up a work 81item pointing to that function and queue that work item on a 82workqueue. 83 84Special purpose threads, called worker threads, execute the functions 85off of the queue, one after the other. If no work is queued, the 86worker threads become idle. These worker threads are managed in so 87called thread-pools. 88 89The cmwq design differentiates between the user-facing workqueues that 90subsystems and drivers queue work items on and the backend mechanism 91which manages thread-pool and processes the queued work items. 92 93The backend is called gcwq. There is one gcwq for each possible CPU 94and one gcwq to serve work items queued on unbound workqueues. 95 96Subsystems and drivers can create and queue work items through special 97workqueue API functions as they see fit. They can influence some 98aspects of the way the work items are executed by setting flags on the 99workqueue they are putting the work item on. These flags include 100things like CPU locality, reentrancy, concurrency limits and more. To 101get a detailed overview refer to the API description of 102alloc_workqueue() below. 103 104When a work item is queued to a workqueue, the target gcwq is 105determined according to the queue parameters and workqueue attributes 106and appended on the shared worklist of the gcwq. For example, unless 107specifically overridden, a work item of a bound workqueue will be 108queued on the worklist of exactly that gcwq that is associated to the 109CPU the issuer is running on. 110 111For any worker pool implementation, managing the concurrency level 112(how many execution contexts are active) is an important issue. cmwq 113tries to keep the concurrency at a minimal but sufficient level. 114Minimal to save resources and sufficient in that the system is used at 115its full capacity. 116 117Each gcwq bound to an actual CPU implements concurrency management by 118hooking into the scheduler. The gcwq is notified whenever an active 119worker wakes up or sleeps and keeps track of the number of the 120currently runnable workers. Generally, work items are not expected to 121hog a CPU and consume many cycles. That means maintaining just enough 122concurrency to prevent work processing from stalling should be 123optimal. As long as there are one or more runnable workers on the 124CPU, the gcwq doesn't start execution of a new work, but, when the 125last running worker goes to sleep, it immediately schedules a new 126worker so that the CPU doesn't sit idle while there are pending work 127items. This allows using a minimal number of workers without losing 128execution bandwidth. 129 130Keeping idle workers around doesn't cost other than the memory space 131for kthreads, so cmwq holds onto idle ones for a while before killing 132them. 133 134For an unbound wq, the above concurrency management doesn't apply and 135the gcwq for the pseudo unbound CPU tries to start executing all work 136items as soon as possible. The responsibility of regulating 137concurrency level is on the users. There is also a flag to mark a 138bound wq to ignore the concurrency management. Please refer to the 139API section for details. 140 141Forward progress guarantee relies on that workers can be created when 142more execution contexts are necessary, which in turn is guaranteed 143through the use of rescue workers. All work items which might be used 144on code paths that handle memory reclaim are required to be queued on 145wq's that have a rescue-worker reserved for execution under memory 146pressure. Else it is possible that the thread-pool deadlocks waiting 147for execution contexts to free up. 148 149 1504. Application Programming Interface (API) 151 152alloc_workqueue() allocates a wq. The original create_*workqueue() 153functions are deprecated and scheduled for removal. alloc_workqueue() 154takes three arguments - @name, @flags and @max_active. @name is the 155name of the wq and also used as the name of the rescuer thread if 156there is one. 157 158A wq no longer manages execution resources but serves as a domain for 159forward progress guarantee, flush and work item attributes. @flags 160and @max_active control how work items are assigned execution 161resources, scheduled and executed. 162 163@flags: 164 165 WQ_NON_REENTRANT 166 167 By default, a wq guarantees non-reentrance only on the same 168 CPU. A work item may not be executed concurrently on the same 169 CPU by multiple workers but is allowed to be executed 170 concurrently on multiple CPUs. This flag makes sure 171 non-reentrance is enforced across all CPUs. Work items queued 172 to a non-reentrant wq are guaranteed to be executed by at most 173 one worker system-wide at any given time. 174 175 WQ_UNBOUND 176 177 Work items queued to an unbound wq are served by a special 178 gcwq which hosts workers which are not bound to any specific 179 CPU. This makes the wq behave as a simple execution context 180 provider without concurrency management. The unbound gcwq 181 tries to start execution of work items as soon as possible. 182 Unbound wq sacrifices locality but is useful for the following 183 cases. 184 185 * Wide fluctuation in the concurrency level requirement is 186 expected and using bound wq may end up creating large number 187 of mostly unused workers across different CPUs as the issuer 188 hops through different CPUs. 189 190 * Long running CPU intensive workloads which can be better 191 managed by the system scheduler. 192 193 WQ_FREEZEABLE 194 195 A freezeable wq participates in the freeze phase of the system 196 suspend operations. Work items on the wq are drained and no 197 new work item starts execution until thawed. 198 199 WQ_RESCUER 200 201 All wq which might be used in the memory reclaim paths _MUST_ 202 have this flag set. This reserves one worker exclusively for 203 the execution of this wq under memory pressure. 204 205 WQ_HIGHPRI 206 207 Work items of a highpri wq are queued at the head of the 208 worklist of the target gcwq and start execution regardless of 209 the current concurrency level. In other words, highpri work 210 items will always start execution as soon as execution 211 resource is available. 212 213 Ordering among highpri work items is preserved - a highpri 214 work item queued after another highpri work item will start 215 execution after the earlier highpri work item starts. 216 217 Although highpri work items are not held back by other 218 runnable work items, they still contribute to the concurrency 219 level. Highpri work items in runnable state will prevent 220 non-highpri work items from starting execution. 221 222 This flag is meaningless for unbound wq. 223 224 WQ_CPU_INTENSIVE 225 226 Work items of a CPU intensive wq do not contribute to the 227 concurrency level. In other words, runnable CPU intensive 228 work items will not prevent other work items from starting 229 execution. This is useful for bound work items which are 230 expected to hog CPU cycles so that their execution is 231 regulated by the system scheduler. 232 233 Although CPU intensive work items don't contribute to the 234 concurrency level, start of their executions is still 235 regulated by the concurrency management and runnable 236 non-CPU-intensive work items can delay execution of CPU 237 intensive work items. 238 239 This flag is meaningless for unbound wq. 240 241 WQ_HIGHPRI | WQ_CPU_INTENSIVE 242 243 This combination makes the wq avoid interaction with 244 concurrency management completely and behave as a simple 245 per-CPU execution context provider. Work items queued on a 246 highpri CPU-intensive wq start execution as soon as resources 247 are available and don't affect execution of other work items. 248 249@max_active: 250 251@max_active determines the maximum number of execution contexts per 252CPU which can be assigned to the work items of a wq. For example, 253with @max_active of 16, at most 16 work items of the wq can be 254executing at the same time per CPU. 255 256Currently, for a bound wq, the maximum limit for @max_active is 512 257and the default value used when 0 is specified is 256. For an unbound 258wq, the limit is higher of 512 and 4 * num_possible_cpus(). These 259values are chosen sufficiently high such that they are not the 260limiting factor while providing protection in runaway cases. 261 262The number of active work items of a wq is usually regulated by the 263users of the wq, more specifically, by how many work items the users 264may queue at the same time. Unless there is a specific need for 265throttling the number of active work items, specifying '0' is 266recommended. 267 268Some users depend on the strict execution ordering of ST wq. The 269combination of @max_active of 1 and WQ_UNBOUND is used to achieve this 270behavior. Work items on such wq are always queued to the unbound gcwq 271and only one work item can be active at any given time thus achieving 272the same ordering property as ST wq. 273 274 2755. Example Execution Scenarios 276 277The following example execution scenarios try to illustrate how cmwq 278behave under different configurations. 279 280 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU. 281 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms 282 again before finishing. w1 and w2 burn CPU for 5ms then sleep for 283 10ms. 284 285Ignoring all other tasks, works and processing overhead, and assuming 286simple FIFO scheduling, the following is one highly simplified version 287of possible sequences of events with the original wq. 288 289 TIME IN MSECS EVENT 290 0 w0 starts and burns CPU 291 5 w0 sleeps 292 15 w0 wakes up and burns CPU 293 20 w0 finishes 294 20 w1 starts and burns CPU 295 25 w1 sleeps 296 35 w1 wakes up and finishes 297 35 w2 starts and burns CPU 298 40 w2 sleeps 299 50 w2 wakes up and finishes 300 301And with cmwq with @max_active >= 3, 302 303 TIME IN MSECS EVENT 304 0 w0 starts and burns CPU 305 5 w0 sleeps 306 5 w1 starts and burns CPU 307 10 w1 sleeps 308 10 w2 starts and burns CPU 309 15 w2 sleeps 310 15 w0 wakes up and burns CPU 311 20 w0 finishes 312 20 w1 wakes up and finishes 313 25 w2 wakes up and finishes 314 315If @max_active == 2, 316 317 TIME IN MSECS EVENT 318 0 w0 starts and burns CPU 319 5 w0 sleeps 320 5 w1 starts and burns CPU 321 10 w1 sleeps 322 15 w0 wakes up and burns CPU 323 20 w0 finishes 324 20 w1 wakes up and finishes 325 20 w2 starts and burns CPU 326 25 w2 sleeps 327 35 w2 wakes up and finishes 328 329Now, let's assume w1 and w2 are queued to a different wq q1 which has 330WQ_HIGHPRI set, 331 332 TIME IN MSECS EVENT 333 0 w1 and w2 start and burn CPU 334 5 w1 sleeps 335 10 w2 sleeps 336 10 w0 starts and burns CPU 337 15 w0 sleeps 338 15 w1 wakes up and finishes 339 20 w2 wakes up and finishes 340 25 w0 wakes up and burns CPU 341 30 w0 finishes 342 343If q1 has WQ_CPU_INTENSIVE set, 344 345 TIME IN MSECS EVENT 346 0 w0 starts and burns CPU 347 5 w0 sleeps 348 5 w1 and w2 start and burn CPU 349 10 w1 sleeps 350 15 w2 sleeps 351 15 w0 wakes up and burns CPU 352 20 w0 finishes 353 20 w1 wakes up and finishes 354 25 w2 wakes up and finishes 355 356 3576. Guidelines 358 359* Do not forget to use WQ_RESCUER if a wq may process work items which 360 are used during memory reclaim. Each wq with WQ_RESCUER set has one 361 rescuer thread reserved for it. If there is dependency among 362 multiple work items used during memory reclaim, they should be 363 queued to separate wq each with WQ_RESCUER. 364 365* Unless strict ordering is required, there is no need to use ST wq. 366 367* Unless there is a specific need, using 0 for @max_active is 368 recommended. In most use cases, concurrency level usually stays 369 well under the default limit. 370 371* A wq serves as a domain for forward progress guarantee (WQ_RESCUER), 372 flush and work item attributes. Work items which are not involved 373 in memory reclaim and don't need to be flushed as a part of a group 374 of work items, and don't require any special attribute, can use one 375 of the system wq. There is no difference in execution 376 characteristics between using a dedicated wq and a system wq. 377 378* Unless work items are expected to consume a huge amount of CPU 379 cycles, using a bound wq is usually beneficial due to the increased 380 level of locality in wq operations and work item execution. 381