====================================Concurrency Managed Workqueue (cmwq)==================================== :Date: September, 2010:Author: Tejun Heo <tj@kernel.org>:Author: Florian Mickler <florian@mickler.org>  Introduction============ There are many cases where an asynchronous process execution contextis needed and the workqueue (wq) API is the most commonly usedmechanism for such cases. When such an asynchronous execution context is needed, a work itemdescribing which function to execute is put on a queue.  Anindependent thread serves as the asynchronous execution context.  Thequeue is called workqueue and the thread is called worker. While there are work items on the workqueue the worker executes thefunctions associated with the work items one after the other.  Whenthere is no work item left on the workqueue the worker becomes idle.When a new work item gets queued, the worker begins executing again.  Why cmwq?========= In the original wq implementation, a multi threaded (MT) wq had oneworker thread per CPU and a single threaded (ST) wq had one workerthread system-wide.  A single MT wq needed to keep around the samenumber of workers as the number of CPUs.  The kernel grew a lot of MTwq users over the years and with the number of CPU cores continuouslyrising, some systems saturated the default 32k PID space just bootingup. Although MT wq wasted a lot of resource, the level of concurrencyprovided was unsatisfactory.  The limitation was common to both ST andMT wq albeit less severe on MT.  Each wq maintained its own separateworker pool.  An MT wq could provide only one execution context per CPUwhile an ST wq one for the whole system.  Work items had to compete forthose very limited execution contexts leading to various problemsincluding proneness to deadlocks around the single execution context. The tension between the provided level of concurrency and resourceusage also forced its users to make unnecessary tradeoffs like libatachoosing to use ST wq for polling PIOs and accepting an unnecessarylimitation that no two polling PIOs can progress at the same time.  AsMT wq don't provide much better concurrency, users which requirehigher level of concurrency, like async or fscache, had to implementtheir own thread pool. Concurrency Managed Workqueue (cmwq) is a reimplementation of wq withfocus on the following goals. * Maintain compatibility with the original workqueue API. * Use per-CPU unified worker pools shared by all wq to provide  flexible level of concurrency on demand without wasting a lot of  resource. * Automatically regulate worker pool and level of concurrency so that  the API users don't need to worry about such details.  The Design========== In order to ease the asynchronous execution of functions a newabstraction, the work item, is introduced. A work item is a simple struct that holds a pointer to the functionthat is to be executed asynchronously.  Whenever a driver or subsystemwants a function to be executed asynchronously it has to set up a workitem pointing to that function and queue that work item on aworkqueue. Special purpose threads, called worker threads, execute the functionsoff of the queue, one after the other.  If no work is queued, theworker threads become idle.  These worker threads are managed in socalled worker-pools. The cmwq design differentiates between the user-facing workqueues thatsubsystems and drivers queue work items on and the backend mechanismwhich manages worker-pools and processes the queued work items. There are two worker-pools, one for normal work items and the otherfor high priority ones, for each possible CPU and some extraworker-pools to serve work items queued on unbound workqueues - thenumber of these backing pools is dynamic. Subsystems and drivers can create and queue work items through specialworkqueue API functions as they see fit. They can influence someaspects of the way the work items are executed by setting flags on theworkqueue they are putting the work item on. These flags includethings like CPU locality, concurrency limits, priority and more.  Toget a detailed overview refer to the API description of``alloc_workqueue()`` below. When a work item is queued to a workqueue, the target worker-pool isdetermined according to the queue parameters and workqueue attributesand appended on the shared worklist of the worker-pool.  For example,unless specifically overridden, a work item of a bound workqueue willbe queued on the worklist of either normal or highpri worker-pool thatis associated to the CPU the issuer is running on. For any worker pool implementation, managing the concurrency level(how many execution contexts are active) is an important issue.  cmwqtries to keep the concurrency at a minimal but sufficient level.Minimal to save resources and sufficient in that the system is used atits full capacity. Each worker-pool bound to an actual CPU implements concurrencymanagement by hooking into the scheduler.  The worker-pool is notifiedwhenever an active worker wakes up or sleeps and keeps track of thenumber of the currently runnable workers.  Generally, work items arenot expected to hog a CPU and consume many cycles.  That meansmaintaining just enough concurrency to prevent work processing fromstalling should be optimal.  As long as there are one or more runnableworkers on the CPU, the worker-pool doesn't start execution of a newwork, but, when the last running worker goes to sleep, it immediatelyschedules a new worker so that the CPU doesn't sit idle while thereare pending work items.  This allows using a minimal number of workerswithout losing execution bandwidth. Keeping idle workers around doesn't cost other than the memory spacefor kthreads, so cmwq holds onto idle ones for a while before killingthem. For unbound workqueues, the number of backing pools is dynamic.Unbound workqueue can be assigned custom attributes using``apply_workqueue_attrs()`` and workqueue will automatically createbacking worker pools matching the attributes.  The responsibility ofregulating concurrency level is on the users.  There is also a flag tomark a bound wq to ignore the concurrency management.  Please refer tothe API section for details. Forward progress guarantee relies on that workers can be created whenmore execution contexts are necessary, which in turn is guaranteedthrough the use of rescue workers.  All work items which might be usedon code paths that handle memory reclaim are required to be queued onwq's that have a rescue-worker reserved for execution under memorypressure.  Else it is possible that the worker-pool deadlocks waitingfor execution contexts to free up.  Application Programming Interface (API)======================================= ``alloc_workqueue()`` allocates a wq.  The original``create_*workqueue()`` functions are deprecated and scheduled forremoval.  ``alloc_workqueue()`` takes three arguments - ``@name``,``@flags`` and ``@max_active``.  ``@name`` is the name of the wq andalso used as the name of the rescuer thread if there is one. A wq no longer manages execution resources but serves as a domain forforward progress guarantee, flush and work item attributes. ``@flags``and ``@max_active`` control how work items are assigned executionresources, scheduled and executed.  ``flags``--------- ``WQ_UNBOUND``  Work items queued to an unbound wq are served by the special  worker-pools which host workers which are not bound to any  specific CPU.  This makes the wq behave as a simple execution  context provider without concurrency management.  The unbound  worker-pools try to start execution of work items as soon as  possible.  Unbound wq sacrifices locality but is useful for  the following cases.   * Wide fluctuation in the concurrency level requirement is    expected and using bound wq may end up creating large number    of mostly unused workers across different CPUs as the issuer    hops through different CPUs.   * Long running CPU intensive workloads which can be better    managed by the system scheduler. ``WQ_FREEZABLE``  A freezable wq participates in the freeze phase of the system  suspend operations.  Work items on the wq are drained and no  new work item starts execution until thawed. ``WQ_MEM_RECLAIM``  All wq which might be used in the memory reclaim paths **MUST**  have this flag set.  The wq is guaranteed to have at least one  execution context regardless of memory pressure. ``WQ_HIGHPRI``  Work items of a highpri wq are queued to the highpri  worker-pool of the target cpu.  Highpri worker-pools are  served by worker threads with elevated nice level.   Note that normal and highpri worker-pools don't interact with  each other.  Each maintains its separate pool of workers and  implements concurrency management among its workers. ``WQ_CPU_INTENSIVE``  Work items of a CPU intensive wq do not contribute to the  concurrency level.  In other words, runnable CPU intensive  work items will not prevent other work items in the same  worker-pool from starting execution.  This is useful for bound  work items which are expected to hog CPU cycles so that their  execution is regulated by the system scheduler.   Although CPU intensive work items don't contribute to the  concurrency level, start of their executions is still  regulated by the concurrency management and runnable  non-CPU-intensive work items can delay execution of CPU  intensive work items.   This flag is meaningless for unbound wq.  ``max_active``-------------- ``@max_active`` determines the maximum number of execution contextsper CPU which can be assigned to the work items of a wq.  For example,with ``@max_active`` of 16, at most 16 work items of the wq can beexecuting at the same time per CPU. Currently, for a bound wq, the maximum limit for ``@max_active`` is512 and the default value used when 0 is specified is 256.  For anunbound wq, the limit is higher of 512 and 4 *``num_possible_cpus()``.  These values are chosen sufficiently highsuch that they are not the limiting factor while providing protectionin runaway cases. The number of active work items of a wq is usually regulated by theusers of the wq, more specifically, by how many work items the usersmay queue at the same time.  Unless there is a specific need forthrottling the number of active work items, specifying '0' isrecommended. Some users depend on the strict execution ordering of ST wq.  Thecombination of ``@max_active`` of 1 and ``WQ_UNBOUND`` used toachieve this behavior.  Work items on such wq were always queued to theunbound worker-pools and only one work item could be active at any giventime thus achieving the same ordering property as ST wq. In the current implementation the above configuration only guaranteesST behavior within a given NUMA node. Instead ``alloc_ordered_queue()`` shouldbe used to achieve system-wide ST behavior.  Example Execution Scenarios=========================== The following example execution scenarios try to illustrate how cmwqbehave under different configurations.  Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU. w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms again before finishing.  w1 and w2 burn CPU for 5ms then sleep for 10ms. Ignoring all other tasks, works and processing overhead, and assumingsimple FIFO scheduling, the following is one highly simplified versionof possible sequences of events with the original wq. ::  TIME IN MSECS	EVENT 0		w0 starts and burns CPU 5		w0 sleeps 15		w0 wakes up and burns CPU 20		w0 finishes 20		w1 starts and burns CPU 25		w1 sleeps 35		w1 wakes up and finishes 35		w2 starts and burns CPU 40		w2 sleeps 50		w2 wakes up and finishes And with cmwq with ``@max_active`` >= 3, ::  TIME IN MSECS	EVENT 0		w0 starts and burns CPU 5		w0 sleeps 5		w1 starts and burns CPU 10		w1 sleeps 10		w2 starts and burns CPU 15		w2 sleeps 15		w0 wakes up and burns CPU 20		w0 finishes 20		w1 wakes up and finishes 25		w2 wakes up and finishes If ``@max_active`` == 2, ::  TIME IN MSECS	EVENT 0		w0 starts and burns CPU 5		w0 sleeps 5		w1 starts and burns CPU 10		w1 sleeps 15		w0 wakes up and burns CPU 20		w0 finishes 20		w1 wakes up and finishes 20		w2 starts and burns CPU 25		w2 sleeps 35		w2 wakes up and finishes Now, let's assume w1 and w2 are queued to a different wq q1 which has``WQ_CPU_INTENSIVE`` set, ::  TIME IN MSECS	EVENT 0		w0 starts and burns CPU 5		w0 sleeps 5		w1 and w2 start and burn CPU 10		w1 sleeps 15		w2 sleeps 15		w0 wakes up and burns CPU 20		w0 finishes 20		w1 wakes up and finishes 25		w2 wakes up and finishes  Guidelines========== * Do not forget to use ``WQ_MEM_RECLAIM`` if a wq may process work  items which are used during memory reclaim.  Each wq with  ``WQ_MEM_RECLAIM`` set has an execution context reserved for it.  If  there is dependency among multiple work items used during memory  reclaim, they should be queued to separate wq each with  ``WQ_MEM_RECLAIM``. * Unless strict ordering is required, there is no need to use ST wq. * Unless there is a specific need, using 0 for @max_active is  recommended.  In most use cases, concurrency level usually stays  well under the default limit. * A wq serves as a domain for forward progress guarantee  (``WQ_MEM_RECLAIM``, flush and work item attributes.  Work items  which are not involved in memory reclaim and don't need to be  flushed as a part of a group of work items, and don't require any  special attribute, can use one of the system wq.  There is no  difference in execution characteristics between using a dedicated wq  and a system wq. * Unless work items are expected to consume a huge amount of CPU  cycles, using a bound wq is usually beneficial due to the increased  level of locality in wq operations and work item execution.  Debugging========= Because the work functions are executed by generic worker threadsthere are a few tricks needed to shed some light on misbehavingworkqueue users. Worker threads show up in the process list as: ::   root      5671  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/0:1]  root      5672  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/1:2]  root      5673  0.0  0.0      0     0 ?        S    12:12   0:00 [kworker/0:0]  root      5674  0.0  0.0      0     0 ?        S    12:13   0:00 [kworker/1:0] If kworkers are going crazy (using too much cpu), there are two typesof possible problems: 	1. Something being scheduled in rapid succession	2. A single work item that consumes lots of cpu cycles The first one can be tracked using tracing: :: 	$ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event	$ cat /sys/kernel/debug/tracing/trace_pipe > out.txt	(wait a few secs)	^C If something is busy looping on work queueing, it would be dominatingthe output and the offender can be determined with the work itemfunction. For the second type of problems it should be possible to just checkthe stack trace of the offending worker thread. :: 	$ cat /proc/THE_OFFENDING_KWORKER/stack The work item's function should be trivially visible in the stacktrace. Non-reentrance Conditions========================= Workqueue guarantees that a work item cannot be re-entrant if the followingconditions hold after a work item gets queued:         1. The work function hasn't been changed.        2. No one queues the work item to another workqueue.        3. The work item hasn't been reinitiated. In other words, if the above conditions hold, the work item is guaranteed to beexecuted by at most one worker system-wide at any given time. Note that requeuing the work item (to the same queue) in the self functiondoesn't break these conditions, so it's safe to do. Otherwise, caution isrequired when breaking the conditions inside a work function.  Kernel Inline Documentations Reference====================================== .. kernel-doc:: include/linux/workqueue.h .. kernel-doc:: kernel/workqueue.c