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Diffstat (limited to 'Documentation/core-api/workqueue.rst')
| -rw-r--r-- | Documentation/core-api/workqueue.rst | 43 |
1 files changed, 29 insertions, 14 deletions
diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst index 3599cf9267b4..ed73c612174d 100644 --- a/Documentation/core-api/workqueue.rst +++ b/Documentation/core-api/workqueue.rst @@ -77,10 +77,12 @@ wants a function to be executed asynchronously it has to set up a work item pointing to that function and queue that work item on a workqueue. -Special purpose threads, called worker threads, execute the functions -off of the queue, one after the other. If no work is queued, the -worker threads become idle. These worker threads are managed in so -called worker-pools. +A work item can be executed in either a thread or the BH (softirq) context. + +For threaded workqueues, special purpose threads, called [k]workers, execute +the functions off of the queue, one after the other. If no work is queued, +the worker threads become idle. These worker threads are managed in +worker-pools. The cmwq design differentiates between the user-facing workqueues that subsystems and drivers queue work items on and the backend mechanism @@ -91,6 +93,12 @@ for high priority ones, for each possible CPU and some extra worker-pools to serve work items queued on unbound workqueues - the number of these backing pools is dynamic. +BH workqueues use the same framework. However, as there can only be one +concurrent execution context, there's no need to worry about concurrency. +Each per-CPU BH worker pool contains only one pseudo worker which represents +the BH execution context. A BH workqueue can be considered a convenience +interface to softirq. + Subsystems and drivers can create and queue work items through special workqueue API functions as they see fit. They can influence some aspects of the way the work items are executed by setting flags on the @@ -106,7 +114,7 @@ unless specifically overridden, a work item of a bound workqueue will be queued on the worklist of either normal or highpri worker-pool that is associated to the CPU the issuer is running on. -For any worker pool implementation, managing the concurrency level +For any thread pool implementation, managing the concurrency level (how many execution contexts are active) is an important issue. cmwq tries to keep the concurrency at a minimal but sufficient level. Minimal to save resources and sufficient in that the system is used at @@ -164,6 +172,17 @@ resources, scheduled and executed. ``flags`` --------- +``WQ_BH`` + BH workqueues can be considered a convenience interface to softirq. BH + workqueues are always per-CPU and all BH work items are executed in the + queueing CPU's softirq context in the queueing order. + + All BH workqueues must have 0 ``max_active`` and ``WQ_HIGHPRI`` is the + only allowed additional flag. + + BH work items cannot sleep. All other features such as delayed queueing, + flushing and canceling are supported. + ``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 @@ -237,15 +256,11 @@ may queue at the same time. Unless there is a specific need for throttling the number of active work items, specifying '0' is recommended. -Some users depend on the strict execution ordering of ST wq. The -combination of ``@max_active`` of 1 and ``WQ_UNBOUND`` used to -achieve this behavior. Work items on such wq were always queued to the -unbound worker-pools and only one work item could be active at any given -time thus achieving the same ordering property as ST wq. - -In the current implementation the above configuration only guarantees -ST behavior within a given NUMA node. Instead ``alloc_ordered_workqueue()`` should -be used to achieve system-wide ST behavior. +Some users depend on strict execution ordering where only one work item +is in flight at any given time and the work items are processed in +queueing order. While the combination of ``@max_active`` of 1 and +``WQ_UNBOUND`` used to achieve this behavior, this is no longer the +case. Use ``alloc_ordered_queue()`` instead. Example Execution Scenarios |