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36 .Nd asynchronous task execution
44 typedef void (*task_fn_t)(void *context, int pending);
46 typedef void (*taskqueue_enqueue_fn)(void *context);
49 STAILQ_ENTRY(task) ta_link; /* link for queue */
50 u_short ta_pending; /* count times queued */
51 u_short ta_priority; /* priority of task in queue */
52 task_fn_t ta_func; /* task handler */
53 void *ta_context; /* argument for handler */
58 .Ft struct taskqueue *
59 .Fn taskqueue_create "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
60 .Ft struct taskqueue *
61 .Fn taskqueue_create_fast "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
63 .Fn taskqueue_free "struct taskqueue *queue"
65 .Fn taskqueue_enqueue "struct taskqueue *queue" "struct task *task"
67 .Fn taskqueue_enqueue_fast "struct taskqueue *queue" "struct task *task"
69 .Fn taskqueue_enqueue_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "int ticks"
71 .Fn taskqueue_cancel "struct taskqueue *queue" "struct task *task" "u_int *pendp"
73 .Fn taskqueue_cancel_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "u_int *pendp"
75 .Fn taskqueue_drain "struct taskqueue *queue" "struct task *task"
77 .Fn taskqueue_drain_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task"
79 .Fn taskqueue_member "struct taskqueue *queue" "struct thread *td"
81 .Fn taskqueue_run "struct taskqueue *queue"
82 .Fn TASK_INIT "struct task *task" "int priority" "task_fn_t func" "void *context"
83 .Fn TASKQUEUE_DECLARE "name"
84 .Fn TASKQUEUE_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
85 .Fn TASKQUEUE_FAST_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
86 .Fn TASKQUEUE_DEFINE_THREAD "name"
87 .Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
88 .Fn TIMEOUT_TASK_INIT "struct taskqueue *queue" "struct timeout_task *timeout_task" "int priority" "task_fn_t func" "void *context"
90 These functions provide a simple interface for asynchronous execution
95 is used to create new queues.
98 include a name that should be unique,
101 flags that specify whether the call to
104 a function that is called from
105 .Fn taskqueue_enqueue
106 when a task is added to the queue,
107 and a pointer to the memory location where the identity of the
108 thread that services the queue is recorded.
109 .\" XXX The rest of the sentence gets lots in relation to the first part.
110 The function called from
111 .Fn taskqueue_enqueue
112 must arrange for the queue to be processed
113 (for instance by scheduling a software interrupt or waking a kernel
115 The memory location where the thread identity is recorded is used
116 to signal the service thread(s) to terminate--when this value is set to
117 zero and the thread is signaled it will terminate.
118 If the queue is intended for use in fast interrupt handlers
119 .Fn taskqueue_create_fast
120 should be used in place of
121 .Fn taskqueue_create .
125 should be used to free the memory used by the queue.
126 Any tasks that are on the queue will be executed at this time after
127 which the thread servicing the queue will be signaled that it should exit.
129 To add a task to the list of tasks queued on a taskqueue, call
130 .Fn taskqueue_enqueue
131 with pointers to the queue and task.
135 then it is simply incremented to reflect the number of times the task
136 was enqueued, up to a cap of USHRT_MAX.
138 the task is added to the list before the first task which has a lower
140 value or at the end of the list if no tasks have a lower priority.
141 Enqueueing a task does not perform any memory allocation which makes
142 it suitable for calling from an interrupt handler.
143 This function will return
145 if the queue is being freed.
148 .Fn taskqueue_enqueue_fast
149 should be used in place of
150 .Fn taskqueue_enqueue
151 when the enqueuing must happen from a fast interrupt handler.
152 This method uses spin locks to avoid the possibility of sleeping in the fast
155 When a task is executed,
156 first it is removed from the queue,
159 is recorded and then the field is zeroed.
162 from the task structure is called with the value of the field
164 as its first argument
167 as its second argument.
172 is called on the task pointer passed to
173 .Fn taskqueue_enqueue .
176 .Fn taskqueue_enqueue_timeout
177 is used to schedule the enqueue after the specified amount of
179 Only non-fast task queues can be used for
185 function is used to cancel a task.
188 count is cleared, and the old value returned in the reference
193 If the task is currently running,
195 is returned, otherwise 0.
196 To implement a blocking
198 that waits for a running task to finish, it could look like:
199 .Bd -literal -offset indent
200 while (taskqueue_cancel(tq, task, NULL) != 0)
201 taskqueue_drain(tq, task);
205 .Fn taskqueue_drain ,
206 the caller is responsible for ensuring that the task is not re-enqueued
207 after being canceled.
210 .Fn taskqueue_cancel_timeout
211 function is used to cancel the scheduled task execution.
215 function is used to wait for the task to finish, and
217 .Fn taskqueue_drain_timeout
218 function is used to wait for the scheduled task to finish.
219 There is no guarantee that the task will not be
220 enqueued after call to
221 .Fn taskqueue_drain .
229 is part of the given taskqueue
237 function will run all pending tasks in the specified
239 Normally this function is only used internally.
242 .Fn TASK_INIT "task" "priority" "func" "context"
243 is provided to initialise a
247 .Fn TIMEOUT_TASK_INIT "queue" "timeout_task" "priority" "func" "context"
248 initializes the timeout_task structure.
254 are simply copied into the task structure fields and the
259 .Fn TASKQUEUE_DECLARE "name" ,
260 .Fn TASKQUEUE_DEFINE "name" "enqueue" "context" "init" ,
261 .Fn TASKQUEUE_FAST_DEFINE "name" "enqueue" "context" "init" ,
263 .Fn TASKQUEUE_DEFINE_THREAD "name"
264 .Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
265 are used to declare a reference to a global queue, to define the
266 implementation of the queue, and declare a queue that uses its own thread.
269 macro arranges to call
271 with the values of its
276 arguments during system initialisation.
278 .Fn taskqueue_create ,
281 argument to the macro is executed as a C statement,
282 allowing any further initialisation to be performed
283 (such as registering an interrupt handler etc.)
286 .Fn TASKQUEUE_DEFINE_THREAD
287 macro defines a new taskqueue with its own kernel thread to serve tasks.
289 .Vt struct taskqueue *taskqueue_name
290 is used to enqueue tasks onto the queue.
292 .Fn TASKQUEUE_FAST_DEFINE
294 .Fn TASKQUEUE_FAST_DEFINE_THREAD
298 .Fn TASKQUEUE_DEFINE_THREAD
299 respectively but taskqueue is created with
300 .Fn taskqueue_create_fast .
301 .Ss Predefined Task Queues
302 The system provides four global taskqueues,
305 .Va taskqueue_swi_giant ,
307 .Va taskqueue_thread .
310 queue is for swi handlers dispatched from fast interrupt handlers,
311 where sleep mutexes cannot be used.
312 The swi taskqueues are run via a software interrupt mechanism.
315 queue runs without the protection of the
318 .Va taskqueue_swi_giant
319 queue runs with the protection of the
324 runs in a kernel thread context, and tasks run from this thread do
328 If the caller wants to run under
330 he should explicitly acquire and release
332 in his taskqueue handler routine.
336 .Fn taskqueue_enqueue
337 with the value of the global taskqueue variable for the queue you wish to
339 .Va ( taskqueue_swi ,
340 .Va taskqueue_swi_giant ,
342 .Va taskqueue_thread ) .
344 .Fn taskqueue_enqueue_fast
345 for the global taskqueue variable
348 The software interrupt queues can be used,
349 for instance, for implementing interrupt handlers which must perform a
350 significant amount of processing in the handler.
351 The hardware interrupt handler would perform minimal processing of the
352 interrupt and then enqueue a task to finish the work.
353 This reduces to a minimum
354 the amount of time spent with interrupts disabled.
356 The thread queue can be used, for instance, by interrupt level routines
357 that need to call kernel functions that do things that can only be done
358 from a thread context.
359 (e.g., call malloc with the M_WAITOK flag.)
361 Note that tasks queued on shared taskqueues such as
363 may be delayed an indeterminate amount of time before execution.
364 If queueing delays cannot be tolerated then a private taskqueue should
365 be created with a dedicated processing thread.
371 This interface first appeared in
373 There is a similar facility called work_queue in the Linux kernel.
375 This manual page was written by