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TASKQUEUE(9)           FreeBSD Kernel Developer's Manual          TASKQUEUE(9)

NAME
     taskqueue - asynchronous task execution

SYNOPSIS
     #include <sys/param.h>
     #include <sys/kernel.h>
     #include <sys/malloc.h>
     #include <sys/queue.h>
     #include <sys/taskqueue.h>

     typedef void (*task_fn_t)(void *context, int pending);)(void *context, int pending);

     typedef void (*taskqueue_enqueue_fn)(void *context);)(void *context);

     struct task {
             STAILQ_ENTRY(task)      ta_link;        /* link for queue */
             u_short                 ta_pending;     /* count times queued */
             u_short                 ta_priority;    /* priority of task in queue */
             task_fn_t               ta_func;        /* task handler */
             void                    *ta_context;    /* argument for handler */
     };

     enum taskqueue_callback_type {
             TASKQUEUE_CALLBACK_TYPE_INIT,
             TASKQUEUE_CALLBACK_TYPE_SHUTDOWN,
     };

     typedef void (*taskqueue_callback_fn)(void *context);)(void *context);

     struct timeout_task;
     struct taskqueue *
     taskqueue_create(const char *name, int mflags,
         taskqueue_enqueue_fn enqueue, void *context);

     struct taskqueue *
     taskqueue_create_fast(const char *name, int mflags,
         taskqueue_enqueue_fn enqueue, void *context);

     int
     taskqueue_start_threads(struct taskqueue **tqp, int count, int pri,
         const char *name, ...);

     int
     taskqueue_start_threads_pinned(struct taskqueue **tqp, int count,
         int pri, int cpu_id, const char *name, ...);

     void
     taskqueue_set_callback(struct taskqueue *queue,
         enum taskqueue_callback_type cb_type, taskqueue_callback_fn callback,
         void *context);

     void
     taskqueue_free(struct taskqueue *queue);

     int
     taskqueue_enqueue(struct taskqueue *queue, struct task *task);

     int
     taskqueue_enqueue_timeout(struct taskqueue *queue,
         struct timeout_task *timeout_task, int ticks);

     int
     taskqueue_cancel(struct taskqueue *queue, struct task *task,
         u_int *pendp);

     int
     taskqueue_cancel_timeout(struct taskqueue *queue,
         struct timeout_task *timeout_task, u_int *pendp);

     void
     taskqueue_drain(struct taskqueue *queue, struct task *task);

     void
     taskqueue_drain_timeout(struct taskqueue *queue,
         struct timeout_task *timeout_task);

     void
     taskqueue_drain_all(struct taskqueue *queue);

     void
     taskqueue_block(struct taskqueue *queue);

     void
     taskqueue_unblock(struct taskqueue *queue);

     int
     taskqueue_member(struct taskqueue *queue, struct thread *td);

     void
     taskqueue_run(struct taskqueue *queue);

     TASK_INIT(struct task *task, int priority, task_fn_t func,
         void *context);

     TASK_INITIALIZER(int priority, task_fn_t func, void *context);

     TASKQUEUE_DECLARE(name);

     TASKQUEUE_DEFINE(name, taskqueue_enqueue_fn enqueue, void *context,
         init);

     TASKQUEUE_FAST_DEFINE(name, taskqueue_enqueue_fn enqueue, void *context,
         init);

     TASKQUEUE_DEFINE_THREAD(name);

     TASKQUEUE_FAST_DEFINE_THREAD(name);

     TIMEOUT_TASK_INIT(struct taskqueue *queue,
         struct timeout_task *timeout_task, int priority, task_fn_t func,
         void *context);

DESCRIPTION
     These functions provide a simple interface for asynchronous execution of
     code.

     The function taskqueue_create() is used to create new queues.  The
     arguments to taskqueue_create() include a name that should be unique, a
     set of malloc(9) flags that specify whether the call to malloc() is
     allowed to sleep, a function that is called from taskqueue_enqueue() when
     a task is added to the queue, and a pointer to the memory location where
     the identity of the thread that services the queue is recorded.  The
     function called from taskqueue_enqueue() must arrange for the queue to be
     processed (for instance by scheduling a software interrupt or waking a
     kernel thread).  The memory location where the thread identity is
     recorded is used to signal the service thread(s) to terminate--when this
     value is set to zero and the thread is signaled it will terminate.  If
     the queue is intended for use in fast interrupt handlers
     taskqueue_create_fast() should be used in place of taskqueue_create().

     The function taskqueue_free() should be used to free the memory used by
     the queue.  Any tasks that are on the queue will be executed at this time
     after which the thread servicing the queue will be signaled that it
     should exit.

     Once a taskqueue has been created, its threads should be started using
     taskqueue_start_threads() or taskqueue_start_threads_pinned().
     taskqueue_start_threads_pinned() takes a cpu_id argument which will cause
     the threads which are started for the taskqueue to be pinned to run on
     the given CPU.  Callbacks may optionally be registered using
     taskqueue_set_callback().  Currently, callbacks may be registered for the
     following purposes:

     TASKQUEUE_CALLBACK_TYPE_INIT      This callback is called by every thread
                                       in the taskqueue, before it executes
                                       any tasks.  This callback must be set
                                       before the taskqueue's threads are
                                       started.

     TASKQUEUE_CALLBACK_TYPE_SHUTDOWN  This callback is called by every thread
                                       in the taskqueue, after it executes its
                                       last task.  This callback will always
                                       be called before the taskqueue
                                       structure is reclaimed.

     To add a task to the list of tasks queued on a taskqueue, call
     taskqueue_enqueue() with pointers to the queue and task.  If the task's
     ta_pending field is non-zero, then it is simply incremented to reflect
     the number of times the task was enqueued, up to a cap of USHRT_MAX.
     Otherwise, the task is added to the list before the first task which has
     a lower ta_priority value or at the end of the list if no tasks have a
     lower priority.  Enqueueing a task does not perform any memory allocation
     which makes it suitable for calling from an interrupt handler.  This
     function will return EPIPE if the queue is being freed.

     When a task is executed, first it is removed from the queue, the value of
     ta_pending is recorded and then the field is zeroed.  The function
     ta_func from the task structure is called with the value of the field
     ta_context as its first argument and the value of ta_pending as its
     second argument.  After the function ta_func returns, wakeup(9) is called
     on the task pointer passed to taskqueue_enqueue().

     The taskqueue_enqueue_timeout() is used to schedule the enqueue after the
     specified amount of ticks.  Only non-fast task queues can be used for
     timeout_task scheduling.  If the ticks argument is negative, the already
     scheduled enqueueing is not re-scheduled.  Otherwise, the task is
     scheduled for enqueueing in the future, after the absolute value of ticks
     is passed.  This function returns -1 if the task is being drained.
     Otherwise, the number of pending calls is returned.

     The taskqueue_cancel() function is used to cancel a task.  The ta_pending
     count is cleared, and the old value returned in the reference parameter
     pendp, if it is non-NULL.  If the task is currently running, EBUSY is
     returned, otherwise 0.  To implement a blocking taskqueue_cancel() that
     waits for a running task to finish, it could look like:

           while (taskqueue_cancel(tq, task, NULL) != 0)
                   taskqueue_drain(tq, task);

     Note that, as with taskqueue_drain(), the caller is responsible for
     ensuring that the task is not re-enqueued after being canceled.

     Similarly, the taskqueue_cancel_timeout() function is used to cancel the
     scheduled task execution.

     The taskqueue_drain() function is used to wait for the task to finish,
     and the taskqueue_drain_timeout() function is used to wait for the
     scheduled task to finish.  There is no guarantee that the task will not
     be enqueued after call to taskqueue_drain().  If the caller wants to put
     the task into a known state, then before calling taskqueue_drain() the
     caller should use out-of-band means to ensure that the task would not be
     enqueued.  For example, if the task is enqueued by an interrupt filter,
     then the interrupt could be disabled.

     The taskqueue_drain_all() function is used to wait for all pending and
     running tasks that are enqueued on the taskqueue to finish.  Tasks posted
     to the taskqueue after taskqueue_drain_all() begins processing, including
     pending enqueues scheduled by a previous call to
     taskqueue_enqueue_timeout(), do not extend the wait time of
     taskqueue_drain_all() and may complete after taskqueue_drain_all()
     returns.

     The taskqueue_block() function blocks the taskqueue.  It prevents any
     enqueued but not running tasks from being executed.  Future calls to
     taskqueue_enqueue() will enqueue tasks, but the tasks will not be run
     until taskqueue_unblock() is called.  Please note that taskqueue_block()
     does not wait for any currently running tasks to finish.  Thus, the
     taskqueue_block() does not provide a guarantee that taskqueue_run() is
     not running after taskqueue_block() returns, but it does provide a
     guarantee that taskqueue_run() will not be called again until
     taskqueue_unblock() is called.  If the caller requires a guarantee that
     taskqueue_run() is not running, then this must be arranged by the caller.
     Note that if taskqueue_drain() is called on a task that is enqueued on a
     taskqueue that is blocked by taskqueue_block(), then taskqueue_drain()
     can not return until the taskqueue is unblocked.  This can result in a
     deadlock if the thread blocked in taskqueue_drain() is the thread that is
     supposed to call taskqueue_unblock().  Thus, use of taskqueue_drain()
     after taskqueue_block() is discouraged, because the state of the task can
     not be known in advance.  The same caveat applies to
     taskqueue_drain_all().

     The taskqueue_unblock() function unblocks the previously blocked
     taskqueue.  All enqueued tasks can be run after this call.

     The taskqueue_member() function returns 1 if the given thread td is part
     of the given taskqueue queue and 0 otherwise.

     The taskqueue_run() function will run all pending tasks in the specified
     queue.  Normally this function is only used internally.

     A convenience macro, TASK_INIT(task, priority, func, context) is provided
     to initialise a task structure.  The TASK_INITIALIZER() macro generates
     an initializer for a task structure.  A macro TIMEOUT_TASK_INIT(queue,
     timeout_task, priority, func, context) initializes the timeout_task
     structure.  The values of priority, func, and context are simply copied
     into the task structure fields and the ta_pending field is cleared.

     Five macros TASKQUEUE_DECLARE(name), TASKQUEUE_DEFINE(name, enqueue,
     context, init), TASKQUEUE_FAST_DEFINE(name, enqueue, context, init), and
     TASKQUEUE_DEFINE_THREAD(name) TASKQUEUE_FAST_DEFINE_THREAD(name) are used
     to declare a reference to a global queue, to define the implementation of
     the queue, and declare a queue that uses its own thread.  The
     TASKQUEUE_DEFINE() macro arranges to call taskqueue_create() with the
     values of its name, enqueue and context arguments during system
     initialisation.  After calling taskqueue_create(), the init argument to
     the macro is executed as a C statement, allowing any further
     initialisation to be performed (such as registering an interrupt handler
     etc.)

     The TASKQUEUE_DEFINE_THREAD() macro defines a new taskqueue with its own
     kernel thread to serve tasks.  The variable struct taskqueue
     *taskqueue_name is used to enqueue tasks onto the queue.

     TASKQUEUE_FAST_DEFINE() and TASKQUEUE_FAST_DEFINE_THREAD() act just like
     TASKQUEUE_DEFINE() and TASKQUEUE_DEFINE_THREAD() respectively but
     taskqueue is created with taskqueue_create_fast().

   Predefined Task Queues
     The system provides four global taskqueues, taskqueue_fast,
     taskqueue_swi, taskqueue_swi_giant, and taskqueue_thread.  The
     taskqueue_fast queue is for swi handlers dispatched from fast interrupt
     handlers, where sleep mutexes cannot be used.  The swi taskqueues are run
     via a software interrupt mechanism.  The taskqueue_swi queue runs without
     the protection of the Giant kernel lock, and the taskqueue_swi_giant
     queue runs with the protection of the Giant kernel lock.  The thread
     taskqueue taskqueue_thread runs in a kernel thread context, and tasks run
     from this thread do not run under the Giant kernel lock.  If the caller
     wants to run under Giant, he should explicitly acquire and release Giant
     in his taskqueue handler routine.

     To use these queues, call taskqueue_enqueue() with the value of the
     global taskqueue variable for the queue you wish to use.

     The software interrupt queues can be used, for instance, for implementing
     interrupt handlers which must perform a significant amount of processing
     in the handler.  The hardware interrupt handler would perform minimal
     processing of the interrupt and then enqueue a task to finish the work.
     This reduces to a minimum the amount of time spent with interrupts
     disabled.

     The thread queue can be used, for instance, by interrupt level routines
     that need to call kernel functions that do things that can only be done
     from a thread context.  (e.g., call malloc with the M_WAITOK flag.)

     Note that tasks queued on shared taskqueues such as taskqueue_swi may be
     delayed an indeterminate amount of time before execution.  If queueing
     delays cannot be tolerated then a private taskqueue should be created
     with a dedicated processing thread.

SEE ALSO
     ithread(9), kthread(9), swi(9)

HISTORY
     This interface first appeared in FreeBSD 5.0.  There is a similar
     facility called work_queue in the Linux kernel.

AUTHORS
     This manual page was written by Doug Rabson.

FreeBSD 11.1-RELEASE-p4          March 1, 2016         FreeBSD 11.1-RELEASE-p4
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