Provided by: freebsd-manpages_7.0-2_all bug

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);

     typedef void (*taskqueue_enqueue_fn)(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 */
     };

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

     void
     taskqueue_free(struct taskqueue *queue);

     struct taskqueue *
     taskqueue_find(const char *name);

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

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

     void
     taskqueue_run(struct taskqueue *queue);

     void
     taskqueue_run_fast(struct taskqueue *queue);

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

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

     TASKQUEUE_DECLARE(name);

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

     TASKQUEUE_DEFINE_THREAD(name);

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.

     The function taskqueue_free() should be used to remove the queue from the
     global list of queues and 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.

     The system maintains a list of all queues which can be searched using
     taskqueue_find().  The first queue whose name matches is returned,
     otherwise NULL.

     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.  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.

     The function taskqueue_enqueue_fast() should be used in place of
     taskqueue_enqueue() when the enqueuing must happen from a fast interrupt
     handler.  This method uses spin locks to avoid the possibility of
     sleeping in the fast interrupt context.

     To execute all the tasks on a queue, call taskqueue_run() or
     taskqueue_run_fast() depending on the flavour of the queue.  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_drain() function is used to wait for the task to finish.
     There is no guarantee that the task will not be enqueued after call to
     taskqueue_drain().

     A convenience macro, TASK_INIT(task, priority, func, context) is provided
     to initialise a task structure.  The values of priority, func, and
     context are simply copied into the task structure fields and the
     ta_pending field is cleared.

     Three macros TASKQUEUE_DECLARE(name), TASKQUEUE_DEFINE(name, enqueue,
     context, init), and TASKQUEUE_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 proc
     *taskqueue_name_proc is defined which contains the kernel thread serving
     the tasks.  The variable struct taskqueue *taskqueue_name is used to
     enqueue tasks onto the queue.

   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 (taskqueue_swi,
     taskqueue_swi_giant, or taskqueue_thread).  Use taskqueue_enqueue_fast()
     for the global taskqueue variable taskqueue_fast.

     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 tqueue in the Linux kernel.

AUTHORS

     This manual page was written by Doug Rabson.

BUGS

     There is no taskqueue_create_fast().