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taskqueue - asynchronous task execution



      taskqueue - asynchronous task execution


      #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 **);
      taskqueue_free(struct taskqueue *queue);
      struct taskqueue *
      taskqueue_find(const char *name);
      taskqueue_enqueue(struct taskqueue *queue, struct task *task);
      taskqueue_enqueue_fast(struct taskqueue *queue, struct task *task);
      taskqueue_run(struct taskqueue *queue);
      taskqueue_run_fast(struct taskqueue *queue);
      taskqueue_drain(struct taskqueue *queue, struct task *task);
      TASK_INIT(struct task *task, int priority, task_fn_t *func,
              void *context);
      TASKQUEUE_DEFINE(name, taskqueue_enqueue_fn enqueue, void *context,


      These functions provide a simple interface for asynchronous execution of
      The function taskqueue_create() is used to create new queues.  The argu‐
      ments 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 pro‐
      cessed (for instance by scheduling a software interrupt or waking a ker‐
      nel 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, other‐
      wise 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 sleep‐
      ing 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
      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 call‐
      ing 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 dis‐
      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.
      ithread(9), kthread(9), swi(9)


      This interface first appeared in FreeBSD 5.0.  There is a similar facil‐
      ity called tqueue in the Linux kernel.


      This manual page was written by Doug Rabson.


      There is no taskqueue_create_fast().


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