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


taskqueueasynchronous 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 */

enum taskqueue_callback_type {

typedef void (*taskqueue_callback_fn)(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);

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

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

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

taskqueue_free(struct taskqueue *queue);

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

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

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

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

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

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

taskqueue_drain_all(struct taskqueue *queue);

taskqueue_block(struct taskqueue *queue);

taskqueue_unblock(struct taskqueue *queue);

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

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

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



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


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

The function () 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 () 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 () 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 () or (). 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 (). Currently, callbacks may be registered for the following purposes:

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.
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 () 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 ().

The () 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.

The () 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 (), the caller is responsible for ensuring that the task is not re-enqueued after being canceled.

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

The () function is used to wait for the task to finish, and the () 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 () 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 () 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 () function unblocks the previously blocked taskqueue. All enqueued tasks can be run after this call.

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

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

A convenience macro, (task, priority, func, context) is provided to initialise a task structure. The () macro generates an initializer for a task structure. A macro (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 (name), (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 () 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.

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

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 () 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.


ithread(9), kthread(9), swi(9)


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


This manual page was written by Doug Rabson.

March 1, 2016 Debian