taskqueue(9) asynchronous task execution


In sys/param.h In sys/kernel.h In sys/malloc.h In sys/queue.h In 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;

Ft struct taskqueue * Fn taskqueue_create const char *name int mflags taskqueue_enqueue_fn enqueue void *context Ft struct taskqueue * Fn taskqueue_create_fast const char *name int mflags taskqueue_enqueue_fn enqueue void *context Ft int Fn taskqueue_start_threads struct taskqueue **tqp int count int pri const char *name ... Ft void Fn taskqueue_set_callback struct taskqueue *queue enum taskqueue_callback_type cb_type taskqueue_callback_fn callback void *context Ft void Fn taskqueue_free struct taskqueue *queue Ft int Fn taskqueue_enqueue struct taskqueue *queue struct task *task Ft int Fn taskqueue_enqueue_fast struct taskqueue *queue struct task *task Ft int Fn taskqueue_enqueue_timeout struct taskqueue *queue struct timeout_task *timeout_task int ticks Ft int Fn taskqueue_cancel struct taskqueue *queue struct task *task u_int *pendp Ft int Fn taskqueue_cancel_timeout struct taskqueue *queue struct timeout_task *timeout_task u_int *pendp Ft void Fn taskqueue_drain struct taskqueue *queue struct task *task Ft void Fn taskqueue_drain_timeout struct taskqueue *queue struct timeout_task *timeout_task Ft void Fn taskqueue_drain_all struct taskqueue *queue Ft void Fn taskqueue_block struct taskqueue *queue Ft void Fn taskqueue_unblock struct taskqueue *queue Ft int Fn taskqueue_member struct taskqueue *queue struct thread *td Ft void Fn taskqueue_run struct taskqueue *queue Fn TASK_INIT struct task *task int priority task_fn_t func void *context Fn TASK_INITIALIZER int priority task_fn_t func void *context Fn TASKQUEUE_DECLARE name Fn TASKQUEUE_DEFINE name taskqueue_enqueue_fn enqueue void *context init Fn TASKQUEUE_FAST_DEFINE name taskqueue_enqueue_fn enqueue void *context init Fn TASKQUEUE_DEFINE_THREAD name Fn TASKQUEUE_FAST_DEFINE_THREAD name Fn 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 Fn taskqueue_create is used to create new queues. The arguments to Fn taskqueue_create include a name that should be unique, a set of malloc(9) flags that specify whether the call to Fn malloc is allowed to sleep, a function that is called from Fn 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 Fn 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 Fn taskqueue_create_fast should be used in place of Fn taskqueue_create .

The function Fn 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 Fn taskqueue_start_threads . Callbacks may optionally be registered using Fn taskqueue_set_callback . 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 Fn 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 Er EPIPE if the queue is being freed.

The function Fn taskqueue_enqueue_fast should be used in place of Fn 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.

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 Fn taskqueue_enqueue .

The Fn 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.

The Fn taskqueue_cancel function is used to cancel a task. The ta_pending count is cleared, and the old value returned in the reference parameter Fa pendp , if it is non- NULL If the task is currently running, EBUSY is returned, otherwise 0. To implement a blocking Fn 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 Fn taskqueue_drain , the caller is responsible for ensuring that the task is not re-enqueued after being canceled.

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

The Fn taskqueue_drain function is used to wait for the task to finish, and the Fn 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 Fn taskqueue_drain . If the caller wants to put the task into a known state, then before calling Fn 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 Fn taskqueue_drain_all function is used to wait for all pending and running tasks that are enqueued on the taskqueue to finish. The caller must arrange that the tasks are not re-enqueued. Note that Fn taskqueue_drain_all currently does not handle tasks with delayed enqueueing.

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

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

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

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

A convenience macro, Fn TASK_INIT task priority func context is provided to initialise a task structure. The Fn TASK_INITIALIZER macro generates an initializer for a task structure. A macro Fn 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 Fn TASKQUEUE_DECLARE name , Fn TASKQUEUE_DEFINE name enqueue context init , Fn TASKQUEUE_FAST_DEFINE name enqueue context init , and Fn TASKQUEUE_DEFINE_THREAD name Fn 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 Fn TASKQUEUE_DEFINE macro arranges to call Fn taskqueue_create with the values of its name enqueue and context arguments during system initialisation. After calling Fn 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 Fn TASKQUEUE_DEFINE_THREAD macro defines a new taskqueue with its own kernel thread to serve tasks. The variable Vt struct taskqueue *taskqueue_name is used to enqueue tasks onto the queue.

Fn TASKQUEUE_FAST_DEFINE and Fn TASKQUEUE_FAST_DEFINE_THREAD act just like Fn TASKQUEUE_DEFINE and Fn TASKQUEUE_DEFINE_THREAD respectively but taskqueue is created with Fn 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 Fn 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 Fn 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.


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


This manual page was written by An Doug Rabson .