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TASKQUEUE (9) | Kernel routines | Unix Manual Pages | :man


taskqueue - asynchronous task execution


Predefined Task Queues
See Also


.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_tta_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"


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.

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
.Vt struct proc *taskqueue_name_proc is defined which contains the kernel thread serving the tasks. The variable
.Vt 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.


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




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