freebsd-skq/share/man/man9/taskqueue.9

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.Dd May 12, 2000
.Dt TASKQUEUE 9
.Os
.Sh NAME
.Nm taskqueue
.Nd asynchronous task execution
.Sh SYNOPSIS
.In sys/param.h
.In sys/kernel.h
.In sys/malloc.h
.In sys/queue.h
.In sys/taskqueue.h
.Bd -literal
typedef void (*task_fn)(void *context, int pending);

typedef void (*taskqueue_enqueue_fn)(void *context);

struct task {
	STAILQ_ENTRY(task)	ta_link;	/* link for queue */
	int			ta_pending;	/* count times queued */
	int			ta_priority;	/* priority of task in queue */
	task_fn			ta_func;	/* task handler */
	void			*ta_context;	/* argument for handler */
};

.Ed
.Ft struct taskqueue *
.Fn taskqueue_create "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
.Ft void
.Fn taskqueue_free "struct taskqueue *queue"
.Ft struct taskqueue *
.Fn taskqueue_find "const char *name"
.Ft int
.Fn taskqueue_enqueue "struct taskqueue *queue" "struct task *task"
.Ft int
.Fn taskqueue_enqueue_fast "struct taskqueue *queue" "struct task *task"
.Ft void
.Fn taskqueue_run "struct taskqueue *queue"
.Fn TASK_INIT "struct task *task" "int priority" "task_fn_t *func" "void *context"
.Fn TASKQUEUE_DECLARE "name"
.Fn TASKQUEUE_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
.Sh DESCRIPTION
These functions provide a simple interface for asynchronous execution
of code.
.Pp
The function
.Fn taskqueue_create
is used to create new queues.
The arguments to
.Fn taskqueue_create
include a name which should be unique,
a set of
.Xr malloc 9
flags which specify whether the call to
.Fn malloc
is allowed to sleep
and a function which is called from
.Fn taskqueue_enqueue
when a task is added to the queue
.\" XXX	The rest of the sentence gets lots in relation to the first part.
to allow the queue to arrange to be run later
(for instance by scheduling a software interrupt or waking a kernel
thread).
.Pp
The function
.Fn 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 which are on the queue will be executed at this time.
.Pp
The system maintains a list of all queues which can be searched using
.Fn taskqueue_find .
The first queue whose name matches is returned, otherwise
.Dv NULL .
.Pp
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
.Va 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
.Va 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.
.Pp
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.
.Pp
To execute all the tasks on a queue,
call
.Fn taskqueue_run .
When a task is executed,
first it is removed from the queue,
the value of
.Va ta_pending
is recorded and then the field is zeroed.
The function
.Va ta_func
from the task structure is called with the value of the field
.Va ta_context
as its first argument
and the value of
.Va ta_pending
as its second argument.
.Pp
A convenience macro,
.Fn TASK_INIT "task" "priority" "func" "context"
is provided to initialise a
.Va task
structure.
The values of
.Va priority ,
.Va func ,
and
.Va context
are simply copied into the task structure fields and the
.Va ta_pending
field is cleared.
.Pp
Two macros
.Fn TASKQUEUE_DECLARE "name"
and
.Fn TASKQUEUE_DEFINE "name" "enqueue" "context" "init"
are used to declare a reference to a global queue
and to define the implementation of the queue.
The
.Fn TASKQUEUE_DEFINE
macro arranges to call
.Fn taskqueue_create
with the values of its
.Va name ,
.Va enqueue
and
.Va context
arguments during system initialisation.
After calling
.Fn taskqueue_create ,
the
.Va 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.)
.Pp
The system provides three global taskqueues,
.Va taskqueue_swi ,
.Va taskqueue_swi_giant ,
and 
.Va taskqueue_thread .
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 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 (
.Va taskqueue_swi ,
.Va taskqueue_swi_giant ,
or
.Va taskqueue_thread
).
.Pp
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.
.Pp
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.)
.Sh HISTORY
This interface first appeared in
.Fx 5.0 .
There is a similar facility called tqueue in the Linux kernel.
.Sh AUTHORS
This man page was written by
.An Doug Rabson .