NAME
taskqueue
—
asynchronous task execution
SYNOPSIS
#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 { TASKQUEUE_CALLBACK_TYPE_INIT, TASKQUEUE_CALLBACK_TYPE_SHUTDOWN, }; 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);
int
taskqueue_start_threads
(struct
taskqueue **tqp, int
count, int pri,
const char *name,
...);
int
taskqueue_start_threads_cpuset
(struct
taskqueue **tqp, int count, int
pri, cpuset_t *mask, const char
*name, ...);
int
taskqueue_start_threads_in_proc
(struct
taskqueue **tqp, int count, int
pri, struct proc *proc, const
char *name, ...);
void
taskqueue_set_callback
(struct
taskqueue *queue, enum
taskqueue_callback_type cb_type,
taskqueue_callback_fn
callback, void
*context);
void
taskqueue_free
(struct
taskqueue *queue);
int
taskqueue_enqueue
(struct
taskqueue *queue, struct
task *task);
int
taskqueue_enqueue_timeout
(struct
taskqueue *queue, struct
timeout_task *timeout_task,
int ticks);
int
taskqueue_enqueue_timeout_sbt
(struct
taskqueue *queue, struct
timeout_task *timeout_task,
sbintime_t sbt,
sbintime_t pr,
int flags);
int
taskqueue_cancel
(struct
taskqueue *queue, struct
task *task, u_int
*pendp);
int
taskqueue_cancel_timeout
(struct
taskqueue *queue, struct
timeout_task *timeout_task,
u_int *pendp);
void
taskqueue_drain
(struct
taskqueue *queue, struct
task *task);
void
taskqueue_drain_timeout
(struct
taskqueue *queue, struct
timeout_task *timeout_task);
void
taskqueue_drain_all
(struct
taskqueue *queue);
void
taskqueue_quiesce
(struct
taskqueue *queue);
void
taskqueue_block
(struct
taskqueue *queue);
void
taskqueue_unblock
(struct
taskqueue *queue);
int
taskqueue_member
(struct
taskqueue *queue, struct
thread *td);
void
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_DECLARE
(name);
TASKQUEUE_DEFINE
(name,
taskqueue_enqueue_fn
enqueue, void
*context,
init);
TASKQUEUE_FAST_DEFINE
(name,
taskqueue_enqueue_fn
enqueue, void
*context,
init);
TASKQUEUE_DEFINE_THREAD
(name);
TASKQUEUE_FAST_DEFINE_THREAD
(name);
TIMEOUT_TASK_INIT
(struct
taskqueue *queue, struct
timeout_task *timeout_task,
int priority,
task_fn_t func,
void *context);
DESCRIPTION
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. If the queue is intended
for use in fast interrupt handlers
taskqueue_create_fast
() should be used in place of
taskqueue_create
().
The function
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
taskqueue_start_threads
(),
taskqueue_start_threads_cpuset
()
or
taskqueue_start_threads_in_proc
().
taskqueue_start_threads_cpuset
() takes a
cpuset argument which will cause the threads which are
started for the taskqueue to be restricted to run on the given CPUs.
taskqueue_start_threads_in_proc
() takes a
proc argument which will cause the threads which are
started for the taskqueue to be assigned to the given kernel process.
Callbacks may optionally be registered using
taskqueue_set_callback
().
Currently, callbacks may be registered for the following purposes:
TASKQUEUE_CALLBACK_TYPE_INIT
- 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.
TASKQUEUE_CALLBACK_TYPE_SHUTDOWN
- 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
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 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
taskqueue_enqueue
().
The
taskqueue_enqueue_timeout
()
function is used to schedule the enqueue after the specified number of
ticks. The
taskqueue_enqueue_timeout_sbt
()
function provides finer control over the scheduling based on
sbt, pr, and
flags, as detailed in
callout(9). 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. This function returns -1 if the task
is being drained. Otherwise, the number of pending calls is returned.
The
taskqueue_cancel
()
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
taskqueue_drain
(),
the caller is responsible for ensuring that the task is not re-enqueued
after being canceled.
Similarly, the
taskqueue_cancel_timeout
()
function is used to cancel the scheduled task execution.
The
taskqueue_drain
()
function is used to wait for the task to finish, and the
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
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
taskqueue_drain_all
()
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
taskqueue_quiesce
()
function is used to wait for the queue to become empty and for all running
tasks to finish. To avoid blocking indefinitely, the caller must ensure by
some mechanism that tasks will eventually stop being posted to the
queue.
The
taskqueue_block
()
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
taskqueue_unblock
()
function unblocks the previously blocked taskqueue. All enqueued tasks can
be run after this call.
The
taskqueue_member
()
function returns 1 if the given thread
td is part of the given taskqueue
queue and 0 otherwise.
The
taskqueue_run
()
function will run all pending tasks in the specified
queue. Normally this function is only used
internally.
A convenience macro,
TASK_INIT
(task,
priority, func,
context) is provided to initialise a
task structure. The
TASK_INITIALIZER
()
macro generates an initializer for a task structure. A macro
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
TASKQUEUE_DECLARE
(name),
TASKQUEUE_DEFINE
(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
TASKQUEUE_DEFINE_THREAD
()
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.
TASKQUEUE_FAST_DEFINE
()
and
TASKQUEUE_FAST_DEFINE_THREAD
()
act just like TASKQUEUE_DEFINE
() and
TASKQUEUE_DEFINE_THREAD
() respectively but taskqueue
is created with
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
taskqueue_enqueue
()
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.
SEE ALSO
HISTORY
This interface first appeared in FreeBSD 5.0. There is a similar facility called work_queue in the Linux kernel.
AUTHORS
This manual page was written by Doug Rabson.