man/man9f/taskq.9f

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 TASKQ 9F "September 12, 2020"
 NAME
taskq, ddi_taskq_create, ddi_taskq_destroy, ddi_taskq_dispatch, ddi_taskq_wait,
ddi_taskq_suspend, taskq_suspended, ddi_taskq_resume - Kernel task queue
operations
 SYNOPSIS
#include <sys/sunddi.h>

ddi_taskq_t *ddi_taskq_create(dev_info_t *dip, const char *name,
 int nthreads, pri_t pri, uint_t cflags);


void ddi_taskq_destroy(ddi_taskq_t *tq);


int ddi_taskq_dispatch(ddi_taskq_t *tq, void (* func)(void *),
 void *arg, uint_t dflags);


void ddi_taskq_wait(ddi_taskq_t *tq);


void ddi_taskq_suspend(ddi_taskq_t *tq);


boolean_t ddi_taskq_suspended(ddi_taskq_t *tq);


void ddi_taskq_resume(ddi_taskq_t *tq);


 INTERFACE LEVEL
illumos DDI specific (illumos DDI)
 PARAMETERS
dip

Pointer to the device's dev_info structure. May be NULL for kernel
modules that do not have an associated dev_info structure.


name

Descriptive string. Only alphanumeric characters can be used in name
and spaces are not allowed. The name should be unique.


nthreads

Number of threads servicing the task queue. Note that the request ordering is
guaranteed (tasks are processed in the order scheduled) if the taskq is
created with a single servicing thread.


pri

Priority of threads servicing the task queue. Drivers and modules should
specify TASKQ_DEFAULTPRI.


cflags

Should pass 0 as flags.


func

Callback function to call.


arg

Argument to the callback function.


dflags

Possible dflags are:
DDI_SLEEP

Allow sleeping (blocking) until memory is available.


DDI_NOSLEEP

Return DDI_FAILURE immediately if memory is not available.


tq

Pointer to a task queue (ddi_taskq_t *).


tp

Pointer to a thread structure.


 DESCRIPTION
A kernel task queue is a mechanism for general-purpose asynchronous task
scheduling that enables tasks to be performed at a later time by another
thread. There are several reasons why you may utilize asynchronous task
scheduling:


1.
You have a task that isn't time-critical, but a current code path that is.


2.
You have a task that may require grabbing locks that a thread already holds.


3.
You have a task that needs to block (for example, to wait for memory), but you
have a thread that cannot block in its current context.


4.
You have a code path that can't complete because of a specific condition,
but also can't sleep or fail. In this case, the task is immediately queued and
then is executed after the condition disappears.


5.
A task queue is just a simple way to launch multiple tasks in parallel.


A task queue consists of a list of tasks, together with one or more threads to
service the list. If a task queue has a single service thread, all tasks are
guaranteed to execute in the order they were dispatched. Otherwise they can be
executed in any order. Note that since tasks are placed on a list, execution of
one task should not depend on the execution of another task or a deadlock
may occur.

The ddi_taskq_create() function creates a task queue instance.

The ddi_taskq_dispatch() function places taskq on the list for
later execution. The dflag argument specifies whether it is allowed sleep
waiting for memory. DDI_SLEEP dispatches can sleep and are guaranteed to
succeed. DDI_NOSLEEP dispatches are guaranteed not to sleep but may fail
(return DDI_FAILURE) if resources are not available.

The ddi_taskq_destroy() function waits for any scheduled tasks to
complete, then destroys the taskq. The caller should guarantee that no
new tasks are scheduled for the closing taskq.

The ddi_taskq_wait() function waits for all previously scheduled tasks to
complete. Note that this function does not stop any new task dispatches.

The ddi_taskq_suspend() function suspends all task execution until
ddi_taskq_resume() is called. Although ddi_taskq_suspend() attempts
to suspend pending tasks, there are no guarantees that they will be suspended.
The only guarantee is that all tasks dispatched after ddi_taskq_suspend()
will not be executed. Because it will trigger a deadlock, the
ddi_taskq_suspend() function should never be called by a task executing
on a taskq.

The ddi_taskq_suspended() function returns B_TRUE if taskq is
suspended, and B_FALSE otherwise. It is intended to ASSERT that the task
queue is suspended.

The ddi_taskq_resume() function resumes task queue execution.
 RETURN VALUES
The ddi_taskq_create() function creates an opaque handle that is used for
all other taskq operations. It returns a taskq pointer on success
and NULL on failure.

The ddi_taskq_dispatch() function returns DDI_FAILURE if it can't
dispatch a task and returns DDI_SUCCESS if dispatch succeeded.

The ddi_taskq_suspended() function returns B_TRUE if taskq is
suspended. Otherwise B_FALSE is returned.
 CONTEXT
All functions may be called from the user or kernel contexts.

Additionally, the ddi_taskq_dispatch function may be called from the
interrupt context only if the DDI_NOSLEEP flag is set.