Workqueue Threads

A workqueue is a kernel object that uses a dedicated thread to process work items in a first in, first out manner. Each work item is processed by calling the function specified by the work item. A workqueue is typically used by an ISR or a high-priority thread to offload non-urgent processing to a lower-priority thread so it does not impact time-sensitive processing.

Any number of workqueues can be defined (limited only by available RAM). Each workqueue is referenced by its memory address.

A workqueue has the following key properties:

A queue of work items that have been added, but not yet processed.
A thread that processes the work items in the queue. The priority of the thread is configurable, allowing it to be either cooperative or preemptive as required.

Regardless of workqueue thread priority the workqueue thread will yield between each submitted work item, to prevent a cooperative workqueue from starving other threads.

A workqueue must be initialized before it can be used. This sets its queue to empty and spawns the workqueue’s thread. The thread runs forever, but sleeps when no work items are available.

Note

The behavior described here is changed from the Zephyr workqueue implementation used prior to release 2.6. Among the changes are:

Precise tracking of the status of cancelled work items, so that the caller need not be concerned that an item may be processing when the cancellation returns. Checking of return values on cancellation is still required.
Direct submission of delayable work items to the queue with K_NO_WAIT rather than always going through the timeout API, which could introduce delays.
The ability to wait until a work item has completed or a queue has been drained.
Finer control of behavior when scheduling a delayable work item, specifically allowing a previous deadline to remain unchanged when a work item is scheduled again.
Safe handling of work item resubmission when the item is being processed on another workqueue.

Using the return values of k_work_busy_get() or k_work_is_pending(), or measurements of remaining time until delayable work is scheduled, should be avoided to prevent race conditions of the type observed with the previous implementation. See also Workqueue Best Practices.

Work Item Lifecycle 

Any number of work items can be defined. Each work item is referenced by its memory address.

A work item is assigned a handler function, which is the function executed by the workqueue’s thread when the work item is processed. This function accepts a single argument, which is the address of the work item itself. The work item also maintains information about its status.

A work item must be initialized before it can be used. This records the work item’s handler function and marks it as not pending.

A work item may be queued (K_WORK_QUEUED) by submitting it to a workqueue by an ISR or a thread. Submitting a work item appends the work item to the workqueue’s queue. Once the workqueue’s thread has processed all of the preceding work items in its queue the thread will remove the next work item from the queue and invoke the work item’s handler function. Depending on the scheduling priority of the workqueue’s thread, and the work required by other items in the queue, a queued work item may be processed quickly or it may remain in the queue for an extended period of time.

A delayable work item may be scheduled (K_WORK_DELAYED) to a workqueue; see Delayable Work.

A work item will be running (K_WORK_RUNNING) when it is running on a work queue, and may also be canceling (K_WORK_CANCELING) if it started running before a thread has requested that it be cancelled.

A work item can be in multiple states; for example it can be:

running on a queue;
marked canceling (because a thread used k_work_cancel_sync() to wait until the work item completed);
queued to run again on the same queue;
scheduled to be submitted to a (possibly different) queue

all simultaneously. A work item that is in any of these states is pending (k_work_is_pending()) or busy (k_work_busy_get()).

A handler function can use any kernel API available to threads. However, operations that are potentially blocking (e.g. taking a semaphore) must be used with care, since the workqueue cannot process subsequent work items in its queue until the handler function finishes executing.

The single argument that is passed to a handler function can be ignored if it is not required. If the handler function requires additional information about the work it is to perform, the work item can be embedded in a larger data structure. The handler function can then use the argument value to compute the address of the enclosing data structure with CONTAINER_OF, and thereby obtain access to the additional information it needs.

A work item is typically initialized once and then submitted to a specific workqueue whenever work needs to be performed. If an ISR or a thread attempts to submit a work item that is already queued the work item is not affected; the work item remains in its current place in the workqueue’s queue, and the work is only performed once.

A handler function is permitted to re-submit its work item argument to the workqueue, since the work item is no longer queued at that time. This allows the handler to execute work in stages, without unduly delaying the processing of other work items in the workqueue’s queue.

Important

A pending work item must not be altered until the item has been processed by the workqueue thread. This means a work item must not be re-initialized while it is busy. Furthermore, any additional information the work item’s handler function needs to perform its work must not be altered until the handler function has finished executing.

Delayable Work 

An ISR or a thread may need to schedule a work item that is to be processed only after a specified period of time, rather than immediately. This can be done by scheduling a delayable work item to be submitted to a workqueue at a future time.

A delayable work item contains a standard work item but adds fields that record when and where the item should be submitted.

A delayable work item is initialized and scheduled to a workqueue in a similar manner to a standard work item, although different kernel APIs are used. When the schedule request is made the kernel initiates a timeout mechanism that is triggered after the specified delay has elapsed. Once the timeout occurs the kernel submits the work item to the specified workqueue, where it remains queued until it is processed in the standard manner.

Note that work handler used for delayable still receives a pointer to the underlying non-delayable work structure, which is not publicly accessible from k_work_delayable. To get access to an object that contains the delayable work object use this idiom:

static void work_handler(struct k_work *work)
{
        struct k_work_delayable *dwork = k_work_delayable_from_work(work);
        struct work_context *ctx = CONTAINER_OF(dwork, struct work_context,
                                                timed_work);
        ...

Triggered Work 

The k_work_poll_submit() interface schedules a triggered work item in response to a poll event (see Polling API), that will call a user-defined function when a monitored resource becomes available or poll signal is raised, or a timeout occurs. In contrast to k_poll(), the triggered work does not require a dedicated thread waiting or actively polling for a poll event.

A triggered work item is a standard work item that has the following added properties:

A pointer to an array of poll events that will trigger work item submissions to the workqueue
A size of the array containing poll events.

A triggered work item is initialized and submitted to a workqueue in a similar manner to a standard work item, although dedicated kernel APIs are used. When a submit request is made, the kernel begins observing kernel objects specified by the poll events. Once at least one of the observed kernel object’s changes state, the work item is submitted to the specified workqueue, where it remains queued until it is processed in the standard manner.

Important

The triggered work item as well as the referenced array of poll events have to be valid and cannot be modified for a complete triggered work item lifecycle, from submission to work item execution or cancellation.

An ISR or a thread may cancel a triggered work item it has submitted as long as it is still waiting for a poll event. In such case, the kernel stops waiting for attached poll events and the specified work is not executed. Otherwise the cancellation cannot be performed.

System Workqueue 

The kernel defines a workqueue known as the system workqueue, which is available to any application or kernel code that requires workqueue support. The system workqueue is optional, and only exists if the application makes use of it.

Important

Additional workqueues should only be defined when it is not possible to submit new work items to the system workqueue, since each new workqueue incurs a significant cost in memory footprint. A new workqueue can be justified if it is not possible for its work items to co-exist with existing system workqueue work items without an unacceptable impact; for example, if the new work items perform blocking operations that would delay other system workqueue processing to an unacceptable degree.

How to Use Workqueues 

Defining and Controlling a Workqueue

A workqueue is defined using a variable of type k_work_q. The workqueue is initialized by defining the stack area used by its thread, initializing the k_work_q, either zeroing its memory or calling k_work_queue_init(), and then calling k_work_queue_start(). The stack area must be defined using K_THREAD_STACK_DEFINE to ensure it is properly set up in memory.

The following code defines and initializes a workqueue:

#define MY_STACK_SIZE 512
#define MY_PRIORITY 5

K_THREAD_STACK_DEFINE(my_stack_area, MY_STACK_SIZE);

struct k_work_q my_work_q;

k_work_queue_init(&my_work_q);

k_work_queue_start(&my_work_q, my_stack_area,
                   K_THREAD_STACK_SIZEOF(my_stack_area), MY_PRIORITY,
                   NULL);

In addition the queue identity and certain behavior related to thread rescheduling can be controlled by the optional final parameter; see k_work_queue_start() for details.

The following API can be used to interact with a workqueue:

k_work_queue_drain() can be used to block the caller until the work queue has no items left. Work items resubmitted from the workqueue thread are accepted while a queue is draining, but work items from any other thread or ISR are rejected. The restriction on submitting more work can be extended past the completion of the drain operation in order to allow the blocking thread to perform additional work while the queue is “plugged”. Note that draining a queue has no effect on scheduling or processing delayable items, but if the queue is plugged and the deadline expires the item will silently fail to be submitted.
k_work_queue_unplug() removes any previous block on submission to the queue due to a previous drain operation.

Submitting a Work Item

A work item is defined using a variable of type k_work. It must be initialized by calling k_work_init(), unless it is defined using K_WORK_DEFINE in which case initialization is performed at compile-time.

An initialized work item can be submitted to the system workqueue by calling k_work_submit(), or to a specified workqueue by calling k_work_submit_to_queue().

The following code demonstrates how an ISR can offload the printing of error messages to the system workqueue. Note that if the ISR attempts to resubmit the work item while it is still queued, the work item is left unchanged and the associated error message will not be printed.

struct device_info {
    struct k_work work;
    char name[16]
} my_device;

void my_isr(void *arg)
{
    ...
    if (error detected) {
        k_work_submit(&my_device.work);
    }
    ...
}

void print_error(struct k_work *item)
{
    struct device_info *the_device =
        CONTAINER_OF(item, struct device_info, work);
    printk("Got error on device %s\n", the_device->name);
}

/* initialize name info for a device */
strcpy(my_device.name, "FOO_dev");

/* initialize work item for printing device's error messages */
k_work_init(&my_device.work, print_error);

/* install my_isr() as interrupt handler for the device (not shown) */
...

The following API can be used to check the status of or synchronize with the work item:

k_work_busy_get() returns a snapshot of flags indicating work item state. A zero value indicates the work is not scheduled, submitted, being executed, or otherwise still being referenced by the workqueue infrastructure.
k_work_is_pending() is a helper that indicates true if and only if the work is scheduled, queued, or running.
k_work_flush() may be invoked from threads to block until the work item has completed. It returns immediately if the work is not pending.
k_work_cancel() attempts to prevent the work item from being executed. This may or may not be successful. This is safe to invoke from ISRs.
k_work_cancel_sync() may be invoked from threads to block until the work completes; it will return immediately if the cancellation was successful or not necessary (the work wasn’t submitted or running). This can be used after k_work_cancel() is invoked (from an ISR)` to confirm completion of an ISR-initiated cancellation.

Scheduling a Delayable Work Item

A delayable work item is defined using a variable of type k_work_delayable. It must be initialized by calling k_work_init_delayable().

For delayed work there are two common use cases, depending on whether a deadline should be extended if a new event occurs. An example is collecting data that comes in asynchronously, e.g. characters from a UART associated with a keyboard. There are two APIs that submit work after a delay:

k_work_schedule() (or k_work_schedule_for_queue()) schedules work to be executed at a specific time or after a delay. Further attempts to schedule the same item with this API before the delay completes will not change the time at which the item will be submitted to its queue. Use this if the policy is to keep collecting data until a specified delay since the first unprocessed data was received;
k_work_reschedule() (or k_work_reschedule_for_queue()) unconditionally sets the deadline for the work, replacing any previous incomplete delay and changing the destination queue if necessary. Use this if the policy is to keep collecting data until a specified delay since the last unprocessed data was received.

If the work item is not scheduled both APIs behave the same. If K_NO_WAIT is specified as the delay the behavior is as if the item was immediately submitted directly to the target queue, without waiting for a minimal timeout (unless k_work_schedule() is used and a previous delay has not completed).

Both also have variants that allow control of the queue used for submission.

The helper function k_work_delayable_from_work() can be used to get a pointer to the containing k_work_delayable from a pointer to k_work that is passed to a work handler function.

The following additional API can be used to check the status of or synchronize with the work item:

k_work_delayable_busy_get() is the analog to k_work_busy_get() for delayable work.
k_work_delayable_is_pending() is the analog to k_work_is_pending() for delayable work.
k_work_flush_delayable() is the analog to k_work_flush() for delayable work.
k_work_cancel_delayable() is the analog to k_work_cancel() for delayable work; similarly with k_work_cancel_delayable_sync().

Synchronizing with Work Items

While the state of both regular and delayable work items can be determined from any context using k_work_busy_get() and k_work_delayable_busy_get() some use cases require synchronizing with work items after they’ve been submitted. k_work_flush(), k_work_cancel_sync(), and k_work_cancel_delayable_sync() can be invoked from thread context to wait until the requested state has been reached.

These APIs must be provided with a k_work_sync object that has no application-inspectable components but is needed to provide the synchronization objects. These objects should not be allocated on a stack if the code is expected to work on architectures with CONFIG_KERNEL_COHERENCE.

Workqueue Best Practices 

Avoid Race Conditions

Sometimes the data a work item must process is naturally thread-safe, for example when it’s put into a k_queue by some thread and processed in the work thread. More often external synchronization is required to avoid data races: cases where the work thread might inspect or manipulate shared state that’s being accessed by another thread or interrupt. Such state might be a flag indicating that work needs to be done, or a shared object that is filled by an ISR or thread and read by the work handler.

For simple flags Atomic Services may be sufficient. In other cases spin locks (k_spinlock) or thread-aware locks (k_sem, k_mutex , …) may be used to ensure data races don’t occur.

If the selected lock mechanism can sleep then allowing the work thread to sleep will starve other work queue items, which may need to make progress in order to get the lock released. Work handlers should try to take the lock with its no-wait path. For example:

static void work_handler(struct work *work)
{
        struct work_context *parent = CONTAINER_OF(work, struct work_context,
                                                   work_item);

        if (k_mutex_lock(&parent->lock, K_NO_WAIT) != 0) {
                /* NB: Submit will fail if the work item is being cancelled. */
                (void)k_work_submit(work);
                return;
        }

        /* do stuff under lock */
        k_mutex_unlock(&parent->lock);
        /* do stuff without lock */
}

Be aware that if the lock is held by a thread with a lower priority than the work queue the resubmission may starve the thread that would release the lock, causing the application to fail. Where the idiom above is required a delayable work item is preferred, and the work should be (re-)scheduled with a non-zero delay to allow the thread holding the lock to make progress.

Note that submitting from the work handler can fail if the work item had been cancelled. Generally this is acceptable, since the cancellation will complete once the handler finishes. If it is not, the code above must take other steps to notify the application that the work could not be performed.

Work items in isolation are self-locking, so you don’t need to hold an external lock just to submit or schedule them. Even if you use external state protected by such a lock to prevent further resubmission, it’s safe to do the resubmit as long as you’re sure that eventually the item will take its lock and check that state to determine whether it should do anything. Where a delayable work item is being rescheduled in its handler due to inability to take the lock some other self-locking state, such as an atomic flag set by the application/driver when the cancel is initiated, would be required to detect the cancellation and avoid the cancelled work item being submitted again after the deadline.

Check Return Values

All work API functions return status of the underlying operation, and in many cases it is important to verify that the intended result was obtained.

Submitting a work item (k_work_submit_to_queue()) can fail if the work is being cancelled or the queue is not accepting new items. If this happens the work will not be executed, which could cause a subsystem that is animated by work handler activity to become non-responsive.
Asynchronous cancellation (k_work_cancel() or k_work_cancel_delayable()) can complete while the work item is still being run by a handler. Proceeding to manipulate state shared with the work handler will result in data races that can cause failures.

Many race conditions have been present in Zephyr code because the results of an operation were not checked.

There may be good reason to believe that a return value indicating that the operation did not complete as expected is not a problem. In those cases the code should clearly document this, by (1) casting the return value to void to indicate that the result is intentionally ignored, and (2) documenting what happens in the unexpected case. For example:

/* If this fails, the work handler will check pub->active and
 * exit without transmitting.
 */
(void)k_work_cancel_delayable(&pub->timer);

However in such a case the following code must still avoid data races, as it cannot guarantee that the work thread is not accessing work-related state.

Don’t Optimize Prematurely

The workqueue API is designed to be safe when invoked from multiple threads and interrupts. Attempts to externally inspect a work item’s state and make decisions based on the result are likely to create new problems.

So when new work comes in, just submit it. Don’t attempt to “optimize” by checking whether the work item is already submitted by inspecting snapshot state with k_work_is_pending() or k_work_busy_get(), or checking for a non-zero delay from k_work_delayable_remaining_get(). Those checks are fragile: a “busy” indication can be obsolete by the time the test is returned, and a “not-busy” indication can also be wrong if work is submitted from multiple contexts, or (for delayable work) if the deadline has completed but the work is still in queued or running state.

A general best practice is to always maintain in shared state some condition that can be checked by the handler to confirm whether there is work to be done. This way you can use the work handler as the standard cleanup path: rather than having to deal with cancellation and cleanup at points where items are submitted, you may be able to have everything done in the work handler itself.

A rare case where you could safely use k_work_is_pending() is as a check to avoid invoking k_work_flush() or k_work_cancel_sync(), if you are certain that nothing else might submit the work while you’re checking (generally because you’re holding a lock that prevents access to state used for submission).

Suggested Uses 

Use the system workqueue to defer complex interrupt-related processing from an ISR to a shared thread. This allows the interrupt-related processing to be done promptly without compromising the system’s ability to respond to subsequent interrupts, and does not require the application to define and manage an additional thread to do the processing.

Configuration Options 

Related configuration options:

API Reference 

Work Queue APIs