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. 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.

A workqueue must be initialized before it can be used. This sets its queue to empty and spawns the workqueue’s thread.

Work Item Lifecycle

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

A work item has the following key properties:

  • 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.

  • A pending flag, which is used by the kernel to signify that the work item is currently a member of a workqueue’s queue.

  • A queue link, which is used by the kernel to link a pending work item to the next pending work item in a workqueue’s queue.

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 submitted 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 a pending work item from its 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 pending work item may be processed quickly or it may remain in the queue for an extended period of time.

A handler function can utilize 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, 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 pending, 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 pending 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 pending. 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.

There is no kernel API that can be used to determine that the handler function has finished executing. Infrastructure that uses work items and needs to know the work item status must manage state in the handler function.

Delayed 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 submitting a delayed work item to a workqueue, rather than a standard work item.

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

  • A delay specifying the time interval to wait before the work item is actually submitted to a workqueue’s queue.

  • A workqueue indicator that identifies the workqueue the work item is to be submitted to.

A delayed work item is initialized and submitted to a workqueue in a similar manner to a standard work item, although different kernel APIs are used. When the submit 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 delayed work item to the specified workqueue, where it remains pending until it is processed in the standard manner.

An ISR or a thread may attempt to cancel a delayed work item. If successful the specified work is not performed. However, attempting to cancel a delayed work item succeeds in only two cases:

  • its timeout has not yet expired and been processed; or

  • it is still pending and the cancellation successfully removes it from the workqueue before the workqueue’s thread gets to it.

Because of the locking used to manage workqueues there are transient states that are sometimes not observable, but if observed will cause the cancellation will fail. In those cases the work item may or may not be invoked. The transient states can be observed and cause failure when:

  • the workqueue or application threads are preemptible;

  • the API is invoked from an ISR; or

  • when the code is run on a multiprocessor system.

Note that both k_delayed_work_submit_to_queue() and k_delayed_work_cancel() attempt to cancel a previously submitted item and can fail. When they fail the work handler of the previous submission may or may not be invoked.

Warning

Because of these race conditions all code that invokes the delayed work API must check return values and be prepared to react when either submission or cancellation fails.

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 pending 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.

Implementation

Defining 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 and then calling k_work_q_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_q_start(&my_work_q, my_stack_area,
               K_THREAD_STACK_SIZEOF(my_stack_area), MY_PRIORITY);

Submitting a Work Item

A work item is defined using a variable of type k_work. It must then be initialized by calling k_work_init().

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 pending, 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) */
...

Submitting a Delayed Work Item

A delayed work item is defined using a variable of type k_delayed_work. It must then be initialized by calling k_delayed_work_init().

An initialized delayed work item can be submitted to the system workqueue by calling k_delayed_work_submit(), or to a specified workqueue by calling k_delayed_work_submit_to_queue(). A delayed work item that has been submitted but not yet consumed by its workqueue can be canceled by calling k_delayed_work_cancel().

Warning

All of these operations can fail as described in Delayed Work.

Suggested Uses

Use the system workqueue to defer complex interrupt-related processing from an ISR to a cooperative 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 an additional thread to do the processing.