Introduction to Task Queues

A common operation in kernel programming is to schedule a task to be performed at a later time, by a different thread. The following examples give some reasons that you might want a different thread to perform a task at a later time:

• Your current code path is time critical. The additional task you want to perform is not time critical.

• The additional task might require grabbing a lock that another thread is currently holding.

• You cannot block in your current context. The additional task might need to block, for example to wait for memory.

• A condition is preventing your code path from completing, but your current code path cannot sleep or fail. You need to queue the current task to execute after the condition disappears.

• You need to launch multiple tasks in parallel.

In each of these cases, a task is executed in a different context. A different context is usually a different kernel thread with a different set of locks held and possibly a different priority. Task queues provide a generic kernel API for scheduling asynchronous tasks.

A task queue is a list of tasks 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 in which they are added to the list. If a task queue has more than one service thread, the order in which the tasks will execute is not known.

If the task queue has more than one service thread, make sure that the execution of one task does not depend on the execution of any other task. Dependencies between tasks can cause a deadlock to occur.