Understanding the Linux Kernel, 3rd Edition

Work Queues

The work queues have been introduced in Linux 2.6 and replace a similar construct called “task queue” used in Linux 2.4. They allow kernel functions to be activated (much like deferrable functions) and later executed by special kernel threads called worker threads .

Despite their similarities, deferrable functions and work queues are quite different. The main difference is that deferrable functions run in interrupt context while functions in work queues run in process context. Running in process context is the only way to execute functions that can block (for instance, functions that need to access some block of data on disk) because, as already observed in the section "Nested Execution of Exception and Interrupt Handlers" earlier in this chapter, no process switch can take place in interrupt context. Neither deferrable functions nor functions in a work queue can access the User Mode address space of a process. In fact, a deferrable function cannot make any assumption about the process that is currently running when it is executed. On the other hand, a function in a work queue is executed by a kernel thread, so there is no User Mode address space to access.

Work queue data structures

The main data structure associated with a work queue is a descriptor called workqueue_struct, which contains, among other things, an array of NR_CPUS elements, the maximum number of CPUs in the system.^[*] Each element is a descriptor of type cpu_workqueue_struct, whose fields are shown in ...