Understanding the Linux Kernel, Second Edition

Softirqs, Tasklets, and Bottom Halves

We mentioned earlier in Section 4.6 that several tasks among those executed by the kernel are not critical: they can be deferred for a long period of time, if necessary. Remember that the interrupt service routines of an interrupt handler are serialized, and often there should be no occurrence of an interrupt until the corresponding interrupt handler has terminated. Conversely, the deferrable tasks can execute with all interrupts enabled. Taking them out of the interrupt handler helps keep kernel response time small. This is a very important property for many time-critical applications that expect their interrupt requests to be serviced in a few milliseconds.

Linux 2.4 answers such a challenge by using three kinds of deferrable and interruptible kernel functions (in short, deferrable functions ^[35]): softirqs , tasklets , and bottom halves . Although these three kinds of deferrable functions work in different ways, they are strictly correlated. Tasklets are implemented on top of softirqs, and bottom halves are implemented by means of tasklets. As a matter of fact, the term “softirq,” which appears in the kernel source code, often denotes all kinds of deferrable functions.

As a general rule, no softirq can be interrupted to run another softirq on the same CPU; the same rule holds for tasklets and bottom halves built on top of softirqs. On a multiprocessor system, however, several deferrable functions can run concurrently on different CPUs. The ...