Understanding Linux Network Internals

Interrupt Handlers

A good deal of the frame handling we discuss in this chapter takes place in response to interrupts from network hardware. The scheduling of functions triggered by interrupts is a complicated topic and deserves some study, even though it doesn't concern networking in particular. Therefore, in this section, we discuss the various ways that interrupts are handled by different network drivers and introduce the concepts of bottom halves and softirqs.

In Chapter 5, we saw how device drivers register their handlers with an IRQ number, but we did not see how hardware interrupts delegate frame processing to software interrupt handlers. This section will describe how an interrupt request associated with the reception of a frame is handled all the way to the point where protocol handlers discussed in Chapter 13 receive their packets. We will see the relationship between hardware IRQs and software IRQs and why the latter category is needed. We will briefly see how interrupts were handled with the old kernels and then compare the old approach to the new one introduced with kernel version 2.4. This discussion will show the advantages of the new model over the old one, especially in the area of performance.

Before launching into softirqs, we need a small introduction to the concept of bottom half handlers . However, I will not go into much detail about them because they are documented in other resources, notably Understanding the Linux Kernel and Linux Device Drivers.

Reasons ...