Chapter 9. Interrupts and Network Drivers

The previous chapters gave an overview of how the initialization of core components in the networking code is taken care of. The remainder of the book offers a feature-by-feature or subsystem-by-subsystem analysis of how networking is implemented, why features were introduced, and, when meaningful, how they interact with each other.

This chapter begins an explanation of how packets travel between the L2 or driver layer and the IP or network layer described in detail in Part V. I'll be referring a lot to the data structures introduced in Chapters 2 and 8, so you should be ready to turn back to those chapters as needed.

Even before the kernel is ready to handle the frame that is coming from or going to the L2 layer, it must deal with the subtle and complex system of interrupts set up to make the handling of thousands of frames per second possible. That is the subject of this chapter.

A couple of other general issues affect the discussion in this chapter:

• When the Linux kernel is compiled with support for symmetric multiprocessing (SMP) and runs on a multiprocessor system, the code for receiving and transmitting packets takes full advantage of that power. The data structures involved are designed with that goal in mind. In this chapter, we will look at one aspect of SMP support in particular: the differences between the new softirq queues and the old backlog queue.

• When talking about the ingress path, I will cover both the old interface, which is still used by most network drivers, and the new interface, called NAPI, which can significantly increase performance under medium to high loads.

In this chapter, you will be given an overview on both bottom half handlers and kernel synchronization mechanisms. However, for a more detailed discussion, you can refer to the other two O'Reilly books, Understanding the Linux Kernel and Linux Device Drivers.