Understanding the Linux Kernel

3.2. Process Switching

In order to control the execution of processes, the kernel must be able to suspend the execution of the process running on the CPU and resume the execution of some other process previously suspended. This activity is called process switching , task switching, or context switching. The following sections describe the elements of process switching in Linux:

• Hardware context

• Hardware support

• Linux code

• Saving the floating point registers

3.2.1. Hardware Context

While each process can have its own address space, all processes have to share the CPU registers. So before resuming the execution of a process, the kernel must ensure that each such register is loaded with the value it had when the process was suspended.

The set of data that must be loaded into the registers before the process resumes its execution on the CPU is called the hardware context. The hardware context is a subset of the process execution context, which includes all information needed for the process execution. In Linux, part of the hardware context of a process is stored in the TSS segment, while the remaining part is saved in the Kernel Mode stack. As we learned in Section 2.3 in Chapter 2, the TSS segment coincides with the tss field of the process descriptor.

We will assume the prev local variable refers to the process descriptor of the process being switched out and next refers to the one being switched in to replace it. We can thus define process switching as the activity consisting ...