Timer interrupts are essential for time-sharing the CPU among runnable processes (that is, those in the TASK_RUNNING state). As we shall see in Chapter 11, each process is usually allowed a quantum of time of limited duration: if the process is not terminated when its quantum expires, the schedule( ) function selects the new process to run.

The counter field of the process descriptor specifies how many ticks of CPU time are left to the process. The quantum is always a multiple of a tick — a multiple of about 10 ms. The value of counter is updated at every tick by update_process_times( ), which is invoked by either the PIT’s timer interrupt handler on uniprocessor systems or the local timer interrupt handler in multiprocessor systems. The code is equivalent to the following:

if (current->pid) {
    --current->counter;
    if (current->counter <= 0) {
        current->counter = 0;
        current->need_resched = 1;
    }
}

The snippet of code starts by making sure the kernel is not handling a process with PID 0 — the swapper process associated with the executing CPU. It must not be time-shared because it is the process that runs on the CPU when no other TASK_RUNNING processes exist (see Section 3.2.2).

When counter becomes smaller than 0, the need_resched field of the process descriptor is set to 1. In this case, the schedule( ) function is invoked before resuming User Mode execution, and other TASK_RUNNING processes will have a chance to resume execution on the CPU.