Understanding the Linux Kernel, 3rd Edition

Functions Used by the Scheduler

The scheduler relies on several functions in order to do its work; the most important are:

scheduler_tick( )

Keeps the time_slice counter of current up-to-date

try_to_wake_up( )

Awakens a sleeping process

recalc_task_prio( )

Updates the dynamic priority of a process

schedule( )

Selects a new process to be executed

load_balance()

Keeps the runqueues of a multiprocessor system balanced

The scheduler_tick( ) Function

We have already explained in the section "Updating Local CPU Statistics" in Chapter 6 how scheduler_tick( ) is invoked once every tick to perform some operations related to scheduling. It executes the following main steps:

1. Stores in the timestamp_last_tick field of the local runqueue the current value of the TSC converted to nanoseconds; this timestamp is obtained from the sched_clock( ) function (see the previous section).

2. Checks whether the current process is the swapper process of the local CPU. If so, it performs the following substeps:

   1. If the local runqueue includes another runnable process besides swapper, it sets the TIF_NEED_RESCHED flag of the current process to force rescheduling. As we’ll see in the section "The schedule( ) Function" later in this chapter, if the kernel supports the hyper-threading technology (see the section "Runqueue Balancing in Multiprocessor Systems" later in this chapter), a logical CPU might be idle even if there are runnable processes in its runqueue, as long as those processes have significantly lower priorities than ...