Scheduler

For more details about processes, check this page.

The scheduler preempts execution of the currently running process to switch context to another process in its run queue, in order to share CPU-time across processes. Each CPU core has its own scheduler.

When a process transitions into Running state, it is inserted into the run queue of a scheduler. The kernel attempts to balance processes across CPU cores.

Likewise, when a process transitions into another state than Running, it is dequeued (removed from its run queue).

Context switching can be triggered by a timer interrupt or when waiting for a resource to become available (for example). It can also be triggered manually by calling schedule.

If there is no process in a scheduler’s run queue, it shall switch to the idle task, which is a kernel thread with PID 0 that puts the CPU in idle state.

Critical sections

Sometimes, we want to be able to process interrupts, but prevent the scheduler from preempting the process.

In order to achieve this, we have to use critical sections. A critical section is entered by calling preempt_disable, and is exited by calling preempt_enable. To ensure correctness, one should prefer using the critical function.

Note that:

• preempt_enable or critical may preempt the execution context before returning
• calling schedule inside a critical section is invalid (for obvious reasons)

Critical sections can be nested. This is handled with a per-CPU counter.