Understanding preemption and context switching is key to fully comprehending scheduling and the impact it has on the kernel in maintaining low latency and consistency. Every process must be preempted either implicitly or explicitly to make way for another process. Preemption might lead to context switching, which requires a low-level architecture-specific operation, carried out by the function context_switch(). There are two primary tasks that need to be done for a processor to switch its context: switch the virtual memory mapping of the old process with the new one, and switch the processor state from that of the old process to the new one. These two tasks are carried out by switch_mm() and switch_to().

Preemption can happen for any of the following reasons:

When a high-priority process becomes runnable. For this, the scheduler will have to periodically check for a high-priority runnable thread. On return from interrupts and system calls, TIF_NEED_RESCHEDULE(kernel-provided...