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Do priority of the processes constitute to preemption when they are in Kernel mode?.
Say there are two processes with priorities 3 and 5. It is quiet obvious that the priority 5 process can be preempted by priority 3 process in user mode. Can that happen when they both are in kernel mode?
What are the other chances of a kernel preemption other than the waiting for IO, syscall takes long time, occurrence of an interrupt to the same CPU etc.
All scheduling decisions happen in kernel mode: when a process is running in user mode, its next scheduling point won’t happen before the kernel takes control again.
Process priorities come into play in a variety of ways when scheduling, but there is one explicit path where higher priority processes can influence scheduling. When a task becomes runnable, the check_preempt_curr hook is called, and if the newly-runnable task has a higher priority than the currently-running task, the hook requests a reschedule. This doesn’t immediately cause a reschedule, but it marks that the next time a reschedule is possible, it should be considered. This will typically happen on exit to userspace (see for example exit_to_usermode_loop on x86), so not long after. The reschedule might also not reschedule the newly-runnable task, if another task is more “deserving” of running when the scheduler runs.
Preemption can happen whenever a preemption point is reached, or on low-latency kernels, whenever the kernel is running code that’s not in a critical section.
See the various preemption configuration options, the __schedule function’s documentation, and the completely fair scheduler’s documentation for details.
