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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.

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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.

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  • I read when the process is about to move to user mode from kernel mode(i.e; exiting a system call), the kernel may schedule and preempt for another process. What if my application calls the system call within a lock(user space lock/mutex) ? Or else if my application never calls a system call, it just loops in a while(1)? For these above two cases where is the opportunity for kernel to preempt my application either in user/kernel mode?
    – Franc M
    May 11, 2020 at 11:59
  • User space locks won’t affect the kernel; they’ll only affect the processes which know about the lock. Any interrupt also involves a user-kernel-user transition, and therefore a scheduling opportunity; even on a system with no activity, the timer interrupt will fire at regular intervals (except on tickless CPUs, but that’s a very specific setup with its own constraints).
    – Stephen Kitt
    May 11, 2020 at 12:14

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