What context does the scheduler code run in?

Question

There are two cases where the scheduler code schedule() is invoked-

1. When a process voluntarily calls schedule()

2. Timer

Answer 1

__schedule() is the main scheduler function.

The main means of driving the scheduler and thus entering this function are:

1. Explicit blocking: mutex, semaphore, waitqueue, etc.

2. TIF_NEED_RESCHED flag is checked on interrupt and userspace return paths. For example, see arch/x86/entry_64.S. To drive preemption between tasks, the scheduler sets the flag in timer interrupt handler scheduler_tick().

3. Wakeups don't really cause entry into schedule(). They add a task to the run-queue and that's it. Now, if the new task added to the run-queue preempts the current task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets called on the nearest possible occasion:

   • If the kernel is preemptible (CONFIG_PREEMPT=y):
     
     • in syscall or exception context, at the next outmost preempt_enable(). (this might be as soon as the wake_up()'s spin_unlock()!)
     • in IRQ context, return from interrupt-handler to preemptible context
   • If the kernel is not preemptible (CONFIG_PREEMPT is not set) then at the next:
     • cond_resched() call
     • explicit schedule() call
     • return from syscall or exception to user-space
     • return from interrupt-handler to user-space

http://lxr.free-electrons.com/source/kernel/sched/core.c#L2389