sched: Document schedule() entry points

/*

 * __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

 */

static void __sched __schedule(void)

{