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    Subject[PATCH] sched: Document schedule() entry points
    Date
    This patch adds a comment on top of the schedule() function to explain
    to scheduler newbies how the main scheduler function is entered.

    Explained-by: Ingo Molnar <mingo@kernel.org>
    Explained-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Pekka Enberg <penberg@kernel.org>
    ---
    kernel/sched/core.c | 34 ++++++++++++++++++++++++++++++++++
    1 files changed, 34 insertions(+), 0 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 468bdd4..9f31bbd 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -3361,6 +3361,40 @@ pick_next_task(struct rq *rq)

    /*
    * __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 is set
    + * 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)
    {
    --
    1.7.7.6


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