Commit-ID: edde96eafc91a510f404e7b82cfc0ecb608505ee Gitweb: http://git.kernel.org/tip/edde96eafc91a510f404e7b82cfc0ecb608505ee Author: Pekka Enberg <penberg@xxxxxxxxxx> AuthorDate: Sat, 4 Aug 2012 11:49:47 +0300 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitDate: Mon, 13 Aug 2012 18:58:15 +0200 sched: Document schedule() entry points This patch adds a comment on top of the schedule() function to explain to scheduler newbies how the main scheduler function is entered. Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxx> Explained-by: Ingo Molnar <mingo@xxxxxxxxxx> Explained-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Signed-off-by: Pekka Enberg <penberg@xxxxxxxxxx> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Link: http://lkml.kernel.org/r/1344070187-2420-1-git-send-email-penberg@xxxxxxxxxx Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> --- 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 fbf1fd0..c9a3655 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3367,6 +3367,40 @@ 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 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) { -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html