Messages in this thread | | | Date | Tue, 31 Jul 2012 10:25:04 -0700 | From | Randy Dunlap <> | Subject | Re: [PATCH] sched: Document schedule() entry points |
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On 07/30/2012 11:15 PM, Pekka Enberg wrote:
> 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
eh?
> + * 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) > {
-- ~Randy
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