Re: [PATCH] sched: do not stop ticks when cpu is not idle
From: Thomas Gleixner
Date: Fri Jul 18 2008 - 09:52:50 EST
On Fri, 18 Jul 2008, Ingo Molnar wrote:
> * eric miao <eric.y.miao@xxxxxxxxx> wrote:
> > Issue: the sched tick would be stopped in some race conditions.
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -4027,7 +4027,8 @@ need_resched_nonpreemptible:
> > rq->nr_switches++;
> > rq->curr = next;
> > ++*switch_count;
> > -
> > + if (rq->curr != rq->idle)
> > + tick_nohz_restart_sched_tick();
> > context_switch(rq, prev, next); /* unlocks the rq */
> applied to tip/sched/urgent, thanks Eric.
> Thomas, Peter, Dmitry, do you concur with the analysis? (commit below)
Yes. I did not understand the issue when Jack pointed it out to me,
but with Erics explanation it's really clear. Thanks for tracking that
> It looks a bit ugly to me in the middle of schedule() - is there no wait
> to solve this within kernel/time/*.c ?
Hmm, yes. I think the proper fix is to enable the tick stop mechanism
in the idle loop and disable it before we go to schedule. That takes
an additional parameter to tick_nohz_stop_sched_tick(), but we then
gain a very clear section where the nohz mimic can be active.
I'll whip up a patch.
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