Re: [PATCH] sched: do not stop ticks when cpu is not idle
From: eric miao
Date: Fri Jul 18 2008 - 10:39:13 EST
On Fri, Jul 18, 2008 at 9:52 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 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
Actually, Jack did most of the analysis and came up with this quick
>> 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.
Sounds great, thanks.
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