linux-kernel - Re: [PATCH] sched: do not stop ticks when cpu is not idle

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Date:	Fri, 18 Jul 2008 15:52:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	eric miao <eric.y.miao@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jack Ren <jack.ren@...vell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle

On Fri, 18 Jul 2008, Ingo Molnar wrote:
> 
> * eric miao <eric.y.miao@...il.com> 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
down.

> 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.

Thanks,

	tglx
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