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Date: Fri, 9 Feb 2018 13:00:53 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Morten Rasmussen <morten.rasmussen@foss.arm.com>,
        Brendan Jackman <brendan.jackman@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed
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On Thu, Feb 08, 2018 at 05:52:00PM +0100, Vincent Guittot wrote:
> On 8 February 2018 at 16:44, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Feb 08, 2018 at 04:05:58PM +0100, Vincent Guittot wrote:
> >> On 8 February 2018 at 15:00, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > On Tue, Feb 06, 2018 at 08:23:05PM +0100, Vincent Guittot wrote:
> >> >
> >> >> @@ -9207,13 +9231,15 @@ void nohz_balance_enter_idle(int cpu)
> >> >>       if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
> >> >>               return;
> >> >>
> >> >> +     rq->has_blocked_load = 1;
> >
> > Should we not set that with rq->lock held? We already clear it while
> > holding rq->lock.
> 
> I think it's safe because it is used to re-enable the periodic decay
> unconditionally.
> It is cleared with  rq->lock held to prevents any update of the cfs_rq
> *_avg  while deciding if we can clear has_blocked_load

Yes, and that clearing must then have observed the new addition. But
please put so in a comment.