linux-kernel - Re: [PATCH] sched: fix clear NOHZ_BALANCE

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Date:	Tue, 4 Jun 2013 13:19:02 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

On Tue, Jun 04, 2013 at 01:11:47PM +0200, Vincent Guittot wrote:
> On 4 June 2013 12:26, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > On Tue, Jun 04, 2013 at 11:36:11AM +0200, Peter Zijlstra wrote:
> >>
> >> The best I can seem to come up with is something like the below; but I think
> >> its ghastly. Surely we can do something saner with that bit.
> >>
> >> Having to clear it at 3 different places is just wrong.
> >
> > We could clear the flag early in scheduler_ipi() and set some
> > specific value in rq->idle_balance that tells we want nohz idle
> > balancing from the softirq, something like this untested:
> 
> I'm not sure that we can have less than 2 places to clear it: cancel
> place or acknowledge place otherwise we can face a situation where
> idle load balance will be triggered 2 consecutive times because
> NOHZ_BALANCE_KICK will be cleared before the idle load balance has
> been done and had a chance to migrate tasks.

I guess it depends what is the minimum value of rq->next_balance, it seems
to be large enough to avoid this kind of incident. Although I don't
know well the whole logic with rq->next_balance and ilb trigger so I must
defer to you.
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