linux-kernel - Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load

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Message-ID: <20160331093408.GB12845@twins.programming.kicks-ass.net>
Date:	Thu, 31 Mar 2016 11:34:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()

On Thu, Mar 31, 2016 at 11:27:22AM +0200, Vincent Guittot wrote:
> On 30 March 2016 at 21:35, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Mar 28, 2016 at 12:38:26PM -0700, Steve Muckle wrote:
> >> Without covering all the paths where CFS utilization changes it's
> >> possible to have to wait up to a tick to act on some changes, since the
> >> tick is the only guaranteed regularly-occurring instance of the hook.
> >> That's an unacceptable amount of latency IMO...
> >
> > Note that even with your patches that might still be the case. Remote
> > wakeups might not happen on the destination CPU at all, so it might not
> > be until the next tick (which always happens locally) that we'll
> > 'observe' the utilization change brought with the wakeups.
> >
> > We could force all the remote wakeups to IPI the destination CPU, but
> > that comes at a significant performance cost.
> 
> Isn't a reschedule ipi already sent in this case ?

In what case? Assuming you talk about a remove wakeup, no. Only if that
wakeup results in a preemption, which isn't a given.

And we really don't want to carry the 'has util increased' information
all the way down to where we make that decision.