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

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Date:	Thu, 31 Mar 2016 09:37:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
	Vincent Guittot <vincent.guittot@...aro.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 Wed, Mar 30, 2016 at 06:42:20PM -0700, Steve Muckle wrote:
> On 03/30/2016 12:35 PM, Peter Zijlstra 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.
> 
> What about only IPI'ing the destination when the utilization change is
> known to require a higher CPU frequency?

Can't, the way the wakeup path is constructed we would be sending the
IPI way before we know about utilization.