Date: Thu, 31 Mar 2016 09:37:43 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steve Muckle <steve.muckle@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, "Rafael J. Wysocki" <rafael@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()
Message-ID: <20160331073743.GF3408@twins.programming.kicks-ass.net>
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 <56F95D10.4070400@linaro.org>
 <56F97856.4040804@arm.com>
 <56F98832.3030207@linaro.org>
 <20160330193544.GD407@worktop>
 <56FC807C.80204@linaro.org>
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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.