Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()
To: Peter Zijlstra <peterz@infradead.org>
References: <1458606068-7476-1-git-send-email-smuckle@linaro.org>
 <56F91D56.4020007@arm.com> <56F95D10.4070400@linaro.org>
 <56F97856.4040804@arm.com> <56F98832.3030207@linaro.org>
 <20160330193544.GD407@worktop>
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>
From: Steve Muckle <steve.muckle@linaro.org>
Message-ID: <56FC807C.80204@linaro.org>
Date: Wed, 30 Mar 2016 18:42:20 -0700
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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?