linux-kernel - Re: [PATCH 5/8] sched/cpufreq: pass sched class into cpufreq_update

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Date:	Wed, 16 Mar 2016 13:39:10 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Juri Lelli <Juri.Lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Michael Turquette <mturquette+renesas@...libre.com>
Subject: Re: [PATCH 5/8] sched/cpufreq: pass sched class into cpufreq_update_util

On Wed, Mar 16, 2016 at 9:53 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Mar 16, 2016 at 09:29:59AM +0100, Vincent Guittot wrote:
>> I wonder if it's really worth passing per sched_class request to
>> sched_util ? sched_util is about selecting a frequency based on the
>> utilization of the CPU, it only needs a value that reflect the whole
>> utilization. Can't we sum  (or whatever the formula we want to apply)
>> utilizations before calling cpufreq_update_util
>
> So I've thought the same; but I'm conflicted, its a shame to compute
> anything if the call then doesn't do anything with it.
>
> And keeping a structure of all the various numbers to pass in also has
> cost of yet another cacheline to touch.

In principle we can use high-order bits of util and max to encode the
information on where they come from.

Of course, that translates to additional ifs in the governor, but I
guess they are unavoidable anyway.