linux-kernel - Re: [RFC] sched: fair: Don't update CPU frequency too frequently

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Message-ID: <20170601122224.c324h4t7y3i4wr6e@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Jun 2017 14:22:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Rafael Wysocki <rjw@...ysocki.net>,
        linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-pm@...r.kernel.org, Juri Lelli <Juri.Lelli@....com>,
        Dietmar.Eggemann@....com, Morten.Rasmussen@....com
Subject: Re: [RFC] sched: fair: Don't update CPU frequency too frequently

On Thu, Jun 01, 2017 at 05:04:27PM +0530, Viresh Kumar wrote:
> This patch relocates the call to utilization hook from
> update_cfs_rq_load_avg() to task_tick_fair().

That's not right. Consider hardware where 'setting' the DVFS is a
'cheap' MSR write, doing that once every 10ms (HZ=100) is absurd.

We spoke about this problem in Pisa, the proposed solution was having
each driver provide a cost metric and the generic code doing a max
filter over the window constructed from that cost metric.