linux-kernel - Re: [RFD PATCH 4/5] sched/cpufreq_schedutil: always consider all CPUs when deciding next freq

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Date:   Thu, 30 Mar 2017 09:58:05 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     peterz@...radead.org, mingo@...hat.com, viresh.kumar@...aro.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        tglx@...utronix.de, vincent.guittot@...aro.org,
        rostedt@...dmis.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, tkjos@...roid.com,
        joelaf@...gle.com, andresoportus@...gle.com,
        morten.rasmussen@....com, dietmar.eggemann@....com,
        patrick.bellasi@....com, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFD PATCH 4/5] sched/cpufreq_schedutil: always consider all
 CPUs when deciding next freq

Hi,

On 30/03/17 00:41, Rafael J. Wysocki wrote:
> On Friday, March 24, 2017 02:08:59 PM Juri Lelli wrote:
> > No assumption can be made upon the rate at which frequency updates get
> > triggered, as there are scheduling policies (like SCHED_DEADLINE) which
> > don't trigger them so frequently.
> > 
> > Remove such assumption from the code.
> 
> But the util/max values for idle CPUs may be stale, no?
> 

Right, that might be a problem. A proper solution I think would be to
remotely update such values for idle CPUs, and I believe Vincent is
working on a patch for that.

As mid-term workarounds, changing a bit the current one, come to my
mind:

 - consider TICK_NSEC (continue) only when SCHED_CPUFREQ_DL is not set
 - remove CFS contribution (without triggering a freq update) when a CPU
   enters IDLE; this might not work well, though, as we probably want
   to keep in blocked util contribution for a bit

What you think is the way to go?

Thanks,

- Juri