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

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Date:   Fri, 31 Mar 2017 08:31:26 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Patrick Bellasi <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

On 30/03/17 22:13, Rafael J. Wysocki wrote:
> On Thu, Mar 30, 2017 at 10:58 AM, Juri Lelli <juri.lelli@....com> wrote:
> > Hi,
> 
> 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?
> 
> Well, do we want SCHED_DEADLINE util contribution to be there even for
> idle CPUs?
> 

DEADLINE util contribution is removed, even if the CPU is idle, by the
reclaiming mechanism when we know (applying GRUB algorithm rules [1])
that it can't be used anymore by a task (roughly speaking). So, we
shouldn't have this problem in the DEADLINE case.

[1] https://marc.info/?l=linux-kernel&m=149029880524038