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 11:03:36 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Juri Lelli <juri.lelli@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "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 Fri, Mar 31, 2017 at 9:31 AM, Juri Lelli <juri.lelli@....com> wrote:
> 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

OK

Why don't you store the contributions from DL and CFS separately, then
(say, as util_dl, util_cfs, respectively) and only discard the CFS one
if delta_ns > TICK_NSEC?