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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 7 Jul 2021 15:53:48 +0200
Message-ID: <CAKfTPtD-i0K=ofKEekoCzAm2eXR+vVzFxR-uc6Em4LMO8W=RFA@mail.gmail.com>
Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision
 of EAS estimated energy
To:     Lukasz Luba <lukasz.luba@arm.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Chris Redpath <Chris.Redpath@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Quentin Perret <qperret@google.com>,
        "open list:THERMAL" <linux-pm@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, segall@google.com,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        CCj.Yeh@mediatek.com
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On Wed, 7 Jul 2021 at 13:02, Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 7/7/21 11:50 AM, Vincent Guittot wrote:
> > On Wed, 7 Jul 2021 at 12:41, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>
> >>
> >>
> >> On 7/7/21 11:32 AM, Vincent Guittot wrote:
> >>> On Wed, 7 Jul 2021 at 12:29, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 7/7/21 11:11 AM, Vincent Guittot wrote:
> >>>>> On Wed, 7 Jul 2021 at 12:06, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>>>
> >>>>
> >>>> [snip]
> >>>>
> >>>>>> No. It's in 0.1uW scale, so 800Watts. Which is 16 CPUs * 64Watts
> >>>>>
> >>>>> Oh! you want 0.1uW precision .... This doesn't seem realistic at all.
> >>>>> I'm not even sure that the power model can even reach an accuracy of
> >>>>> 1mW
> >>>>>
> >>>>
> >>>> True, the EM is registering platform with 1mW precision, but 1uW
> >>>
> >>> Do you mean 1uW or 0.1uW ?
> >>
> >> In this patch set I've proposed 0.1uW, but I'm open to drop one
> >> order of magnitude. The 1uW still be good.
> >
> > I don't want to underestimate the capabilities of the power model but
> > I don't see which benefit you will get with 0.1uW precision
> > With a 1uW precision the long type currently used for the returned
> > value is fine for 32bits machine AFAICT
> >
>
> For 1uW and 1.2Watts for one core, 4 CPUs in cluster we get:
> (1200 * 1000) * (4 * 1024) = ~4.9bln
> so it would need div 64 version

But as stated before, this is an internal computation step and doesn't
have to be reflected in the returned value which can stay a long