linux-kernel - Re: [PATCH v2 2/2] cpufreq: schedutil: consolidate capacity margin calculation

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Message-ID: <20171002111345.GC18885@leoy-ThinkPad-T440>
Date:   Mon, 2 Oct 2017 19:13:45 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2 2/2] cpufreq: schedutil: consolidate capacity margin
 calculation

Hi Joel,

On Sun, Oct 01, 2017 at 06:02:24PM -0700, Joel Fernandes wrote:
> Hi Leo,
> 
> On Sun, Oct 1, 2017 at 5:30 PM, Leo Yan <leo.yan@...aro.org> wrote:
> > Scheduler CFS class has variable 'capacity_margin' to calculate the
> 
> s/calculate/represent/ ?
> 
> > capacity margin, and schedutil governor also needs to compensate the
> > same margin for frequency tipping point. Below are formulas used in
> > CFS class and schedutil governor separately:
> >
> > CFS:       U` = U * capacity_margin / 1024 = U * 1.25
> 
> You should mention in the commit message, at the moment
> capacity_margin is 1280 which makes U` = 1.25.
> 
> > Schedutil: U` = U + U >> 2 = U + U * 0.25  = U * 1.25
> >
> > This patch consolidates the capacity margin calculation so let
> > schedutil to use same formula with CFS class. As result this can avoid
> 
> As a result.
> 
> > the mismatch issue between schedutil and CFS class after change
> > 'capacity_margin' to other values.
> 
> This didn't make sense to me. May be you meant:
> 
> This patch consolidates the usage of the capacity margin value and
> lets schedutil use the same formula as the CFS class. Thus we can
> avoid the mismatch between schedutil and CFS class if
> 'capacity_margin' is changed to other values in the future.

Yeah, thanks for the suggestion. And accept all upper comment.
Will refine for new patch.

Thanks,
Leo Yan

> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Morten Rasmussen <morten.rasmussen@....com>
> > Cc: Chris Redpath <Chris.Redpath@....com>
> > Cc: Joel Fernandes <joelaf@...gle.com>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Cc: Patrick Bellasi <patrick.bellasi@....com>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 6 ++++--
> >  kernel/sched/sched.h             | 1 +
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 9209d83..13cc243 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -155,7 +155,8 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> >   *
> >   * next_freq = C * curr_freq * util_raw / max
> >   *
> > - * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
> > + * Take C = capacity_margin / 1024 = 1.25, so it's for the frequency tipping
> > + * point at (util / max) = 0.8.
> 
> The above comment assumes capacity_margin is 1280. If for any reason
> the capacity_margin is changed to something else, then the comment
> wont make sense anymore.
> 
> thanks,
> 
> - Joel