linux-kernel - Re: [PATCH v5 01/10] sched/pelt: Move pelt related code in a dedicated file

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Date:   Tue, 29 May 2018 17:02:29 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v5 01/10] sched/pelt: Move pelt related code in a
 dedicated file

Hi Quentin,

On 29 May 2018 at 16:55, Quentin Perret <quentin.perret@....com> wrote:
>
> On Friday 25 May 2018 at 19:04:55 (+0100), Patrick Bellasi wrote:
> > On 25-May 15:26, Quentin Perret wrote:
> > > And also, I understand these functions are large, but if we _really_
> > > want to inline them even though they're big, why not putting them in
> > > sched-pelt.h ?
> >
> > Had the same tought at first... but then I recalled that header is
> > generated from a script. Thus, eventually, it should be a different one.
>
> Ah, good point. This patch already introduces a pelt.h so I guess that
> could work as well.
>
> >
> > > We probably wouldn't accept that for everything, but
> > > those PELT functions are used all over the place, including latency
> > > sensitive code paths (e.g. task wake-up).
> >
> > We should better measure the overheads, if any, and check what
> > (a modern) compiler does. Maybe some hackbench run could help on the
> > first point.
>
> FWIW, I ran a few hackbench tests today on my Intel box:
>  - Intel i7-6700 (4 cores / 8 threads) @ 3.40GHz
>  - Base kernel: today's tip/sched/core "2539fc82aa9b sched/fair: Update
>    util_est before updating schedutil"
>  - Compiler: GCC 7.3.0

Which cpufreq governor are you using ?

>
> The tables below summarize the results for:
> perf stat --repeat 10 perf bench sched messaging --pipe --thread -l 50000 --group G
>
> Without patch:
>   +---+-------+----------+---------+
>   | G | Tasks | Duration | Stddev  |
>   +---+-------+----------+---------+
>   | 1 | 40    | 3.906    | +-0.84% |
>   | 2 | 80    | 8.569    | +-0.77% |
>   | 4 | 160   | 16.384   | +-0.46% |
>   | 8 | 320   | 33.686   | +-0.42% |
>   +---+-------+----------+---------+
>
> With patch:

Just to make sure. You mean only this patch and not the whole patchset ?

>   +---+-------+----------------+---------+
>   | G | Tasks | Duration       | Stddev  |
>   +---+-------+----------------+---------+
>   | 1 | 40    | 3.953 (+1.2%)  | +-1.43% |
>   | 2 | 80    | 8.646 (+0.9%)  | +-0.32% |
>   | 4 | 160   | 16.390 (+0.0%) | +-0.38% |
>   | 8 | 320   | 33.992 (+0.9%) | +-0.27% |
>   +---+-------+----------------+---------+
>
> So there is (maybe) a little something on my box, but not so significant
> IMHO ... :)
>
> Thanks,
> Quentin