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 15:55:06 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, viresh.kumar@...aro.org,
        valentin.schneider@....com
Subject: Re: [PATCH v5 01/10] sched/pelt: Move pelt related code in a
 dedicated file

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

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:
  +---+-------+----------------+---------+
  | 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