linux-kernel - Re: [PATCH 1/4] sched: consolidate and cleanup access to CPU's max compute capacity

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Date:   Fri, 15 Sep 2023 15:20:25 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, sudeep.holla@....com,
        gregkh@...uxfoundation.org, rafael@...nel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, viresh.kumar@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-pm@...r.kernel.org,
        conor.dooley@...rochip.com, suagrfillet@...il.com,
        ajones@...tanamicro.com, lftan@...nel.org
Subject: Re: [PATCH 1/4] sched: consolidate and cleanup access to CPU's max
 compute capacity

On Thu, 14 Sept 2023 at 22:46, Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> On 01/09/2023 15:03, Vincent Guittot wrote:
> > Remove struct rq cpu_capacity_orig field and use arch_scale_cpu_capacity()
> > instead.
> >
> > Scheduler uses 3 methods to get access to the CPU's max compute capacity:
> > - arch_scale_cpu_capacity(cpu) which is the default way to get CPU's capacity.
> > - cpu_capacity_orig field which is periodically updated with
> >   arch_scale_cpu_capacity().
> > - capacity_orig_of(cpu) which encapsulates rq->cpu_capacity_orig
> >
> > There is no real need to save the value returned by arch_scale_cpu_capacity()
> > in struct rq. arch_scale_cpu_capacity() returns:
> > - either a per_cpu variable.
> > - or a const value for systems which have only one capacity.
> >
> > Remove cpu_capacity_orig and use arch_scale_cpu_capacity() everywhere.
> >
> > No functional changes.
> >
> > some tests of Arm64:
> > small SMP device (hikey): no noticeable changes
> > HMP device (RB5): hackbench shows minor improvement (1-2%)
> > large smp (thx2): hackbench and tbench shows minor improvement (1%)
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> Next to util_fits_cpu() which uses capacity_orig as a local variable
> (which is fine) there is sis() referring to capacity_orig in a comment.
>
> Documentation/scheduler/sched-capacity.rst uses the term `capacity_orig`
> in chapter 1.2 to explain the difference between CPU's maximum
> (attainable) capacity and capacity as the former reduced by pressure.

ok, I will have a look at those references to capacity_orig

>
> Not sure if you want to change those refs as well with this patch?
> People might get confused about the term `capacity_orig` pretty soon.
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Thanks