linux-kernel - Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum

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Date:   Wed, 8 Apr 2020 19:03:57 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum

On Wed, 8 Apr 2020 at 18:31, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 08.04.20 14:29, Vincent Guittot wrote:
> > On Wed, 8 Apr 2020 at 11:50, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> [...]
>
> >>  /**
> >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> >> index 8344757bba6e..74b0c0fa4b1b 100644
> >> --- a/kernel/sched/topology.c
> >> +++ b/kernel/sched/topology.c
> >> @@ -2052,12 +2052,17 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >>         /* Attach the domains */
> >>         rcu_read_lock();
> >>         for_each_cpu(i, cpu_map) {
> >> +               unsigned long cap = arch_scale_cpu_capacity(i);
> >
> > Why do you replace the use of rq->cpu_capacity_orig by
> > arch_scale_cpu_capacity(i) ?
> > There is nothing about this change in the commit message
>
> True. And I can change this back.
>
> It seems though that the solution is not sufficient because of the
> 'rd->span &nsub cpu_active_mask' issue discussed under patch 2/4.
>
> But this remind me of another question I have.
>
> Currently we use arch_scale_cpu_capacity() more often (16 times) than
> capacity_orig_of()/rq->cpu_capacity_orig .
>
> What's hindering us to remove rq->cpu_capacity_orig and the code around
> it and solely rely on arch_scale_cpu_capacity()? I mean the arch
> implementation should be fast.

Or we can do the opposite and only use capacity_orig_of()/rq->cpu_capacity_orig.

Is there a case where the max cpu capacity changes over time ? So I
would prefer to use cpu_capacity_orig which is a field of scheduler
instead of always calling an external arch specific function