linux-kernel - Re: [RFC PATCH 3/3] sched/fair: Traverse cpufreq policies to detect capacity inversion

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Message-ID: <CAKfTPtCZYGEvDBe5X1v7TiNZag0atUozGKip6EAgvZDWyo8e-g@mail.gmail.com>
Date:   Sun, 4 Dec 2022 12:35:39 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>, Wei Wang <wvw@...gle.com>,
        Xuewen Yan <xuewen.yan94@...il.com>,
        Hank <han.lin@...iatek.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>
Subject: Re: [RFC PATCH 3/3] sched/fair: Traverse cpufreq policies to detect
 capacity inversion

On Sat, 3 Dec 2022 at 15:33, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 12/02/22 15:57, Vincent Guittot wrote:
>
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 7c0dd57e562a..4bbbca85134b 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -8856,23 +8856,20 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
> > >          *   * Thermal pressure will impact all cpus in this perf domain
> > >          *     equally.
> > >          */
> > > -       if (sched_energy_enabled()) {
> > > +       if (static_branch_unlikely(&sched_asym_cpucapacity)) {
> > >                 unsigned long inv_cap = capacity_orig - thermal_load_avg(rq);
> > > -               struct perf_domain *pd = rcu_dereference(rq->rd->pd);
> > > +               struct cpufreq_policy *policy, __maybe_unused *policy_n;
> > >
> > >                 rq->cpu_capacity_inverted = 0;
> > >
> > > -               SCHED_WARN_ON(!rcu_read_lock_held());
> > > -
> > > -               for (; pd; pd = pd->next) {
> > > -                       struct cpumask *pd_span = perf_domain_span(pd);
> > > +               for_each_active_policy_safe(policy, policy_n) {
> >
> > So you are looping all cpufreq policy (and before the perf domain) in
> > the period load balance. That' really not something we should or want
> > to do
>
> Why is it not acceptable in the period load balance but acceptable in the hot
> wake up path in feec()? What's the difference?

This patch loops on all cpufreq policy in sched softirq, how can this
be sane ? and not only in eas mode but also in the default asymmetric
performance  one.

This inverted detection doesn't look like the right way to fix your
problem IMO. That being said, i agree that I haven't made any other
proposal apart that I think that you should use a different rules for
task and for overutilized and part of your problem comes from this.

Then this make eas and util_fits_cpu even more Arm specific and I
still hope to merge sched_asym_cpucapacity and asym_packing a some
levels because they looks  more and more similar but each side is
trying to add some SoC specific policy

Vincent

>
>
> Thanks!
>
> --
> Qais Yousef