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

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Message-ID: <20221203143323.w32boxa6asqvvdnp@airbuntu>
Date:   Sat, 3 Dec 2022 14:33:23 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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 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?


Thanks!

--
Qais Yousef