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Date:   Wed, 24 Feb 2021 13:46:11 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Yue Hu <zbestahu@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yue Hu <huyue2@...ong.com>
Subject: Re: [PATCH] cpufreq: schedutil: Don't consider freq reduction to busy
 CPU if need_freq_update is set

On Wed, Feb 24, 2021 at 3:24 AM Yue Hu <zbestahu@...il.com> wrote:
>
> On Mon, 22 Feb 2021 15:30:34 +0100
> "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>
> > On Mon, Feb 22, 2021 at 2:57 PM Yue Hu <zbestahu@....com> wrote:
> > >
> > > On Mon, 22 Feb 2021 11:00:14 +0530
> > > Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > > On 19-02-21, 19:45, Yue Hu wrote:
> > > > > We will set next_f to next_freq(previous freq) if next_f is
> > > > > reduced for busy CPU. Then the next sugov_update_next_freq() will check
> > > > > if next_freq matches next_f if need_freq_update is not set.
> > > > > Obviously, we will do nothing for the case. And The related check to
> > > > > fast_switch_enabled and raw_spin_{lock,unlock} operations are
> > > > > unnecessary.
> > > >
> > > > Right, but we will still need sugov_update_next_freq() to have the
> > > > same implementation regardless and so I am not sure if we should add
> > >
> > > Yes, sugov_update_next_freq() should be keeping current logic for corner case.
> > >
> > > > this change:
> > > >
> > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > > index 41e498b0008a..7289e1adab73 100644
> > > > --- a/kernel/sched/cpufreq_schedutil.c
> > > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > > @@ -362,6 +362,9 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time,
> > > >          * recently, as the reduction is likely to be premature then.
> > > >          */
> > > >         if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> > > > +               if (!sg_policy->need_freq_update)
> > >
> > > The initial purpose about code of `next_f = sg_policy->next_freq` here (for special CPU busy
> > > case) should be skipping the freq update.
> > >
> > > Since commit 600f5badb78c ("cpufreq: schedutil: Don't skip freq update when limits change"),
> > > we add the check to busy CPU for not skipping the update, we need to update the freq using
> > > computed one because limits change.
> > >
> > > After commit 23a881852f3e ("cpufreq: schedutil: Don't skip freq update if need_freq_update
> > > is set"), we removed the need_freq_update check(no issue of commit 600f5badb78c anymore?)
> > > and introduce to always do an update in sugov_update_next_freq() if need_freq_update is set
> > > even though current freq == sg_policy->next_freq because of corner case issue. But that is
> > > conflict with original purpose of the freq skip code (next_f = sg_policy->next_freq) of
> > > busy CPU.
> >
> > That's because we realized that it was not always a good idea to skip
> > the update even if next_f == sg_policy->next_freq.
> >
> > That's why CPUFREQ_NEED_UPDATE_LIMITS has been introduced and the
> > current flow is a result of subsequent code rearrangements.
>
> ok, care about unnecessary(should be) behaviors(fast_switch_enabled and raw_spin_{lock,unlock})
> if need_freq_update is unset?
>
> If we care, i will send another patch (which is different from above change for busy CPU).

Please send a patch and we'll see (this is how things go).

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