linux-kernel - Re: [PATCH 1/2] cpufreq: schedutil: Fix superfluous updates caused by need_freq

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Message-ID: <Z_coNmh-CabcfIWD@sultan-box.localdomain>
Date: Wed, 9 Apr 2025 19:08:54 -0700
From: Sultan Alsawaf <sultan@...neltoast.com>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Stephan Gerhold <stephan.gerhold@...aro.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	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>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
	Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 1/2] cpufreq: schedutil: Fix superfluous updates caused
 by need_freq_update

On Thu, Apr 10, 2025 at 10:06:41AM +0800, Xuewen Yan wrote:
> On Thu, Apr 10, 2025 at 9:49 AM Sultan Alsawaf <sultan@...neltoast.com> wrote:
> >
> > On Wed, Apr 09, 2025 at 07:48:05PM +0800, Xuewen Yan wrote:
> > > Or can we modify it as follows?
> > >
> > > -->8--
> > >
> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index 1a19d69b91ed..0e8d3b92ffe7 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -83,7 +83,7 @@ static bool sugov_should_update_freq(struct
> > > sugov_policy *sg_policy, u64 time)
> > >
> > >         if (unlikely(sg_policy->limits_changed)) {
> > >                 sg_policy->limits_changed = false;
> > > -               sg_policy->need_freq_update =
> > > cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > > +               sg_policy->need_freq_update = true;
> > >                 return true;
> > >         }
> > >
> > > @@ -95,11 +95,15 @@ static bool sugov_should_update_freq(struct
> > > sugov_policy *sg_policy, u64 time)
> > >  static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> > >                                    unsigned int next_freq)
> > >  {
> > > -       if (sg_policy->need_freq_update)
> > > +       if (sg_policy->need_freq_update) {
> > >                 sg_policy->need_freq_update = false;
> > > -       else if (sg_policy->next_freq == next_freq)
> > > -               return false;
> > > +               if (cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > > +                       goto change;
> > > +       }
> > >
> > > +       if (sg_policy->next_freq == next_freq)
> > > +               return false;
> > > +change:
> > >         sg_policy->next_freq = next_freq;
> > >         sg_policy->last_freq_update_time = time;
> >
> > If CPUFREQ_NEED_UPDATE_LIMITS isn't specified, then there's no need to request a
> > frequency switch from the driver when the current frequency is exactly the same
> > as the next frequency.
> 
> Yes, the following check would return false:
> 
>  +       if (sg_policy->next_freq == next_freq)
>  +               return false;

But what does that change fix? In fact, that change causes a limits update to
trigger a frequency switch request to the driver even when the new frequency is
the same as the current one.

Sultan