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Date: Tue, 20 Feb 2024 14:28:25 +0000
From: Qais Yousef <qyousef@layalina.io>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH] sched: cpufreq: Rename map_util_perf to
 apply_dvfs_headroom
Message-ID: <20240220142825.odzabxo3tnkhiepa@airbuntu>
References: <20240205022006.2229877-1-qyousef@layalina.io>
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 <20240220135745.h5mlvutle6wn6eim@airbuntu>
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On 02/20/24 15:22, Vincent Guittot wrote:
> On Tue, 20 Feb 2024 at 14:57, Qais Yousef <qyousef@layalina.io> wrote:
> >
> > On 02/14/24 08:32, Vincent Guittot wrote:
> > > On Mon, 5 Feb 2024 at 03:20, Qais Yousef <qyousef@layalina.io> wrote:
> > > >
> > > > We are providing headroom for the utilization to grow until the next
> > > > decision point to pick the next frequency. Give the function a better
> > > > name and give it some documentation. It is not really mapping anything.
> > >
> > > The renaming makes sense.
> > >
> > > >
> > > > Also move it to sched.h. This function relies on updating util signal
> > >
> > > I don't see the benefit of moving it the sched.h as it is only used by
> > > cpufreq_schedutil()
> >
> > Hehe what's for me the reason to move it for you it's the reason not to :-)
> >
> > (I believe you meant cpufreq_schedutil.c)
> >
> > It doesn't make sense outside of schedutil, does it? I can't see it being
> > suitable for consumption by other governors for example as it is not generic
> > enough.
> >
> > And the headroom definition needs to evolve. And the tight coupling to util
> > which is a scheduler internal metric will make it hard once it's part of
> > cpufreq. The headroom IMO is a property of the governor.
> 
> In this case make it part of cpufreq_schedutil.c if this is the
> governor that can use it. I don't like sched.h because It gives the
> impression that scheduler can play with it whereas it's a property of
> the cpufreq governor

Okay will do

Thanks!

> 
> >
> > We can defer the moving for now if you insist. But I think it's inevitable?
> >
> > >
> > >
> > > > appropriately to give a headroom to grow. This is more of a scheduler
> > > > functionality than cpufreq. Move it to sched.h where all the other util
> > > > handling code belongs.
> > > >
> > > > Signed-off-by: Qais Yousef <qyousef@layalina.io>
> > > > ---
> > > >  include/linux/sched/cpufreq.h    |  5 -----
> > > >  kernel/sched/cpufreq_schedutil.c |  2 +-
> > > >  kernel/sched/sched.h             | 17 +++++++++++++++++
> > > >  3 files changed, 18 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> > > > index bdd31ab93bc5..d01755d3142f 100644
> > > > --- a/include/linux/sched/cpufreq.h
> > > > +++ b/include/linux/sched/cpufreq.h
> > > > @@ -28,11 +28,6 @@ static inline unsigned long map_util_freq(unsigned long util,
> > > >  {
> > > >         return freq * util / cap;
> > > >  }
> > > > -
> > > > -static inline unsigned long map_util_perf(unsigned long util)
> > > > -{
> > > > -       return util + (util >> 2);
> > > > -}
> > > >  #endif /* CONFIG_CPU_FREQ */
> > > >
> > > >  #endif /* _LINUX_SCHED_CPUFREQ_H */
> > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > > index 95c3c097083e..abbd1ddb0359 100644
> > > > --- a/kernel/sched/cpufreq_schedutil.c
> > > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > > @@ -179,7 +179,7 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
> > > >                                  unsigned long max)
> > > >  {
> > > >         /* Add dvfs headroom to actual utilization */
> > > > -       actual = map_util_perf(actual);
> > > > +       actual = apply_dvfs_headroom(actual);
> > > >         /* Actually we don't need to target the max performance */
> > > >         if (actual < max)
> > > >                 max = actual;
> > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > > index e58a54bda77d..0da3425200b1 100644
> > > > --- a/kernel/sched/sched.h
> > > > +++ b/kernel/sched/sched.h
> > > > @@ -3002,6 +3002,23 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
> > > >                                  unsigned long min,
> > > >                                  unsigned long max);
> > > >
> > > > +/*
> > > > + * DVFS decision are made at discrete points. If CPU stays busy, the util will
> > > > + * continue to grow, which means it could need to run at a higher frequency
> > > > + * before the next decision point was reached. IOW, we can't follow the util as
> > > > + * it grows immediately, but there's a delay before we issue a request to go to
> > > > + * higher frequency. The headroom caters for this delay so the system continues
> > > > + * to run at adequate performance point.
> > > > + *
> > > > + * This function provides enough headroom to provide adequate performance
> > > > + * assuming the CPU continues to be busy.
> > > > + *
> > > > + * At the moment it is a constant multiplication with 1.25.
> > > > + */
> > > > +static inline unsigned long apply_dvfs_headroom(unsigned long util)
> > > > +{
> > > > +       return util + (util >> 2);
> > > > +}
> > > >
> > > >  /*
> > > >   * Verify the fitness of task @p to run on @cpu taking into account the
> > > > --
> > > > 2.34.1
> > > >