linux-kernel - Re: [PATCH] sched/fair: Fix frequency selection for non invariant case

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Message-ID: <CAKfTPtAR-yWG8odEv-b8XoZQa05OiYd3PLn56MA+AqPb4h=NJA@mail.gmail.com>
Date: Wed, 14 Feb 2024 18:19:35 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Jon Hunter <jonathanh@...dia.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, wkarny@...il.com, 
	torvalds@...ux-foundation.org, qyousef@...alina.io, tglx@...utronix.de, 
	rafael@...nel.org, viresh.kumar@...aro.org, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org, 
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>, Thierry Reding <treding@...dia.com>, 
	Sasha Levin <sashal@...dia.com>, Laxman Dewangan <ldewangan@...dia.com>, 
	Shardar Mohammed <smohammed@...dia.com>
Subject: Re: [PATCH] sched/fair: Fix frequency selection for non invariant case

Hi John,

On Wed, 14 Feb 2024 at 18:12, Jon Hunter <jonathanh@...dia.com> wrote:
>
> Hi Vincent,
>
> On 14/01/2024 18:36, Vincent Guittot wrote:
> > When frequency invariance is not enabled, get_capacity_ref_freq(policy)
> > returns the current frequency and the performance margin applied by
> > map_util_perf(), enabled the utilization to go above the maximum compute
> > capacity and to select a higher frequency than the current one.
> >
> > The performance margin is now applied earlier in the path to take into
> > account some utilization clampings and we can't get an utilization higher
> > than the maximum compute capacity.
> >
> > We must use a frequency above the current frequency to get a chance to
> > select a higher OPP when the current one becomes fully used. Apply
> > the same margin and returns a frequency 25% higher than the current one in
> > order to switch to the next OPP before we fully use the cpu at the current
> > one.
> >
> > Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Closes: https://lore.kernel.org/lkml/CAHk-=wgWcYX2oXKtgvNN2LLDXP7kXkbo-xTfumEjmPbjSer2RQ@mail.gmail.com/
> > Reported-by: Wyes Karny <wkarny@...il.com>
> > Closes: https://lore.kernel.org/lkml/20240114091240.xzdvqk75ifgfj5yx@wyes-pc/
> > Fixes: 9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor performance estimation")
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Tested-by: Wyes Karny <wkarny@...il.com>
> > ---
> >   kernel/sched/cpufreq_schedutil.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 95c3c097083e..d12e95d30e2e 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -133,7 +133,11 @@ unsigned long get_capacity_ref_freq(struct cpufreq_policy *policy)
> >       if (arch_scale_freq_invariant())
> >               return policy->cpuinfo.max_freq;
> >
> > -     return policy->cur;
> > +     /*
> > +      * Apply a 25% margin so that we select a higher frequency than
> > +      * the current one before the CPU is full busy
> > +      */
> > +     return policy->cur + (policy->cur >> 2);
> >   }
> >
> >   /**
>
>
> We have also observed a performance degradation on our Tegra platforms
> with v6.8-rc1. Unfortunately, the above change does not fix the problem
> for us and we are still seeing a performance issue with v6.8-rc4. For
> example, running Dhrystone on Tegra234 I am seeing the following ...
>
> Linux v6.7:
> [ 2216.301949] CPU0: Dhrystones per Second: 31976326 (18199 DMIPS)
> [ 2220.993877] CPU1: Dhrystones per Second: 49568123 (28211 DMIPS)
> [ 2225.685280] CPU2: Dhrystones per Second: 49568123 (28211 DMIPS)
> [ 2230.364423] CPU3: Dhrystones per Second: 49632220 (28248 DMIPS)
>
> Linux v6.8-rc4:
> [   44.661686] CPU0: Dhrystones per Second: 16068483 (9145 DMIPS)
> [   51.895107] CPU1: Dhrystones per Second: 16077457 (9150 DMIPS)
> [   59.105410] CPU2: Dhrystones per Second: 16095436 (9160 DMIPS)
> [   66.333297] CPU3: Dhrystones per Second: 16064000 (9142 DMIPS)
>
> If I revert this change and the following ...
>
>   b3edde44e5d4 ("cpufreq/schedutil: Use a fixed reference frequency")
>   f12560779f9d ("sched/cpufreq: Rework iowait boost")
>   9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor
>
> ... then the perf is similar to where it was ...
>
> Linux v6.8-rc4 plus reverts:
> [   31.768189] CPU0: Dhrystones per Second: 48421678 (27559 DMIPS)
> [   36.556838] CPU1: Dhrystones per Second: 48401324 (27547 DMIPS)
> [   41.343343] CPU2: Dhrystones per Second: 48421678 (27559 DMIPS)
> [   46.163347] CPU3: Dhrystones per Second: 47679814 (27137 DMIPS)
>
> All CPUs are running with the schedutil CPUFREQ governor. We have not
> looked any further but wanted to report this in case you have any more
> thoughts or suggestions for us to try.

Have you tried this :
https://lore.kernel.org/lkml/20240117190545.596057-1-vincent.guittot@linaro.org/

It's in driver-core-linus' branch and should be sent to Linus soon

Vincent

>
> Thanks
> Jon
>
> --
> nvpublic