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

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Date: Wed, 14 Feb 2024 09:19:42 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Jon Hunter <jonathanh@...dia.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>, 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, 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

On Wed, 14 Feb 2024 at 09:12, Jon Hunter <jonathanh@...dia.com> wrote:
>
> 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 ...

Ok, guys, this whole scheduler / cpufreq rewrite seems to have been
completely buggered.

Please tell me why we shouldn't just revert things as per above?

Sure, the problem _I_ experienced is fixed, but apparently there are
others just lurking, and they are even bigger degradations than the
one I saw.

We're now at rc4, we're not releasing a 6.8 with the above kinds of
numbers. So either there's another obvious one-liner fix, or we need
to revert this whole thing.

Yes, dhrystones is a truly crappy benchmark, but partly _because_ it's
such a horribly bad benchmark it's also a very simple case. It's pure
CPU load with absolutely nothing interesting going on. Regressing on
that by a factor of three is a sign of complete failure.

                  Linus