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

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Date: Tue, 16 Jan 2024 10:59:49 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
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
Subject: Re: [PATCH] sched/fair: Fix frequency selection for non invariant
 case


* Vincent Guittot <vincent.guittot@...aro.org> 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);
>  }

I've updated the changelog to better express what was broken and how we 
fixed it. Ack?

	Ingo

==========================>
From: Vincent Guittot <vincent.guittot@...aro.org>
Date: Sun, 14 Jan 2024 19:36:00 +0100
Subject: [PATCH] sched/fair: Fix frequency selection for non-invariant case

Linus reported a ~50% performance regression on single-threaded
workloads on his AMD Ryzen system, and bisected it to:

  9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor performance estimation")

When frequency invariance is not enabled, get_capacity_ref_freq(policy)
is supposed to return the current frequency and the performance margin
applied by map_util_perf(), enabling the utilization to go above the
maximum compute capacity and to select a higher frequency than the current one.

After the changes in 9c0b4bb7f630, the performance margin was applied
earlier in the path to take into account utilization clampings and
we couldn't get a utilization higher than the maximum compute capacity,
and the CPU remained 'stuck' at lower frequencies.

To fix this, 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 return 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.

[ mingo: Clarified the changelog. ]

Fixes: 9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor performance estimation")
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Bisected-by: Linus Torvalds <torvalds@...ux-foundation.org>
Reported-by: Wyes Karny <wkarny@...il.com>
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Tested-by: Wyes Karny <wkarny@...il.com>
Link: https://lore.kernel.org/r/20240114183600.135316-1-vincent.guittot@linaro.org
---
 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..eece6244f9d2 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 fully busy:
+	 */
+	return policy->cur + (policy->cur >> 2);
 }
 
 /**