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sched/fair: Fix frequency selection for non-invariant case

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

  9c0b4bb7f6 ("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 9c0b4bb7f6, 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: 9c0b4bb7f6 ("sched/cpufreq: Rework schedutil governor performance estimation")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Bisected-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Wyes Karny <wkarny@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Wyes Karny <wkarny@gmail.com>
Link: https://lore.kernel.org/r/20240114183600.135316-1-vincent.guittot@linaro.org
This commit is contained in:
Vincent Guittot 2024-01-14 19:36:00 +01:00 committed by Ingo Molnar
parent bfe8eb3b85
commit e37617c8e5
1 changed files with 5 additions and 1 deletions
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6
kernel/sched/cpufreq_schedutil.c
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@ -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);
}
/**