Re: [PATCH 6.6 8/8] sched/fair: Fix frequency selection for non-invariant case

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Aug 16, 2025 at 02:16:18AM +0800, Wentao Guan wrote:
> From: Vincent Guittot <vincent.guittot@linaro.org>
> 
> 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@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
> (cherry picked from commit e37617c8e53a1f7fcba6d5e1041f4fd8a2425c27)
> ---
>  kernel/sched/cpufreq_schedutil.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

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