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

[Qais Yousef]

On 01/14/24 19:36, 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@xxxxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/CAHk-=wgWcYX2oXKtgvNN2LLDXP7kXkbo-xTfumEjmPbjSer2RQ@xxxxxxxxxxxxxx/
> Reported-by: Wyes Karny <wkarny@xxxxxxxxx>
> 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@xxxxxxxxxx>
> Tested-by: Wyes Karny <wkarny@xxxxxxxxx>
> ---
>  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 think we can do better, but this does re-instate the previous behavior at
least for this merge window. So FWIW

Reviewed-and-tested-by: Qais Yousef <qyousef@xxxxxxxxxxx>

>  }
>  
>  /**
> -- 
> 2.34.1
>