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

[Vincent Guittot]

On Tue, 16 Jan 2024 at 10:59, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Vincent Guittot <vincent.guittot@xxxxxxxxxx> 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've updated the changelog to better express what was broken and how we
> fixed it. Ack?

Looks good

Thanks

>
>         Ingo
>
> ==========================>
> From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxx>
> Bisected-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Reported-by: Wyes Karny <wkarny@xxxxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Tested-by: Wyes Karny <wkarny@xxxxxxxxx>
> Link: https://lore.kernel.org/r/20240114183600.135316-1-vincent.guittot@xxxxxxxxxx
> ---
>  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);
>  }
>
>  /**