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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Wed, 14 Feb 2024 08:32:47 +0100
Message-ID: <CAKfTPtBoapJtwD3DByd06CE07MD3eGhfJVyZ01cRLyKLO8fJ7w@mail.gmail.com>
Subject: Re: [PATCH] sched: cpufreq: Rename map_util_perf to apply_dvfs_headroom
To: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>, 
	"Rafael J. Wysocki" <rafael@kernel.org>, Viresh Kumar <viresh.kumar@linaro.org>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, linux-kernel@vger.kernel.org, 
	linux-pm@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Mon, 5 Feb 2024 at 03:20, Qais Yousef <qyousef@layalina.io> wrote:
>
> We are providing headroom for the utilization to grow until the next
> decision point to pick the next frequency. Give the function a better
> name and give it some documentation. It is not really mapping anything.

The renaming makes sense.

>
> Also move it to sched.h. This function relies on updating util signal

I don't see the benefit of moving it the sched.h as it is only used by
cpufreq_schedutil()


> appropriately to give a headroom to grow. This is more of a scheduler
> functionality than cpufreq. Move it to sched.h where all the other util
> handling code belongs.
>
> Signed-off-by: Qais Yousef <qyousef@layalina.io>
> ---
>  include/linux/sched/cpufreq.h    |  5 -----
>  kernel/sched/cpufreq_schedutil.c |  2 +-
>  kernel/sched/sched.h             | 17 +++++++++++++++++
>  3 files changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index bdd31ab93bc5..d01755d3142f 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -28,11 +28,6 @@ static inline unsigned long map_util_freq(unsigned long util,
>  {
>         return freq * util / cap;
>  }
> -
> -static inline unsigned long map_util_perf(unsigned long util)
> -{
> -       return util + (util >> 2);
> -}
>  #endif /* CONFIG_CPU_FREQ */
>
>  #endif /* _LINUX_SCHED_CPUFREQ_H */
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 95c3c097083e..abbd1ddb0359 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -179,7 +179,7 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
>                                  unsigned long max)
>  {
>         /* Add dvfs headroom to actual utilization */
> -       actual = map_util_perf(actual);
> +       actual = apply_dvfs_headroom(actual);
>         /* Actually we don't need to target the max performance */
>         if (actual < max)
>                 max = actual;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e58a54bda77d..0da3425200b1 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3002,6 +3002,23 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
>                                  unsigned long min,
>                                  unsigned long max);
>
> +/*
> + * DVFS decision are made at discrete points. If CPU stays busy, the util will
> + * continue to grow, which means it could need to run at a higher frequency
> + * before the next decision point was reached. IOW, we can't follow the util as
> + * it grows immediately, but there's a delay before we issue a request to go to
> + * higher frequency. The headroom caters for this delay so the system continues
> + * to run at adequate performance point.
> + *
> + * This function provides enough headroom to provide adequate performance
> + * assuming the CPU continues to be busy.
> + *
> + * At the moment it is a constant multiplication with 1.25.
> + */
> +static inline unsigned long apply_dvfs_headroom(unsigned long util)
> +{
> +       return util + (util >> 2);
> +}
>
>  /*
>   * Verify the fitness of task @p to run on @cpu taking into account the
> --
> 2.34.1
>