linux-kernel - Re: [PATCH 3/4] cpufreq/schedutil: use a fixed reference frequency

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Message-ID: <CAKfTPtCM_dsWcD6qru_nLYpDH9_OoziMy5d3zAofXNsXzG94xQ@mail.gmail.com>
Date:   Mon, 25 Sep 2023 14:06:32 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, sudeep.holla@....com,
        gregkh@...uxfoundation.org, rafael@...nel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        viresh.kumar@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-pm@...r.kernel.org, conor.dooley@...rochip.com,
        suagrfillet@...il.com, ajones@...tanamicro.com, lftan@...nel.org
Subject: Re: [PATCH 3/4] cpufreq/schedutil: use a fixed reference frequency

On Thu, 21 Sept 2023 at 11:19, Ionela Voinescu <ionela.voinescu@....com> wrote:
>
> On Friday 01 Sep 2023 at 15:03:11 (+0200), Vincent Guittot wrote:
> > cpuinfo_max_freq can change at runtime because of boost as example. This
> > implies that the value could not be the same than the one that has been
> > used when computing the capacity of a CPU.
> >
> > The new arch_scale_freq_ref() returns a fixed and coherent frequency
> > reference that can be used when computing a frequency based on utilization.
> >
> > Use this arch_scale_freq_ref() when available and fallback to
> > cpuinfo.max_freq otherwise.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 29 +++++++++++++++++++++++++++--
> >  1 file changed, 27 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 4492608b7d7f..9996ef429e2b 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -114,6 +114,31 @@ static void sugov_deferred_update(struct sugov_policy *sg_policy)
> >       }
> >  }
> >
> > +#ifdef arch_scale_freq_ref
> > +/**
> > + * arch_scale_freq_ref_policy - get the reference frequency of a given CPU that
> > + * has been used to correlate frequency and compute capacity.
> > + * @cpu: the CPU in question.
> > + *
> > + * Return: the reference CPU frequency.
> > + */
> > +static __always_inline
> > +unsigned long  arch_scale_freq_ref_policy(struct cpufreq_policy *policy)
>
> This should not be an arch_ function as it's only a wrapper over an
> arch_ function and not a function that different architectures might
> implement differently usually in architecture specific code.

I expect this function to disappear at some point once all arch will
use it that why I named it arch_* but I can rename it

>
> > +{
> > +     return arch_scale_freq_ref(policy->cpu);
>
> It might make it easier to read if arch_scale_freq_ref() had a default
> implementation that returned 0.

I will use the suggestion made by Peter to have only one function


>
> Then this code would simply become:
>
> freq = arch_scale_freq_ref(policy->cpu);
> if (freq)
>         return freq;
> else if (arch_scale_freq_invariant())
>         return ..
> ..
>
> This approach is similar to the use of arch_freq_get_on_cpu() in
> cpufreq.c, and, as there, having a chosen maximum frequency of 0 would
> not be a valid value.
>
> > +}
> > +#else
> > +static __always_inline
> > +unsigned long  arch_scale_freq_ref_policy(struct cpufreq_policy *policy)
> > +{
> > +     if (arch_scale_freq_invariant())
> > +             return policy->cpuinfo.max_freq;
> > +
> > +
> > +     return policy->cur;
> > +}
> > +#endif
> > +
> >  /**
> >   * get_next_freq - Compute a new frequency for a given cpufreq policy.
> >   * @sg_policy: schedutil policy object to compute the new frequency for.
> > @@ -139,11 +164,11 @@ static void sugov_deferred_update(struct sugov_policy *sg_policy)
> >  static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >                                 unsigned long util, unsigned long max)
> >  {
> > +     unsigned int freq;
> >       struct cpufreq_policy *policy = sg_policy->policy;
> > -     unsigned int freq = arch_scale_freq_invariant() ?
> > -                             policy->cpuinfo.max_freq : policy->cur;
> >
> >       util = map_util_perf(util);
> > +     freq = arch_scale_freq_ref_policy(policy);
>
> Given its single use here, it would likely be better to place the code
> above directly here, rather than create a wrapper over a few lines of
> code.
>
> Hope it helps,
> Ionela.
>
> >       freq = map_util_freq(util, freq, max);
> >
> >       if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> > --
> > 2.34.1
> >
> >