linux-kernel - Re: [PATCH v3 1/5] cpufreq: Add a cpufreq pressure feedback for the scheduler

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Message-ID: <CAKfTPtC7sM1ZeW4iGN6fzvF9bR0juYePycX3GeWzYQwRBR7XkA@mail.gmail.com>
Date: Mon, 8 Jan 2024 17:46:16 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	sudeep.holla@....com, rafael@...nel.org, viresh.kumar@...aro.org, 
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	corbet@....net, gregkh@...uxfoundation.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
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	qyousef@...alina.io
Subject: Re: [PATCH v3 1/5] cpufreq: Add a cpufreq pressure feedback for the scheduler

On Mon, 8 Jan 2024 at 17:35, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 08/01/2024 14:48, Vincent Guittot wrote:
> > Provide to the scheduler a feedback about the temporary max available
> > capacity. Unlike arch_update_thermal_pressure, this doesn't need to be
> > filtered as the pressure will happen for dozens ms or more.
>
> Is this then related to the 'medium pace system pressure' you mentioned
> in your OSPM '23 talk?
>
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  drivers/cpufreq/cpufreq.c | 36 ++++++++++++++++++++++++++++++++++++
> >  include/linux/cpufreq.h   | 10 ++++++++++
> >  2 files changed, 46 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 44db4f59c4cc..fa2e2ea26f7f 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -2563,6 +2563,40 @@ int cpufreq_get_policy(struct cpufreq_policy *policy, unsigned int cpu)
> >  }
> >  EXPORT_SYMBOL(cpufreq_get_policy);
> >
> > +DEFINE_PER_CPU(unsigned long, cpufreq_pressure);
> > +
> > +/**
> > + * cpufreq_update_pressure() - Update cpufreq pressure for CPUs
> > + * @policy: cpufreq policy of the CPUs.
> > + *
> > + * Update the value of cpufreq pressure for all @cpus in the policy.
> > + */
> > +static void cpufreq_update_pressure(struct cpufreq_policy *policy)
> > +{
> > +     unsigned long max_capacity, capped_freq, pressure;
> > +     u32 max_freq;
> > +     int cpu;
> > +
> > +     /*
> > +      * Handle properly the boost frequencies, which should simply clean
> > +      * the thermal pressure value.
>                ^^^^^^^
> IMHO, this is a copy & paste error from topology_update_thermal_pressure()?
>
> > +      */
> > +     if (max_freq <= capped_freq) {
>
> max_freq seems to be uninitialized.

argh yes, I made crap while cleaning up
both max_freq and capped_freq are uninitialized

>
> > +             pressure = 0;
>
> Is this x86 (turbo boost) specific? IMHO at arm we follow this max freq
> (including boost) relates to 1024 in capacity? Or haven't we made this
> discussion yet?

This is not x86 specific. We can have capped_freq > max_freq on Arm too

Also this bypass all calculation below when max_freq == capped_freq
which is the most common case

>
> > +     } else {
> > +             cpu = cpumask_first(policy->related_cpus);
> > +             max_capacity = arch_scale_cpu_capacity(cpu);
> > +             capped_freq = policy->max;
> > +             max_freq = arch_scale_freq_ref(cpu);
> > +
> > +             pressure = max_capacity -
> > +                        mult_frac(max_capacity, capped_freq, max_freq);
> > +     }
> > +
> > +     for_each_cpu(cpu, policy->related_cpus)
> > +             WRITE_ONCE(per_cpu(cpufreq_pressure, cpu), pressure);
> > +}
> > +
>
> [...]
>