linux-kernel - Re: cpufreq: intel_pstate: map utilization into the pstate range

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Date:   Tue, 28 Dec 2021 19:16:45 +0100 (CET)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
cc:     Francisco Jerez <currojerez@...eup.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: cpufreq: intel_pstate: map utilization into the pstate range



On Tue, 28 Dec 2021, Rafael J. Wysocki wrote:

> On Tue, Dec 28, 2021 at 6:46 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
> >
> >
> >
> > On Tue, 28 Dec 2021, Rafael J. Wysocki wrote:
> >
> > > On Tue, Dec 28, 2021 at 5:58 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
> > > >
> > > > I looked a bit more into why pstate 20 is always using the least energy. I
> > > > have just one thread spinning for 10 seconds, I use a fixed value for the
> > > > pstate, and I measure the energy usage with turbostat.
> > >
> > > How exactly do you fix the pstate?
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index e7af18857371..19440b15454c 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -400,7 +402,7 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
> >                 sg_cpu->util = prev_util;
> >
> >         cpufreq_driver_adjust_perf(sg_cpu->cpu, map_util_perf(sg_cpu->bw_dl),
> > -                                  map_util_perf(sg_cpu->util), sg_cpu->max);
> > +                                  sysctl_sched_fixedfreq, sg_cpu->max);
>
> This is just changing the "target" hint given to the processor which
> may very well ignore it, though.

It doesn't seem to ignore it.  I also print the current frequency on every
clock tick, and it is as it should be.  This is done in the function
arch_scale_freq_tick in arch/x86/kernel/smpboot.c, where I added:

        trace_printk("freq %lld\n", div64_u64((cpu_khz * acnt), mcnt));


>
> >
> >         sg_cpu->sg_policy->last_freq_update_time = time;
> >  }
> >
> > ------------------------------
> >
> > sysctl_sched_fixedfreq is a variable that I added to sysfs.
>
> If I were trying to fix a pstate, I would set scaling_max_freq and
> scaling_min_freq in sysfs for all CPUs to the same value.
>
> That would cause intel_pstate to set HWP min and max to the same value
> which should really cause the pstate to be fixed, at least outside the
> turbo range of pstates.

OK, I can try that, thanks.

julia