linux-kernel - Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance

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Date:   Tue, 8 Oct 2019 11:32:38 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Giovanni Gherdovich <ggherdovich@...e.cz>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        Len Brown <lenb@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <qperret@...rret.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance

On Tue, Oct 8, 2019 at 9:43 AM Giovanni Gherdovich <ggherdovich@...e.cz> wrote:
>
> On Thu, 2019-10-03 at 19:53 +0200, Rafael J. Wysocki wrote:
> > On Thursday, October 3, 2019 2:15:37 PM CEST Peter Zijlstra wrote:
> > > On Thu, Oct 03, 2019 at 12:27:52PM +0200, Rafael J. Wysocki wrote:
> > > > On Wednesday, October 2, 2019 2:29:25 PM CEST Giovanni Gherdovich wrote:
> > > > > +static bool turbo_disabled(void)
> > > > > +{
> > > > > +       u64 misc_en;
> > > > > +       int err;
> > > > > +
> > > > > +       err = rdmsrl_safe(MSR_IA32_MISC_ENABLE, &misc_en);
> > > > > +       if (err)
> > > > > +               return false;
> > > > > +
> > > > > +       return (misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE);
> > > > > +}
> > > >
> > > > This setting may be updated by the platform firmware (BIOS) in some cases
> > > > (see kernel.org BZ 200759, for example), so in general checking it once
> > > > at the init time is not enough.
> > >
> > > Is there anything sane we can do if the BIOS frobs stuff like that under
> > > our feet? Other than yell bloody murder, that is?
> >
> > Sane?  No, I don't think so.
> >
> > Now, in principle *something* could be done to fix things up in the _PPC
> > notify handler, but I guess we would just end up disabling the scale
> > invariance code altogether in those cases.
>
> I'm looking at how to react to turbo being disabled at run time, assuming a
> _PPC notification is triggered in that case.
>
> I don't think the correct action would be to disable scale invariance: if the
> turbo range is not available, then max frequency is max_P, and scale
> invariance can go on using that. The case max_freq=max_P is represented by
> arch_max_freq=1024 in this patch (because arch_max_freq=max_freq*1024/max_P).

OK, so now you have the case when the BIOS enables turbo on the fly,
but then the scale invariance is not going to be enabled AFAICS.

> Since the variable arch_max_freq is global to all CPUs, the fact that the _PPC
> notification is sent to just one CPU is not a concern: the CPU receiving the
> notif will set arch_max_freq=1024 (Srinivas was worried about this in another
> message).
>
> This looks like a job for the ->update_limits callback you added to "struct
> cpufreq_driver" in response to the mentioned kernel.org BZ 200759.
> I see that only intel_pstate implements it, it's not clear to me yet if I'll
> have to give an ->update_limits to acpi_cpufreq as well to treat this case.

If you want acpi_cpufreq and intel_pstate to be consistent with each
other in that repsect, then yes.