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

From: Giovanni Gherdovich <ggherdovich@suse.cz>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Peter Zijlstra <peterz@infradead.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
	Len Brown <lenb@kernel.org>,
	x86@kernel.org, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Mel Gorman <mgorman@techsingularity.net>,
	Matt Fleming <matt@codeblueprint.co.uk>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Juri Lelli <juri.lelli@redhat.com>, Paul Turner <pjt@google.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Quentin Perret <qperret@qperret.net>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Doug Smythies <dsmythies@telus.net>
Subject: Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance
Date: Tue, 08 Oct 2019 09:48:39 +0200	[thread overview]
Message-ID: <1570520919.25071.2.camel@suse.cz> (raw)
In-Reply-To: <1990043.dY4KdrEkPr@kreacher>

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).

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.

Giovanni