Re: [PATCH v4 1/6] x86,sched: Add support for frequency invariance

From: Giovanni Gherdovich <ggherdovich@suse.cz>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ionela Voinescu <ionela.voinescu@arm.com>,
	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>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	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 v4 1/6] x86,sched: Add support for frequency invariance
Date: Thu, 19 Dec 2019 21:27:44 +0100	[thread overview]
Message-ID: <1576787264.8929.10.camel@suse.cz> (raw)
In-Reply-To: <20191218193429.GH11457@worktop.programming.kicks-ass.net>

On Wed, 2019-12-18 at 20:34 +0100, Peter Zijlstra wrote:
> On Fri, Dec 06, 2019 at 12:57:46PM +0100, Giovanni Gherdovich wrote:
> > > On Wednesday 13 Nov 2019 at 13:46:49 (+0100), Giovanni Gherdovich wrote:
> > > I see above that you enable the static key (and therefore frequency
> > > invariance before setting the max frequency ratio (if possible) and
> > > before you initialise the counter references. Is there any reason for
> > > doing this?
> > This is a fair point; mine was a deliberate choice but you're the second
> > person making this remark (Peter Zijlstra also suggested I find the max
> > frequency before I set the static key), so it appears this design is
> > unpopular
> > enough to warrant a change in v5.
> 
> You actually 'fix' this in the next patch. I thought it was a patch
> management 'fail' that it didn't end up in this patch.

Uhm. I'm not sure I agree; let me paste the function intel_set_cpu_max_freq
after the entire series is applied:

> static void intel_set_cpu_max_freq(void)
> {
>         u64 ratio = 1, turbo_ratio = 1;
> 
>         if (slv_set_cpu_max_freq(&ratio, &turbo_ratio))
>                 goto set_value;
> 
>         if (glm_set_cpu_max_freq(&ratio, &turbo_ratio))
>                 goto set_value;
> 
>         if (knl_set_cpu_max_freq(&ratio, &turbo_ratio))
>                 goto set_value;
> 
>         if (skx_set_cpu_max_freq(&ratio, &turbo_ratio))
>                 goto set_value;
> 
>         core_set_cpu_max_freq(&ratio, &turbo_ratio);
>

let's say that all functions return false; as I don't check the return value of
the last one, you can very well end up here with 'ratio' and 'turbo_ratio'
that are still untouched, =1 since their initialization, and I would go on and
set the static key anyway (because I previously checked X86_FEATURE_APERFMPERF).

Right?

> set_value:
>         arch_max_turbo_freq = div_u64(turbo_ratio * SCHED_CAPACITY_SCALE,
>                                         ratio);
>         set_arch_max_freq(turbo_disabled());
>         static_branch_enable(&arch_scale_freq_key);
> }

But again, not only people disagree with this behavior, it's probably a little
misleading too in how it's written. Changing in v5.

Giovanni