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Date:   Tue, 26 Jan 2021 11:02:45 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Giovanni Gherdovich <ggherdovich@suse.cz>
Cc:     Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Jon Grimm <Jon.Grimm@amd.com>,
        Nathan Fontenot <Nathan.Fontenot@amd.com>,
        Yazen Ghannam <Yazen.Ghannam@amd.com>,
        Thomas Lendacky <Thomas.Lendacky@amd.com>,
        Suthikulpanit Suravee <Suravee.Suthikulpanit@amd.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pu Wen <puwen@hygon.cn>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Michael Larabel <Michael@phoronix.com>, x86@kernel.org,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-acpi@vger.kernel.org
Subject: Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max_boost
 in schedutil invariant formula
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On Tue, Jan 26, 2021 at 10:28:30AM +0100, Giovanni Gherdovich wrote:
> On Mon, 2021-01-25 at 11:04 +0100, Peter Zijlstra wrote:
> > On Fri, Jan 22, 2021 at 09:40:38PM +0100, Giovanni Gherdovich wrote:
> > > This workload is constant in time, so instead of using the PELT sum we can
> > > pretend that scale invariance is obtained with
> > > 
> > >     util_inv = util_raw * freq_curr / freq_max1        [formula-1]
> > > 
> > > where util_raw is the PELT util from v5.10 (which is to say, not invariant),
> > > and util_inv is the PELT util from v5.11-rc4. freq_max1 comes from
> > > commit 976df7e5730e ("x86, sched: Use midpoint of max_boost and max_P for
> > > frequency invariance on AMD EPYC") and is (P0+max_boost)/2 = (2.25+3.4)/2 =
> > > 2.825 GHz.  Then we have the schedutil formula
> > > 
> > >     freq_next = 1.25 * freq_max2 * util_inv            [formula-2]
> > > 
> > > Here v5.11-rc4 uses freq_max2 = P0 = 2.25 GHz (and this patch changes it to
> > > 3.4 GHz).
> > > 
> > > Since all cores are busy, there is no boost available. Let's be generous and say
> > > the tasks initially get P0, i.e. freq_curr = 2.25 GHz. Combining the formulas
> > > above and taking util_raw = 825/1024 = 0.8, freq_next is:
> > > 
> > >     freq_next = 1.25 * 2.25 * 0.8 * 2.25 / 2.825 = 1.79 GHz
> > 
> > Right, so here's a 'problem' between schedutil and cpufreq, they don't
> > use the same f_max at all times.
> > 
> > And this is also an inconsistency between acpi_cpufreq and intel_pstate
> > (passive). IIRC the intel_pstate cpufreq drivers uses 4C/1C/P0 resp,
> > while ACPI seems to stick to P0 f_max.
> 
> That's correct. A different f_max is used depending on the occasion. Let me
> rephrase with:
> 
> cpufreq core asks the driver what's the f_max. What's the answer?
> 
> intel_pstate says: 1C

Oh indeed it does...

> acpi_cpufreq says: P0
> 
> scheduler asks the freq-invariance machinery what's f_max, because it needs to
> compute f_curr/f_max. What's the answer?
> 
> Intel CPUs: 4C in most cases, 1C on Atom, something else on Xeon Phi.
> AMD CPUs: (P0 + 1C) / 2.

Right, and thwn freq-invariance uses larger f_max than cpufreq uses for
frequency selection, we under select exactly like you found.