Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max_boost in schedutil invariant formula

[Giovanni Gherdovich <ggherdovich@suse.cz>]

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
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.


Legend:
1C = 1-core boost
4C = 4-cores boost
P0 = max non-boost P-States

> 
> Rafael; should ACPI change that behaviour rather than adding yet another
> magic variable?