Messages in this thread | | | Subject | Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max_boost in schedutil invariant formula | From | Giovanni Gherdovich <> | Date | Tue, 26 Jan 2021 10:28:30 +0100 |
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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?
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