linux-kernel - Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

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Date:   Mon, 26 Oct 2020 14:52:37 -0500
From:   "Fontenot, Nathan" <Nathan.Fontenot@....com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Giovanni Gherdovich <ggherdovich@...e.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Mel Gorman <mgorman@...e.de>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Ingo Molnar <mingo@...hat.com>,
        kernel-janitors@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Gilles Muller <Gilles.Muller@...ia.fr>,
        srinivas.pandruvada@...ux.intel.com,
        Linux PM <linux-pm@...r.kernel.org>,
        Len Brown <len.brown@...el.com>, puwen@...on.cn,
        yazen.ghannam@....com, kim.phillips@....com,
        suravee.suthikulpanit@....com
Subject: Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

On 10/23/2020 12:46 PM, Tom Lendacky wrote:
> On 10/23/20 2:03 AM, Peter Zijlstra wrote:
>> On Thu, Oct 22, 2020 at 10:10:35PM +0200, Giovanni Gherdovich wrote:
>>> * for the AMD EPYC machines we haven't yet implemented frequency invariant
>>>    accounting, which might explain why schedutil looses to ondemand on all
>>>    the benchmarks.
>>
>> Right, I poked the AMD people on that a few times, but nothing seems to
>> be forthcoming :/ Tom, any way you could perhaps expedite the matter?
> 
> Adding Nathan to the thread to help out here.
> 
> Thanks,
> Tom

Thanks Tom, diving in...

> 
>>
>> In particular we're looking for some X86_VENDOR_AMD/HYGON code to run in
>>
>>    arch/x86/kernel/smpboot.c:init_freq_invariance()
>>
>> The main issue is finding a 'max' frequency that is not the absolute max
>> turbo boost (this could result in not reaching it very often) but also
>> not too low such that we're always clipping.

I've started looking into this and have a lead but need to confirm that the
frequency value I'm getting is not an absolute max.

>>
>> And while we're here, IIUC AMD is still using acpi_cpufreq, but AFAIK
>> the chips have a CPPC interface which could be used instead. Is there
>> any progress on that?
>>

Correct, AMD uses acpi_cpufreq. The newer AMD chips do have a CPPC interface
(not sure how far back 'newer' covers). I'll take a look at schedutil and
cppc_cpufreq and the possibility of transitioning to them for AMD.

-Nathan