linux-kernel - Re: [PATCH RFC] sched/fair: Penalty the cfs task which executes mwait/hlt

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Date:   Tue, 14 Jan 2020 18:53:19 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        KarimAllah <karahmed@...zon.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        christopher.s.hall@...el.com, hubert.chrzaniuk@...el.com,
        Len Brown <len.brown@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH RFC] sched/fair: Penalty the cfs task which executes mwait/hlt

On Mon, 13 Jan 2020 at 20:36, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Jan 13, 2020 at 12:52:20PM +0100, Paolo Bonzini wrote:
> > On 13/01/20 11:43, Peter Zijlstra wrote:
> > > So the very first thing we need to get sorted is that MPERF/TSC ratio
> > > thing. TurboStat does it, but has 'funny' hacks on like:
> > >
> > >   b2b34dfe4d9a ("tools/power turbostat: KNL workaround for %Busy and Avg_MHz")
> > >
> > > and I imagine that there's going to be more exceptions there. You're
> > > basically going to have to get both Intel and AMD to commit to this.
> > >
> > > IFF we can get concensus on MPERF/TSC, then yes, that is a reasonable
> > > way to detect a VCPU being idle I suppose. I've added a bunch of people
> > > who seem to know about this.
> > >
> > > Anyone, what will it take to get MPERF/TSC 'working' ?
> >
> > Do we really need MPERF/TSC for this use case, or can we just track
> > APERF as well and do MPERF/APERF to compute the "non-idle" time?
>
> So MPERF runs at fixed frequency (when !IDLE and typically the same
> frequency as TSC), APERF runs at variable frequency (when !IDLE)
> depending on DVFS state.
>
> So APERF/MPERF gives the effective frequency of the core, but since both
> stop during IDLE, it will not be a good indication of IDLE.
>
> Otoh, TSC doesn't stop in idle (.oO this depends on
> X86_FEATURE_CONSTANT_TSC) and therefore the MPERF/TSC ratio gives how
> much !idle time there was between readings.

Do you have a better solution to penalty vCPU process which mwait/hlt
executed inside? :)

    Wanpeng