linux-kernel - Re: cputime takes cstate into consideration

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Date:   Tue, 9 Jul 2019 11:27:54 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Raslan, KarimAllah" <karahmed@...zon.de>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kernellwp@...il.com" <kernellwp@...il.com>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: cputime takes cstate into consideration

On 7/9/19 5:38 AM, Peter Zijlstra wrote:
> On Mon, Jul 08, 2019 at 07:00:08PM -0700, Ankur Arora wrote:
>> On 2019-06-26 12:23 p.m., Thomas Gleixner wrote:
>>> On Wed, 26 Jun 2019, Raslan, KarimAllah wrote:
>>>> On Wed, 2019-06-26 at 10:54 -0400, Konrad Rzeszutek Wilk wrote:
>>>>> There were some ideas that Ankur (CC-ed) mentioned to me of using the perf
>>>>> counters (in the host) to sample the guest and construct a better
>>>>> accounting idea of what the guest does. That way the dashboard
>>>>> from the host would not show 100% CPU utilization.
>>>>
>>>> You can either use the UNHALTED cycles perf-counter or you can use MPERF/APERF
>>>> MSRs for that. (sorry I got distracted and forgot to send the patch)
>>>
>>> Sure, but then you conflict with the other people who fight tooth and nail
>>> over every single performance counter.
>> How about using Intel PT PwrEvt extensions? This should allow us to
>> precisely track idle residency via just MWAIT and TSC packets. Should
>> be pretty cheap too. It's post Cascade Lake though.
> 
> That would fully claim PT just for this stupid accounting thing and be
> completely Intel specific.
> 
> Just stop this madness already.
I see the point about just accruing guest time (in mwait or not) as
guest CPU time.
But, to take this madness a little further, I'm not sure I see why it
fully claims PT. AFAICS, we should be able to enable PwrEvt and whatever
else simultaneously.

Ankur

>