linux-kernel - Re: cputime takes cstate into consideration

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Message-ID: <20190626192100.GP3419@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Jun 2019 21:21:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "kernellwp@...il.com" <kernellwp@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "ankur.a.arora@...cle.com" <ankur.a.arora@...cle.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: cputime takes cstate into consideration

On Wed, Jun 26, 2019 at 06:55:36PM +0000, Raslan, KarimAllah wrote:

> If the host is completely in no_full_hz mode and the pCPU is dedicated to a 
> single vCPU/task (and the guest is 100% CPU bound and never exits), you would 
> still be ticking in the host once every second for housekeeping, right? Would 
> not updating the mwait-time once a second be enough here?

People are trying very hard to get rid of that remnant tick. Lets not
add dependencies to it.

IMO this is a really stupid issue, 100% time is correct if the guest
does idle in pinned vcpu mode.