linux-kernel - Re: cputime takes cstate into consideration

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Message-ID: <20190626200126.GQ3419@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Jun 2019 22:01:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
Cc:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "kernellwp@...il.com" <kernellwp@...il.com>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "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 07:27:35PM +0000, Raslan, KarimAllah wrote:
> On Wed, 2019-06-26 at 21:21 +0200, Peter Zijlstra wrote:
> > 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.
> 
> One use case for proper accounting (obviously for a slightly relaxed definition 
> or *proper*) is *external* monitoring of CPU utilization for scaling group
> (i.e. more VMs will be launched when you reach a certain CPU utilization).
> These external monitoring tools needs to account CPU utilization properly.

That's utter nonsense; what's the point of exposing mwait to guests if
you're not doing vcpu pinning. For overloaded guests mwait makes no
sense what so ever.