linux-kernel - Re: cputime takes cstate into consideration

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Message-ID: <alpine.DEB.2.21.1906262115530.32342@nanos.tec.linutronix.de>
Date:   Wed, 26 Jun 2019 21:19:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
cc:     "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>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "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, 26 Jun 2019, Raslan, KarimAllah wrote:
> On Wed, 2019-06-26 at 20:41 +0200, Thomas Gleixner wrote:
> > The host doesn't know what the guest CPUs are doing. And if you have a full
> > zero exit setup and the guest is computing stuff or doing that network
> > offloading thing then they will notice the 100/s vmexits and complain.
> 
> 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?

It maybe that it 'still' does that, but the goal is to fix that by doing
remote accounting. I think Frederic is pretty close to that.

Then your 'lets do accounting' on the housekeeping tick falls apart.

And even with that tick every second, the nohz full people take every
shortcut to go back into the guest ASAP. Doing a dozen MSR reads will
surely not find many enthusiastic supporters.

Thanks,

	tglx