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Message-ID: <alpine.DEB.2.21.1906262206160.32342@nanos.tec.linutronix.de>
Date: Wed, 26 Jun 2019 22:09:03 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
cc: "Raslan, KarimAllah" <karahmed@...zon.de>,
"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>,
"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, 26 Jun 2019, Peter Zijlstra wrote:
> 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.
I think you misunderstood. The guests are pinned. What they can do today is
monitor the guests utilization time through mwait/vmexit. If that goes over
a certain threshold they can automatically launch more VMs to spread the
load.
With MWAIT in the guest this is gone...
Thanks,
tglx
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