linux-kernel - Re: cputime takes cstate into consideration

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Message-ID: <1561575336.25880.7.camel@amazon.de>
Date:   Wed, 26 Jun 2019 18:55:36 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
CC:     "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, 2019-06-26 at 20:41 +0200, Thomas Gleixner wrote:
> On Wed, 26 Jun 2019, Konrad Rzeszutek Wilk wrote:
> > 
> > On Wed, Jun 26, 2019 at 06:16:08PM +0200, Peter Zijlstra wrote:
> > > 
> > > On Wed, Jun 26, 2019 at 10:54:13AM -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.
> > > 
> > > But then you generate extra noise and vmexits on those cpus, just to get
> > > this accounting sorted, which sounds like a bad trade.
> > 
> > Considering that the CPUs aren't doing anything and if you do say the 
> > IPIs "only" 100/second - that would be so small but give you a big benefit
> > in properly accounting the guests.
> 
> 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?

> 
> > 
> > But perhaps there are other ways too to "snoop" if a guest is sitting on
> > an MWAIT?
> 
> No idea.
> 
> Thanks,
> 
> 	tglx
> 
> 



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