linux-kernel - Re: cputime takes cstate into consideration

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Date:   Wed, 26 Jun 2019 20:41:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        KarimAllah <karahmed@...zon.de>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: cputime takes cstate into consideration

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.

> But perhaps there are other ways too to "snoop" if a guest is sitting on
> an MWAIT?

No idea.

Thanks,

	tglx