linux-kernel - Re: cputime takes cstate into consideration

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Date:   Tue, 10 Dec 2019 08:44:25 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "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>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: cputime takes cstate into consideration

On Thu, 27 Jun 2019 at 03:27, Raslan, KarimAllah <karahmed@...zon.de> 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.

Except cputime accounting, the other gordian knot is qemu main loop,
libvirt, kthreads etc can't be offload to the other hardware like
smart nic, these stuff will contend with vCPUs even if MWAIT/HLT
instructions are executing in the guest. There is a HLT activity state
in CPU VMCS which indicates the logical processor is inactive because
it executed the HLT instruction, but SDM 24.4.2 mentioned that
execution of the MWAIT instruction may put a logical processor into an
inactive state, however, this VMCS field never reflects this state.

    Wanpeng