Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 26 Jun 2019 21:29:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     "Raslan, KarimAllah" <karahmed@amazon.de>,
        "konrad.wilk@oracle.com" <konrad.wilk@oracle.com>,
        "boris.ostrovsky@oracle.com" <boris.ostrovsky@oracle.com>,
        "joao.m.martins@oracle.com" <joao.m.martins@oracle.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "kernellwp@gmail.com" <kernellwp@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mtosatti@redhat.com" <mtosatti@redhat.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "ankur.a.arora@oracle.com" <ankur.a.arora@oracle.com>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>
Subject: Re: cputime takes cstate into consideration
In-Reply-To: <20190626192100.GP3419@hirez.programming.kicks-ass.net>
Message-ID: <alpine.DEB.2.21.1906262123440.32342@nanos.tec.linutronix.de>
References: <CANRm+Cyge6viybs63pt7W-cRdntx+wfyOq5EWE2qmEQ71SzMHg@mail.gmail.com> <alpine.DEB.2.21.1906261211410.32342@nanos.tec.linutronix.de> <20190626145413.GE6753@char.us.oracle.com> <20190626161608.GM3419@hirez.programming.kicks-ass.net>
 <20190626183016.GA16439@char.us.oracle.com> <alpine.DEB.2.21.1906262038040.32342@nanos.tec.linutronix.de> <1561575336.25880.7.camel@amazon.de> <20190626192100.GP3419@hirez.programming.kicks-ass.net>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="8323329-1592477559-1561577343=:32342"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

  This message is in MIME format.  The first part should be readable text,
  while the remaining parts are likely unreadable without MIME-aware tools.

--8323329-1592477559-1561577343=:32342
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8BIT

On Wed, 26 Jun 2019, 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.

Correct. We are going to see the same issue with UMWAIT/UMONITOR. If the
timeout is set long enough by the admin, then a task can stay in user mode
UMWAIT for a very long time. And we're going to account that as user time.

That's not any different with a guest.

You might go there and establish a shared page with the guest where the
guest drops his internal accounting information. For trusted guests that
might be a good approximation. For untrusted ones not so much, but then you
just have to say, you occupy the CPU 100% in guest mode. If you idle there,
none of my problems.

Thanks,

	tglx

--8323329-1592477559-1561577343=:32342--