From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rik van Riel <riel@redhat.com>
Subject: Re: vtime accounting
Date: Fri, 24 Mar 2017 11:04:38 -0400
Message-ID: <1490367878.28917.2.camel@redhat.com>
References: <20170308105700.GA109453@lvm>
 <20170313162259.GE18298@potion>
 <20170314082601.GC1277@cbox>
 <20170314165858.GA5435@potion>
 <20170314183913.GF1277@cbox>
 <20170314202721.GD5432@potion>
         <20170315084303.GJ1277@cbox>
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Cc: Christoffer Dall <christoffer.dall@linaro.org>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        Marc Zyngier <marc.zyngier@arm.com>, fweisbec@redhat.com
To: Christoffer Dall <cdall@linaro.org>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx1.redhat.com ([209.132.183.28]:39058 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1756898AbdCXPEr (ORCPT <rfc822;kvm@vger.kernel.org>);
        Fri, 24 Mar 2017 11:04:47 -0400
In-Reply-To: <20170315084303.GJ1277@cbox>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Wed, 2017-03-15 at 09:43 +0100, Christoffer Dall wrote:

> Yes, I think the key is whether you end up taking a timer interrupt
> before or after switchign PF_VCPU.  So you can spend X jiffies in the
> guest, come back, change PF_VCPU (timer still hasen't expired), and
> then
> the timer expires immediately afterwards, and the whole block of
> jiffies
> that are incremented as a result of the timer gets accounted as
> kernel
> time.
> 
> (Note that jiffies on clocksourced architectures is an arbitrary
> number,
> which somehow scales according to the clocksource frequency,
> depending
> on the CONFIG_HZ, so a single timer interrupt can increment jiffies
> by
> more than 1.
> 

That really should not happen with nohz_full.

The housekeeping CPU should get a timer interrupt
every jiffy, unless something changed recently that
I am not aware of.

Frederic, am I totally off base?