From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753337Ab2DPLcg (ORCPT ); Mon, 16 Apr 2012 07:32:36 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:43621 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753172Ab2DPLcf convert rfc822-to-8bit (ORCPT ); Mon, 16 Apr 2012 07:32:35 -0400 Message-Id: <4F8C1F6D020000780007E11A@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.0 Date: Mon, 16 Apr 2012 12:32:29 +0100 From: "Jan Beulich" To: "David Vrabel" Cc: "Thomas Gleixner" , "xen-devel" , "Konrad Rzeszutek Wilk" , , "Tim Deegan" , "Sheng Yang" Subject: Re: [PATCH] xen: always set the sched clock as unstable References: <1334341255-11153-1-git-send-email-david.vrabel@citrix.com> In-Reply-To: <1334341255-11153-1-git-send-email-david.vrabel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> On 13.04.12 at 20:20, David Vrabel wrote: > From: David Vrabel > > The sched clock was considered stable based on the capabilities of the > underlying hardware. This does not make sense for Xen PV guests as: > a) the hardware TSC is not used directly as the clock source; and b) > guests may migrate to hosts with different hardware capabilities. > > It is not clear to me whether the Xen clock source is supposed to be > stable and whether it should be stable across migration. For a clock > source to be stable it must be: a) monotonic; c) synchronized across > CPUs; and c) constant rate. > > There have also been reports of systems with apparently unstable > clocks where clearing sched_clock_stable has fixed problems with > migrated VMs hanging. > > So, always set the sched clock as unstable when using the Xen clock > source. > > Signed-off-by: David Vrabel > --- > arch/x86/xen/time.c | 1 + > 1 files changed, 1 insertions(+), 0 deletions(-) > > diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c > index 0296a95..8469b5a 100644 > --- a/arch/x86/xen/time.c > +++ b/arch/x86/xen/time.c > @@ -473,6 +473,7 @@ static void __init xen_time_init(void) > do_settimeofday(&tp); > > setup_force_cpu_cap(X86_FEATURE_TSC); > + sched_clock_stable = 0; This, unfortunately, is not sufficient afaict: If a CPU gets brought up post-boot, the variable may need to be cleared again. Instead you ought to call mark_tsc_unstable(). Jan > > xen_setup_runstate_info(cpu); > xen_setup_timer(cpu); > -- > 1.7.2.5