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Re: [Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 domU time
From: Mark Adams <mark@xxxxxxxxxxxxxxxxxx>
Date: Tue, 26 Oct 2010 10:22:54 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On Thu, Oct 07, 2010 at 07:04:18AM -0700, Dan Magenheimer wrote:
> Hi Jeremy and Mark --
> Oddly, I saw that "clocksource tsc unstable" message myself
> on a busy 2.6.36-rc5 PV domain yesterday.  While it is possible
> that this reflects a hardware problem, the fact that you
> saw it on a Nehalem+ Intel processor makes it very unlikely.
> The "s" and "t" debug keys (the output of which can be seen via
> "xm debug-key s; xm dmesg | tail" in dom0) can help diagnose
> the problem if it is indeed a hardware problem or BIOS
> problem or the result of a CPU hot-add... all unlikely.
> It IS possible that the code that emulates tsc is broken
> somewhere, but I don't think tsc should be emulated by
> default for dom0 on a Nehalem+ box... and even if it is,
> it is directly based on Xen system time which, if it went
> awry, would probably cause major problems.
> Looking through the Linux code that prints that message (in
> kernel/time/clocksource.c) it appears that the message
> appears if the tsc deviates from the "watchdog clocksource",
> which in PV domains is "xen" (or more precisely pvclock
> I think).  So most likely, this is a symptom of a problem
> with pvclock or the watchdog code in the pvops kernel, not
> an indicator that the tsc is actually unstable.
> Dan

Is there any more information I can provide to help with debugging this?
We haven't had the problem since. It could just be a coincidence but it
happened around the time that daylight savings occurred in the US (we
are in the UK).


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