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

To: Mark Adams <mark@xxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 domU time
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 26 Oct 2010 10:03:07 -0700
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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 On 10/26/2010 02:22 AM, Mark Adams wrote:
> 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).

In Linux/Xen it shouldn't have any effect since the clocks are always
maintained in UTC, then timezone details are applied much later in
usermode.  But Windows has a bad habit of setting the hardware RTC to
local time, and mucking about with it for DST changes - but that would
only be relevant if you booted Windows on your host machine (I don't
think there's any way for a Windows guest's time to leak into the
host/dom0's timebase).

Unfortunately these kinds of time problems can be notoriously hard to
pin down and diagnose.


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