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Re: [Xen-devel] [patch 14/33] xen: xen time implementation

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [patch 14/33] xen: xen time implementation
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Wed, 06 Jun 2007 12:52:59 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>, Andi Kleen <ak@xxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>
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On 6/6/07 12:00, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> If the error across CPUS is +/- just a few microseconds at worst then having
>> the clocksource clamp to no less than the last timestamp returned seems a
>> reasonable fix. Time won't 'stop' for longer than the cross-CPU error, and
>> that should always be a tiny value.
> 
> Are you sure this is also true when e.g. a CPU gets throttled due to thermal
> conditions? It is my understanding that both the duty cycle adjustment and
> the frequency reduction would yield a reduced rate TSC, which would be
> accounted for only the next time the local clock gets calibrated. Otherwise,
> immediate calibration (and vcpu update) would need to be forced out of the
> thermal interrupt.

Yes, this could be an issue. Is there any way to get an interrupt or MCE
when thermal throttling occurs?

 -- Keir


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