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

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Date:	Wed, 06 Jun 2007 12:52:59 +0100
From:	Keir Fraser <keir@...source.com>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	<virtualization@...ts.osdl.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [patch 14/33] xen: xen time implementation




On 6/6/07 12:00, "Jan Beulich" <jbeulich@...ell.com> 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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