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

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Date:	Wed, 06 Jun 2007 12:20:07 +0200
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<virtualization@...ts.osdl.org>,
	"Xen-devel" <xen-devel@...ts.xensource.com>,
	"Chris Wright" <chrisw@...s-sol.org>, "Andi Kleen" <ak@...e.de>,
	"lkml" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [patch 14/33] xen: xen time implementation

>>  But I think that a clock source can be expected to be
>> monotonic anyway, which Xen's interpolation mechanism doesn't
>> guarantee across multiple CPUs. (I'm actually beginning to think that
>> this might also be the reason for certain test suites occasionally reporting
>> timeouts to fire early.)
>>   
>
>Does the kernel expect the tsc clocksource to be completely monotonic
>across cpus?  Any form of cpu-local clocksource is going to have this
>problem; I wonder if clocksources can really only be useful if they're
>always referring to a single system-wide time reference - seems like a
>bit of a limitation.

I suppose so - the clock source's rating gets set to zero if it can be predicted
that the TSCs aren't all synchronized, which should pretty much exclude the
use of this clock source for any other than fallback if there's really nothing
else available.

Jan

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