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Re: [Xen-devel] Time skew on HP DL785 (and possibly other boxes)

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Time skew on HP DL785 (and possibly other boxes)
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Sun, 05 Apr 2009 14:27:48 +0100
Cc: "john.v.morris@xxxxxx" <john.v.morris@xxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 05/04/2009 13:17, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

>> One concern I have however, is Intel's
>> was added by them to prevent TSCs from diverging due to Cx deep sleep
>> states, by observing that usually all TSCs will tick at the
>> same exact rate,
> Here one correction is, that constant tsc logic is introduced for
> P-states instead of C-states, to have TSC always stepping in
> constant pace on a given processor, regardless of whatever
> opertion point is being requested by cpufreq governor. It
> doesn't say anything that all TSCs tick at same rate however.

Then changeset 18923 is indeed broken and should be reverted? The problem is
this changeset doesn't just affect the cases it is meant to 'fix' (usage of
C states for CPUs without no-stop TSC). Apart from the fact it can be broken
for systems with that type of CPU as well, it's actually enabled for any
modern CPU (anything advertising the constant-tsc feature). Probably I
shouldn't have checked in that patch in the first place.

 -- Keir

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