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RE: [Xen-devel] Time went backwards

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, "Niraj Tolia" <ntolia@xxxxxxxxx>, "Xen Developers" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Time went backwards
From: "Langsdorf, Mark" <mark.langsdorf@xxxxxxx>
Date: Fri, 15 Feb 2008 12:04:00 -0600
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Thread-topic: [Xen-devel] Time went backwards
> > I have been experimenting with frequency scaling on a 4 core (2
> > sockets) AMD Opteron processors. The versions of Xen I have tested
> > with include xen-testing as well as xen-unstable (cs 
> 17042). For both
> > these versions, I am using the linux 2.6.18-8 tree and have passed
> > cpufreq=dom0-kernel to the xen command line.
> > 
> > However, I get plenty of "Time went backwards" whenever I attempt to
> > change frequency. Does anyone know how this can be fixed?
> > 
> > % grep name /proc/cpuinfo | head -1
> > model name      : Dual-Core AMD Opteron(tm) Processor 2216 HE
> > 
> > powernow-k8: error - out of sync, fix 0x10 0x10, vid 0xe 0xc
> > powernow-k8: error - out of sync, fix 0x10 0x2, vid 0xc 0x12
> Presumably you have cpufreq=dom0-kernel on your Xen command 
> line? Apart from
> that noone else has reported 'out of sync' messages from the powernow
> driver. Mark may have some idea what that could be about.

He's got a dual-core system, but the logical grouping of the
cores doesn't match the physical grouping.  Something like
cores 0,1 share frequencies, but PowerNow! believes that 
cores 0 and 2 do.  So when a frequency change on core 0
occurs, the Xen hypervisor gets alerted that cores 0 and 2
have new frequencies.  This causes two errors with Xen -
the HV believes core 1 has the old frequency, when it
has the new frequency, and it believes core 2 has the new
frequency, when it really has the old frequency.  Time errors
will accumulate rapidly.

Niraj -

Please email me privately.  I have some more time to work on 
this issue and some new ideas on how to get information about it.

-Mark Langsdorf
Operating System Research Center

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