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Re: [Xen-devel] pv_ops dom0 clock only runs under load

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] pv_ops dom0 clock only runs under load
From: Milan Holzäpfel <listen@xxxxxxxx>
Date: Fri, 6 Mar 2009 22:27:52 +0100
Cc: Milan Holzäpfel <mail@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On Fri, 6 Mar 2009 21:46:49 +0100
Milan Holzäpfel <listen@xxxxxxxx> wrote:

> On Thu, 05 Mar 2009 21:41:44 -0800
> Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> > Milan Holzäpfel wrote:
> > 
> > > The clock only seems to run if the system is under load (e.g. run
> > > "while true; do echo -n; done" in a shell).  
> > > With the CPU loaded and the clock running everything seems to work
> > > quite well (hard disk and network access, that is.)
> > >
> > > Changing the "xen" clocksource to "tsc" via sysfs didn't make a
> > > difference.  Enabling or disabling the "tickless" option doesn't make a
> > > difference either. 
> > >   
> > 
> > That's pretty mysterious; it does suggest your cpu's tsc is not ticking.
> Excerpt from the dmesg (2.6.29 pvops with / without Xen)
> [    4.870697] Marking TSC unstable due to TSC halts in idle
> Saw it scrolling by.
> [...]
> By the way, I booted the 2.6.29 pvops enabled kernel without Xen and
> the tsc timesource only increments when the CPU was loaded, too.  On
> 2.6.18 (without Xen) it worked normally (also with the BIOS options
> enabled).
> [...]
> I will try whether disabling kernel stuff (there is something called
> "cpuidle" compiled in) makes the tsc clocksource work again. 

Booting with idle=poll appended to the kernel command line makes the
TSC work again. 

The "xen" clocksource of the 2.6.29 pvops kernel (under Xen 3.3.1)
works again.  2.6.29 pvops on "bare metal" chooses TSC as default time
source, which works too. 

Doesn't seem like a real solution, does it? The CPU doesn't seem to run
at full load all the time though (temperature stays quite low).

Another question: The xm dmesg I posted reports "(XEN) Platform timer
is 14.318MHz HPET".  Why is the TSC used for time keeping then? The
HPET works perfectly on every kernel not running under Xen. 


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