Re: [Xen-devel] time jumping back and forth

[Keir F. <Kei...@cl...>]

> 
> I am have been seeing xen0 time jump around all weekend.   I have not been
> able to pin down a way to reproduce the problem.   
> 
> It will appear like the clocks are drifting unlocked from ntp and then
> suddenly most are right on time.  A while later some clocks are off an
> hour and some are fine.   Sometimes checking or setting hwclock makes
> thing jump.  Some hosts seem more prone to it than others.
> 
> This is with the today's pull, changeset 1.1347 and running ntpd 4.2.0a
> I'm wondering if ntpd is having trouble synchronizing to its server.

Sounds possible. I think the problem is that the kernel's STA_UNSYNC
flag is changing, and so DOM0 is sometimes pulling wallclock time from
Xen and sometimes from the hardware clock or an NTP server. If any of
these is wildly out (e.g., the CMOS clock, by an hour because of
daylight saving) then time will be very screwed.

I've checked in a fix which stops DOM0 from ever pulling time from Xen
-- after boot time, no good can ever come of it as Xen's timebase is
no more accurate than DOM0's.

More generally -- if anyone sees problems with time jumping in DOM >0,
it may be because you are running ntpd in that domain. It is only
necessary to run ntpd in DOM0, but if you *must* run ntpd in others
then add 'independent_wallclock' to the kernel command line when you
boot them.

 -- Keir