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Re: [Xen-devel] Xen 4 TSC problems

To: Keir Fraser <keir.xen@xxxxxxxxx>
Subject: Re: [Xen-devel] Xen 4 TSC problems
From: Olivier Hanesse <olivier.hanesse@xxxxxxxxx>
Date: Wed, 23 Feb 2011 20:04:25 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Mark Adams <mark@xxxxxxxxxxxxxxxxxx>
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I am sorry for the lack of information.
Every domUs on the dom0 are affected by this bug at the exact same time.

And I had this bug on a dozen servers (all running on the same hw) since October (when I switched from Xen 3.2 to 4.0).



Le 23/02/2011 18:19, Keir Fraser a écrit :
On 23/02/2011 16:16, "Dan Magenheimer"<dan.magenheimer@xxxxxxxxxx>  wrote:

It¹s very unlikely this is a problem with TSC. It is most likely a Xen (or
possibly a PV Linux) problem where a guest (or dom0) either ³goes out to
lunch² for a long period, or some other timer gets stuck.  The ³clocksource
tsc unstable² message is a side effect of this... it¹s very likely the TSC
that IS stable and correct and the other clocksource (pvclock) has lost/gained
50 minutes!

Mark Adams cc¹ed and his original xen-devel posting below.  The fact that two
different users (possibly on the same processor/system type?) have submitted
the message with a delta so similar would lead me to believe there is some
timer that is ³wrapping².  And since pvclock is usually the clocksource for
dom0, and pvclock is driven!  by Xen¹s ³system time², a reasonable guess is
that the timer that is wrapping is in Xen itself.

Mark¹s delta = -2999660303788 ns
Your delta = -2999660334211 ns

Googling, I see the HPET wraparound is ~306 seconds and this delta is about
3000 seconds, so that may be a bad guess.

Keir, any thoughts on this?  Do you recall any post-4.0 patches that may have
fixed this?
I've never seen a 3000s wrap, and I don't know of anything that would have
fixed a bug like this. If this is a Xen time wrap of some kind then it would
affect all running guests; it's not clear here whether only one, or all,
guests see the wrap.




From: Olivier Hanesse [mailto:olivier.hanesse@xxxxxxxxx]
Sent: Wednesday, February 23, 2011 3:50 AM
To: xen-devel@xxxxxxxxxxxxxxxxxx!  m; Xen Users
Subject: [Xen-devel] Xen 4 TSC problems


I've got an issue about time keeping with Xen 4.0 (Debian squeeze release).

My problem is here (hopefully I amn't the only one, so there might be a bug
somewhere) : http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=599161#50

After some times,  I got this error : Clocksource tsc unstable (delta =
-2999660334211 ns). It has happened on several servers.

Looking at the output of "xm debug-key s;"

(XEN) TSC has constant rate, deep Cstates possible, so not reliable, warp=2850

I am using a "Intel(R) Xeon(R) CPU L5420  @ 2.50GHz", which has the
"constant_tsc", but not the "nonstop_tsc" one.

On other systems with a newer cpu with "nonstop_tsc", I don't have this issue
(systems are running the same distros with same config).

I tried to boot with "max_cstate=0", but nothing changed, my TSC isn't
reliable and after some times, I will got the "50min" issue again.

I don't unders!  tand how a system can do a jump of "50min" in the future. Why
50min ? it is not 40min, not 1 hour, it is always 50min.

I don't know how to make my TSC "reliable" (I already disable everything about
Powerstate in BIOS Settings).

Any ideas ?



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