Re: Xen 4 TSC problems

[Olivier Hanesse <olivier.hanesse@gmail.com>]

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).

Regards

Olivier

Le 23/02/2011 18:19, Keir Fraser a écrit :
> On 23/02/2011 16:16, "Dan Magenheimer"<dan.magenheimer@oracle.com>  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.
>
>   K.
>
>> Thanks,
>> Dan
>>
>> References:
>> http://lists.xensource.com/archives/html/xen-devel/2010-10/msg00210.html
>> https://lkml.org/lkml/2010/10/26/126
>>
>>
>> From: Olivier Hanesse [mailto:olivier.hanesse@gmail.com]
>> Sent: Wednesday, February 23, 2011 3:50 AM
>> To: xen-devel@lists.xensource.co!  m; Xen Users
>> Subject: [Xen-devel] Xen 4 TSC problems
>>
>>
>> Hello
>>
>>
>>
>> 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
>> (count=3)
>>
>>
>>
>> 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 ?
>>
>>
>>
>> Regards
>>
>>
>>
>> Olivier
>>
>