Re: Xen 4 TSC problems

[Keir Fraser <keir.xen@gmail.com>]

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
>