[Xen-devel] Still Time went backwards problems Xen 3.3.1/2.6.18.8

["Carsten Schiers" <carsten@xxxxxxxxxx>]

Still have problems with "Time went backwards" messages. Now I am nearly 
completely at
Xen 3.3.1 and Xen kernel 2.6.18.8 for Dom0 and nearly all DomUs. There 
is only one DomU
That is on a customized special kernel for Endian (2.6.21.7-2).

Q1: Can a DomU kernel version anyhow be responsible for that effect?

Attached you can see the last two days. I noticed that they occur every 
five minutes, 
so it's probably connected with the munin cron job that collects xen 
domain usage and
cpufreq-stats. Nothing more. Delta is still quite huge, around 40 ms. 
Value is negative.

Q2: Is that any better than if it's positive?
Q3: Will the delta be reset or will it go up and down all the time?

Currently I use both cpufreq=dom0 and cpuidle. 

Q4: Can the problem eventually be related to either of them only?

Right now, I disabled cpuidle for the time being, to see whether there 
is a difference.
If not, I intend to use xen-unstable next. I attached the current xm 
dmesg and xm info,
so don't be surprised to miss cpuidle.

Well, I am not sure whether I do too much work on this issue, but I 
think the log message
is there for a reason. 

Best Regards,
Carsten.

P.S.: There are combinations that are much worse that this. Especially 
running Xen 3.3.1
and actual Lenny kernel seems to be worse.

-----Ursprüngliche Nachricht-----
Von: Langsdorf, Mark [mailto:mark.langsdorf@xxxxxxx] 
Gesendet: Donnerstag, 15. Januar 2009 20:27
An: Carsten Schiers
Betreff: RE: RE: RE: RE: [Xen-devel] AMD P-States not recognized for Xen 
3.3 and 3.4

If you have a single socket system, then fixing the
out of sync issue will probably not fix the TSC issue.

If you have a dual or quad socket system, fixing the
out of sync error should reduce the frequency and
magnitude of the TSC issues.  At least, that's the
best I can decipher from my notes.

Also, most of the work I did on cpufreq was in 2007, 
before Dan M. greatly redid Xen hypervisor time-keeping.
If an up-to-date kernel/hypervisor still has TSC problems,
it may be possible to fix them.  I don't really have 
time to research it myself, but if you do have problems, 
please contact me and I'll see what can be done.

-Mark Langsdorf
Operating System Research Center
AMD

> -----Original Message-----
> From: Carsten Schiers [mailto:carsten@xxxxxxxxxx] 
> Sent: Thursday, January 15, 2009 1:17 PM
> To: Langsdorf, Mark
> Subject: AW: RE: RE: RE: [Xen-devel] AMD P-States not 
> recognized for Xen 3.3 and 3.4
> 
> Thanks Mark, don't want to get on your nerve, but this will 
> only fix the
> powernow-k8 out of synch issue but not the fact that my 4050e 
> will have
> TSC issues between the two cores when switching frequencies, right? 
> 
> Am I right that there is no cure for that currently? And that 
> I am not 
> safe
> to ignore them, because some kernel wizard is synchronizing 
> them? 'Cause
> up to now I only found a synch in boot.c.
> 
> Sorry for all those questions, it's quite complicated. My 
> best times as
> code producer habe been some 15 years ago...
> 
> Thanks,
> Carsten.
> 
> -----Ursprüngliche Nachricht-----
> Von: Langsdorf, Mark [mailto:mark.langsdorf@xxxxxxx] 
> Gesendet: Donnerstag, 15. Januar 2009 18:23
> An: Carsten Schiers
> Betreff: RE: RE: RE: [Xen-devel] AMD P-States not recognized 
> for Xen 3.3 
> and 3.4
> 
> > Thank you Mark. Please excuse my small knowledge in 
> > kernel/xen hacking.  I understood that I should either 
> > 
> >   a) create a build of the actual xen-unstable.hg together with 
> > linux-2.6.18-xen.hg kernel (where you think to have fixed 
> the problem)
> >
> > or 
> > 
> >   b) to include the mentioned fixes into my environment 
> (which is the 
> > 3.2.1 Xen and Bastian Blank's (waldi) kernel (based on 2.6.18, too),
> > right?
> > 
> > I'll try out what will be more easy, but I think it should be a).
> 
> Right.  Using the latest unstable kernel & hypervisor gives
> you all the patches.  If you're backporting support into
> the waldi kernel, you need to make sure you backport all
> the relevant patches.
> 
> -Mark Langsdorf
> Operating System Research Center
> AMD
> 
> 
> 
> 
>

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