RE: [Xen-devel] xen tsc problems?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 13 Jul 2010, Dan Magenheimer wrote:
> > From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> > 
> > On 13/07/2010 15:37, "Stefano Stabellini"
> > <Stefano.Stabellini@xxxxxxxxxxxxx>
> > wrote:
> > 
> > > Does this mean that the host has some serious tsc issues?
> > > Can this be a symptom of a bug in xen?
> > > Suggestion are welcome.
> > 
> > The 's' and 't' debug key handlers will be useful to get an idea of how
> > stable host TSCs are.
> > 
> >  -- Keir
> 
> Also you can try max_cstate=0 as a Xen boot parameter to rule
> out power management screwing up the tsc.
> 
> > > Does this mean that the host has some serious tsc issues?
> 
> Probably.  But the default tsc_mode (0) is intended to hide all
> such issues.  Could you check the 's' debug-key output to
> ensure your guest is actually running with tsc_mode=0?
> 

this is the output of 's' and 't' without max_cstate=0:

(XEN) Synced stime skew: max=245ns avg=202ns samples=2 current=160ns
(XEN) Synced cycles skew: max=615 avg=577 samples=2 current=540 
(XEN) TSC has constant rate, deep Cstates possible, so not reliable, warp=0 
(count=2)                                         
(XEN) dom3(hvm): mode=0,ofs=0x2b2e19a77ea,khz=3000048,inc=1,vtsc count: 1211682 
total

this is the output of 's' and 't' with max_cstate=0:

(XEN) Synced stime skew: max=110ns avg=105ns samples=2 current=110ns
(XEN) Synced cycles skew: max=1020 avg=652 samples=2 current=285
(XEN) TSC has constant rate, no deep Cstates, passed warp test, deemed 
reliable, warp=0 (count=2)
(XEN) dom2(hvm): mode=0,ofs=0xb748091f5,khz=3000032,inc=1,vtsc count: 758954 
total

I still get the same warning from the guest.


I started to wonder why the guest is seeing such a big tsc warp when xen
is seeing 0, so I added more tracing and eventually I found out that the
value of v->arch.hvm_vcpu.stime_offset is significantly different
between the two vcpus and the difference increases after the scaling.
Then I added timer_mode=1 to my vm config file and the problem went
away.
I think that delay_for_missed_ticks shouldn't cause tsc scew in
the guest.



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