Re: Clocksource tsc unstable (delta = -4398046474878 ns)

[Athanasius <kvm@miggy.org>]

On Tue, Mar 30, 2010 at 10:08:28AM +0200, Sebastian Hetze wrote:
> On Mon, Mar 29, 2010 at 11:31:13AM +0100, Athanasius wrote:
> >   I didn't see any such messages but I've had a recent experience with
> > the time on one KVM host leaping *forwards* approx. 5 and 2.5 hours in
> > two separate incidents.  Eerily the exact jumps, as best I can tell from
> > logs are of 17592 and 8796 seconds, give or take a second or two.  If
> > you look at these as nanoseconds then that's 'exactly' 2^44 and 2^43
> > nanoseconds.
> >   What I've done that seems to have avoided this happening again is drop
> > KVM_CLOCK kernel option from the kvm guests' kernel.
> 
> To my understanding, kvm-clock is the best and most reliable clocksource
> available, so I do not think it is a good idea to disable it.
> 
> There is a lot of bit shift operation happening with the clocksources,
> so there may be a real bug hidden somewhere in the code.
> Somehow ntp adjustment is involved, can this cause such huge steps?
> Im my case, I actually have NTP running in the guest. However, the
> statistics show a pretty stable timing here.

  This is one thing thing to note, I *was* running ntpd in the affected
guest (and rather obviously, I still am).  If there's some bad
interaction between KVM_CLOCK and ntpd it needs documenting in the
first instance and preferably also fixing.

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	   "And it's me who is my enemy. Me who beats me up.
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