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RE: [Xen-devel] Timer going backwards and Unable to handle kernel NULLpointer

>>> "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxx> 28.05.07 22:03 >>>
>> I've got a Thunder K8SRE with two dual core Opteron processors and
>> eight 1 GB SDRAM sticks in it.
>> Booting in to SMP mode resulted in this:
>> May 16 00:43:37 weebl kernel: Timer ISR/3: Time went backwards:
>> delta=-12967620 delta_cpu=217015619 shadow=46263756493 off=423276508
>> processed=46700000000 cpu_processed=46470016761
>> May 16 00:43:37 weebl kernel:  0: 46680000000
>> May 16 00:43:37 weebl kernel:  1: 46700016761
>> May 16 00:43:37 weebl kernel:  2: 45670016761
>> May 16 00:43:37 weebl kernel:  3: 46470016761
>The only time I've seen this is on a system that was overheating and
>bouncing in and out of thermal throttling. Any chance that could be
>happening here?

I've been seeing these pretty regularly on a single-socket dual-core Athlon
system for the last couple of months, and only on Friday finally found time
to start looking into these. Besides the messages above, I also see hangs
in about every other boot attempt but only if I do *not* use serial output
(which makes debugging a little harder), and never once initial boot finished
- this is why I finally needed to find time to look into the problem. I shall
note though that the kernel we use does not disable CONFIG_GENERIC_TIME
and makes use of a Xen clocksource as posted by Jeremy among the
paravirt ops patches.
What happens when the hang occurs (in do_nanosleep context) is that the
time read/interpolated from the Xen provided values is in the past compared
to the last value read (and cached inside the kernel), resulting in a huge
timeout value rather than the intended 50ms one.
Without having collected data proving this (will do later today), I currently
think that the interpolation parameters are too imprecise until the first time
local_time_calibration() runs on each CPU, i.e. during little less than the 
second of dom0's life).


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