RE: [Xen-devel] [PATCH] Fix softlockup issue after vcpu hotplug

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

Actually I'm a bit interested in this case, where watchdog thread 
depends on timer interrupt to be awaken, while next timer interval 
depends on soft timer wheel. For the new online cpu, all its 
processes previously running have been migrated to others before 
offline. Thus when just coming back online, there may be no 
meaningful timer wheel and few activities on that vcpu. In this case, 
a (LONG_MAX >> 1) may be returned as a big timeout.

So saying this new watchdog model, simply walking timer wheel is 
not enough. Maybe we can force max timeout value to 1s in safe_halt 
to special this case? I'll make a try on this. But this will make current 
tick-less model to a bit tick-ful back. :-)

Thanks,
Kevin


>From: Tian Kevin
>Sent: 2007年1月30日 20:12
>
>>
>>Perhaps there is a bug in our cpu onlining code -- a big timeout like that
>>does need investigating. I don't think we can claim this bug is
>>root-caused
>>yet so it's premature to be applying patches.
>>
>
>Agree. I'll do more investigation on this point. Just quickly compared
>the watchdog thread between 2.6.18 and 2.6.16. Previously in 2.6.16,
>an explicit schedule timeout with 1s is used, while 2.6.18 wakes up
>the watchdog thread per second from timer interrupt (softlockup_tick).
>One distinct difference on this change is, watchdog thread in 2.6.16
>will have a soft timer registered while 2.6.18 not. I'm doubting that
>this may make some difference to decision of next_timer_interrupt.
>
>By the way, do you think whether scheduler may do something to
>punish new-online vcpu? Just from code, I didn't see that since new
>awaken vcpu is always boosted... However in the actual, I found
>that virtual timer interrupt number increased slowly for that cpu by
>'cat /proc/interrupts'. Sometimes it may even freeze for dozen of
>seconds. But yes, this may the phenomenon instead of reason. :-)
>
>Thanks,
>Kevin
>

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