Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

Avi Kivity
2009-01-20T12:37:26Z

Ingo Molnar wrote:
> * Kevin Shanahan <kmshanah-biM/RbsGxha6c6uEtOJ/EA@public.gmane.org> wrote:
>
>
>> On Mon, 2009-01-19 at 22:45 +0100, Rafael J. Wysocki wrote:
>>
>>> This message has been generated automatically as a part of a report
>>> of regressions introduced between 2.6.27 and 2.6.28.
>>>
>>> The following bug entry is on the current list of known regressions
>>> introduced between 2.6.27 and 2.6.28. Please verify if it still should
>>> be listed and let me know (either way).
>>>
>>>
>>> Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12465
>>> Subject : KVM guests stalling on 2.6.28 (bisected)
>>> Submitter : Kevin Shanahan <kmshanah-biM/RbsGxha6c6uEtOJ/EA@public.gmane.org>
>>> Date : 2009-01-17 03:37 (3 days old)
>>>
>> Yes, please keep this on the list.
>>
>
> This only seems to occur under KVM, right? I.e. you tested it with -no-kvm
> and the problem went away, correct?
>
> This suggests some sort of KVM-specific problem. Scheduler latencies in
> the seconds that occur under normal load situations are noticed and
> reported quickly - and there are no such open regressions currently.
>
>

Not necessarily. -no-kvm runs with only one thread, compared to kvm
that runs with 1 + nr_cpus threads.

> Avi, can you reproduce these latencies?

No.

> A possibly theory would be some
> sort of guest wakeup problem/race triggered by a shift in
> preemption/scheduling patterns. Or something related to preempt-notifiers
> (which KVM is using). A genuine scheduler bug is in the cards too, but the
> KVM-only angle of this bug gives it a low probability.
>

Can we trace task wakeups somehow? (latency between wakeup and actually
running).

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