From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)
Date: Tue, 20 Jan 2009 14:37:26 +0200
Message-ID: <4975C586.8090605@redhat.com>
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To: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
Cc: Kevin Shanahan <kmshanah-biM/RbsGxha6c6uEtOJ/EA@public.gmane.org>, "Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>, Linux Kernel Mailing List <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Kernel Testers List <kernel-testers-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Kevin Shanahan <kmshanah-IiIpDuVlHfMLO379cgqW9odd74u8MsAO@public.gmane.org>, Mike Galbraith <efault-Mmb7MZpHnFY@public.gmane.org>, Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>

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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