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Date: Mon, 9 Aug 2010 16:21:21 +0200
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
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        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] lockup_detector: Make DETECT_HUNT_TASK default depend
 on LOCKUP_DETECTOR
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On Mon, Aug 09, 2010 at 09:49:17AM -0400, Don Zickus wrote:
> > > Why s390 doesn't want the softlockup detector to begin with?
> > 
> > If I remember correctly then we disabled that back then because we got
> > false positives. The reason for those were that the softlockup detector
> > did not take steal time into account.
> > E.g. if a guest cpu runs for 10 seconds, but the hypervisor would steal
> > 9 seconds in order to run other guest cpus this specific cpu would still
> > think it ran for 10 seconds and therefore would generate invalid warnings.
> 
> I have learned recently that is applies to all virtual machines including
> KVM, Xen and VMWare(?).  However, you only see this when you overload the
> hypervisor with lots of guests.  Which is why you normally don't see this
> on those types of guests.

On s390 you always run virtualized and usually even as a 2nd level guest.
Overloading a machine is quite common here.
The problem we have is that you can't tell afterwards if a warning was valid
or invalid due to overloading. Imho it is just pointless without taking steal
time into account and that's why we disabled it.