linux-kernel - Re: [PATCH 2/3] [BUGFIX] x86/x86_64: fix CPU offlining triggered inactive device IRQ interrruption

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Message-ID: <m1r601fg9l.fsf@fess.ebiederm.org>
Date:	Thu, 09 Apr 2009 18:29:10 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Gary Hade <garyhade@...ibm.com>
Cc:	mingo@...e.hu, mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
	x86@...nel.org, linux-kernel@...r.kernel.org, lcm@...ibm.com
Subject: Re: [PATCH 2/3] [BUGFIX] x86/x86_64: fix CPU offlining triggered inactive device IRQ interrruption

Gary Hade <garyhade@...ibm.com> writes:

> Impact: Eliminates a race that can leave the system in an
>         unusable state
>
> During rapid offlining of multiple CPUs there is a chance
> that an IRQ affinity move destination CPU will be offlined
> before the IRQ affinity move initiated during the offlining
> of a previous CPU completes.  This can happen when the device
> is not very active and thus fails to generate the IRQ that is
> needed to complete the IRQ affinity move before the move
> destination CPU is offlined.  When this happens there is an
> -EBUSY return from __assign_irq_vector() during the offlining
> of the IRQ move destination CPU which prevents initiation of
> a new IRQ affinity move operation to an online CPU.  This
> leaves the IRQ affinity set to an offlined CPU.
>
> I have been able to reproduce the problem on some of our
> systems using the following script.  When the system is idle
> the problem often reproduces during the first CPU offlining
> sequence.

You appear to be focusing on the IBM x460 and x3835.  Can you describe
what kind of interrupt setup you are running.

You may be the first person to actually hit the problems with cpu offlining
and irq migration that have theoretically been present for a long.

Eric
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