linux-kernel - Re: Lost IPIs during CPU Hotplug

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Message-ID: <20130521145243.GA29150@osiris>
Date:	Tue, 21 May 2013 16:52:43 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Jens Axboe <jaxboe@...ionio.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Shaohua Li <shli@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: Lost IPIs during CPU Hotplug

On Mon, May 20, 2013 at 02:37:43PM +0200, Heiko Carstens wrote:
> I just got a dump from a system running a 3.0.something kernel, however I
> think the problem exists with current kernels as well.
> 
> Testcase was some I/O intense workload together with cpu hotplug stress.
> 
> When trying to bring a cpu online we got an endless loop on the cpu that
> issued the cpu_up and called smp_call_function_single() within its cpu
> hotplug notifier:

[...]

> It looks to me like the IPI(s) was lost when cpu 3 was brought down before:

[...]

> So it looks to me like yet another CPU_DYING cpu hotplug notifier is needed
> for the generic smp code, which looks for pending IPIs on the to be brought
> down cpu and executes them.
> 
> Does that make sense?

Ok, I was able to reproduce it. And the fix should be in the s390 specific
arch code within __cpu_disable() just before the cpu gets removed from the
cpu online mask.
No idea why this never has been seen before.

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