linux-kernel - Re: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop

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Message-ID: <87ppkzk5zi.fsf@tassilo.jf.intel.com>
Date:	Wed, 02 Apr 2014 10:15:29 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de,
	paul.gortmaker@...driver.com, JBeulich@...e.com, prarit@...hat.com,
	drjones@...hat.com, toshi.kani@...com, riel@...hat.com,
	gong.chen@...ux.intel.com
Subject: Re: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop

Igor Mammedov <imammedo@...hat.com> writes:

> Hang is observed on virtual machines during CPU hotplug,
> especially in big guests with many CPUs. (It reproducible
> more often if host is over-committed).
>
> It happens because master CPU gives up waiting on
> secondary CPU and allows it to run wild. As result
> AP causes locking or crashing system. For example
> as described here: https://lkml.org/lkml/2014/3/6/257
>
> If master CPU have sent STARTUP IPI successfully,
> make it wait indefinitely till AP boots.


But what happens on a real machine when the other CPU is dead?

I've seen that. Kernel still boots. With your patch it would
hang.

I don't think you can do that. It needs to have some timeout.
Maybe a longer or configurable one?

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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