linux-kernel - Re: [V4 PATCH 4/4] x86/apic: Introduce noextnmi boot option

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Message-ID: <20151001062733.GL2881@worktop.programming.kicks-ass.net>
Date:	Thu, 1 Oct 2015 08:27:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
Cc:	Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
Subject: Re: [V4 PATCH 4/4] x86/apic: Introduce noextnmi boot option

On Thu, Oct 01, 2015 at 02:33:18AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > On Fri, Sep 25, 2015 at 08:28:11PM +0900, Hidehiro Kawai wrote:
> > > This patch introduces new boot option "noextnmi" which disables
> > > external NMI.  This option is useful for the dump capture kernel
> > > so that an HA application or administrator wouldn't mistakenly
> > > shoot down the kernel by NMI.
> > 
> > So that they can get really stuck when the crash kernel crashes, right?
> > ;-)
> 
> No, it is different from my intention.
> 
> `mistakenly' in the above means; they issue NMI due to a misconception
> that the monitored host is stuck in the 1st kernel while it is actually
> taking a crash dump in the 2nd kernel.  To avoid this kind of accident,
> there is a tool such as fence_kdump which notifies "I'm taking a crash
> dump, so don't send NMI" to the HA clustering software.  However, there
> is a time window between kernel panic and the notification.
> 
> "noextnmi" allows users to avoid this kind of accident all the time of
> 2nd kernel.

Yes yes, I understand. But if the crash kernel also gets stuck they have
no means of recovery, right? (other than power cycling the hardware)

Just playing devils advocate here, I don't actually object to the patch.
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