linux-kernel - Re: [Help Test] kdump, x86, acpi: Reproduce CPU0 SMI corruption issue after unsetting BSP flag

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Date:	Mon, 19 Aug 2013 15:46:26 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Mitchell, Lisa (MCLinux in Fort Collins)" <lisa.mitchell@...com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, bhelgaas@...gle.com,
	Jingbai Ma <jingbai.ma@...com>
Subject: Re: [Help Test] kdump, x86, acpi: Reproduce CPU0 SMI corruption
 issue after unsetting BSP flag

On Sun, 18 Aug 2013 19:59:53 -0700
"Eric W. Biederman" <ebiederm@...ssion.com> wrote:

> 
> 
> 
> >
> >Sorry Eric, I'm not clear to what you mean by ``short one core''...
> >Which are you suggesting? Disabling BSP if crash happens on AP is
> >reasonable?
> >Or restricting cpus to a single one only just as the current kdump
> >configuration is reasonable?
> 
> I am suggesting we start every cpu except the BSP from the AP we started on.
> 
> N-1 cpus seems like a good tradeoff between performance and reliability for those who need it.

FWIW a large customers of ours is fine with such a limitation. And I
have already tested this approach manually (starting the kdump kernel
with maxcpus=1 and hot-plugging the remaining APs from user-space).

Now that this approach is in line with upstream efforts, I'm going to
test it on some more machines and see if there are any troubles.

@Hatayama-san:
> BTW, I have question that does normal kdump work well if crash happens
> on some AP? I wonder the same issue could happen on the 2nd kernel.

I'm not sure what you mean. Normal kdump starts with "maxcpus=1", and
yes, that works even if the secondary kernel is booted from an AP. OTOH
I suspect that not having any BSP in the system may be the cause of some
mysterious random reboots and/or hangs experienced by some customers.

I'll try setting the BSP flag on the boot CPU unconditionally and see
if it makes any difference.

Petr Tesarik
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