linux-kernel - RE: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP

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Date:	Tue, 16 Oct 2012 04:51:36 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"x86@...nel.org" <x86@...nel.org>
CC:	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"Brown, Len" <len.brown@...el.com>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>
Subject: RE: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP

> -----Original Message-----
> From: HATAYAMA Daisuke [mailto:d.hatayama@...fujitsu.com]
> Sent: Monday, October 15, 2012 9:35 PM
> To: linux-kernel@...r.kernel.org; kexec@...ts.infradead.org;
> x86@...nel.org
> Cc: mingo@...e.hu; tglx@...utronix.de; hpa@...or.com; Brown, Len; Yu,
> Fenghua; vgoyal@...hat.com; ebiederm@...ssion.com;
> grant.likely@...retlab.ca; rob.herring@...xeda.com;
> d.hatayama@...fujitsu.com
> Subject: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP
> 
> Multiple CPUs are useful for CPU-bound processing like compression and
> I do want to use compression to generate crash dump quickly. But now
> we cannot wakeup the 2nd and later cpus in the kdump 2nd kernel if
> crash happens on AP. If crash happens on AP, kexec enters the 2nd
> kernel with the AP, and there BSP in the 1st kernel is expected to be
> haling in the 1st kernel or possibly in any fatal system error state.
> 
> To wake up AP, we use the method called INIT-INIT-SIPI. INIT causes
> BSP to jump into BIOS init code. A typical visible behaviour is hang
> or immediate reset, depending on the BIOS init code.
> 
> AP can be initiated by INIT even in a fatal state: MP spec explains
> that processor-specific INIT can be used to recover AP from a fatal
> system error. On the other hand, there's no method for BSP to recover;
> it might be possible to do so by NMI plus any hand-coded reset code
> that is carefully designed, but at least I have no idea in this
> direction now.

In my BSP hotplug patchset, BPS is waken up by NMI. The patchset is
not in tip tree yet.

BSP hotplug patchset can be found at https://lkml.org/lkml/2012/10/12/336

> 
> Therefore, the idea I do in this patch set is simply to disable BSP if
> vboot cpu is AP.
> 

The BSP hotplug patchset will be useful for you goal. With the BSP hotplug
patcheset, you can wake up BSP and don't need to disable it.

> My motivation is to use multiple CPUs in order to quickly generate
> crash dump on the machine with huge amount of memory. I assume such
> machine tends to also have a lot of CPUs. So disabling one CPU would
> be no problem.

Luckily you don't need to disable any CPU to archive your goal with
the BSP hotplug pachest:)

On a dual core/single thread machine, this means you get 100% performance
boost with BSP's help.

Plus crash dump kernel code is better structured by not treating BSP
specially.

Thanks.

-Fenghua