Re: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP

[HATAYAMA Daisuke]

From: "Yu, Fenghua" <fenghua.yu@xxxxxxxxx>
Subject: RE: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP
Date: Tue, 16 Oct 2012 04:51:36 +0000

>> -----Original Message-----
>> From: HATAYAMA Daisuke [mailto:d.hatayama@xxxxxxxxxxxxxx]
>> Sent: Monday, October 15, 2012 9:35 PM
>> To: linux-kernel@xxxxxxxxxxxxxxx; kexec@xxxxxxxxxxxxxxxxxxx;
>> x86@xxxxxxxxxx
>> Cc: mingo@xxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; Brown, Len; Yu,
>> Fenghua; vgoyal@xxxxxxxxxx; ebiederm@xxxxxxxxxxxx;
>> grant.likely@xxxxxxxxxxxx; rob.herring@xxxxxxxxxxx;
>> d.hatayama@xxxxxxxxxxxxxx
>> 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.
> 

Hello Fenghua.

I've of course noticed your patch set and locally tested, but I saw
NMI to BSP failed in the 2nd kernel. I'll send a log to you later.

BTW, I tested with your previous v8 patch set. Did you change
something during v8 to v9 relevant to this issue?

Thanks.
HATAYAMA, Daisuke

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