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

From: HATAYAMA Daisuke
Date: Tue Oct 16 2012 - 01:16:00 EST

From: HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx>
Subject: Re: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP
Date: Tue, 16 Oct 2012 14:03:13 +0900

> 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
>>> 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?

I've fogetten saying one comment that your patch distinguish BSP by
CPU#0. CPU#0 is assigned to the boot cpu, which can be AP in the kdump
2nd kernel. Distinguishing BSP by CPU#0 is not enough here.

I have my local patch set based on your v8 patch doing this, but NMI
to BSP failed. I guess this comes from the difference of BSP states:
halting in play dead in your NMI method and halting in the 1st kernel
on crash or possibly in a fatal system error on actual situation.


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