Date:	Wed, 30 Oct 2013 14:06:43 +0800
From:	Baoquan He <bhe@...hat.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
Cc:	fengguang.wu@...el.com, jingbai.ma@...com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	bp@...en8.de, ebiederm@...ssion.com, akpm@...ux-foundation.org,
	hpa@...ux.intel.com, vgoyal@...hat.com
Subject: Re: [PATCH v4 0/3] x86, apic, kexec: Add disable_cpu_apic kernel
 parameter

On 10/30/13 at 09:44am, HATAYAMA Daisuke wrote:
> (2013/10/29 23:21), Baoquan He wrote:
> >Hi,
> >
> >I am reviewing this patchset, and found there's a cpu0 hotplug feature
> >posted by intel which we can borrow an idea from. In that implementation,
> >CPU0 is waken up by nmi not INIT to avoid the realmode bootstrap code
> >execution. I tried it by below patch which includes one line of change.
> >
> >By console printing, I got the boot cpu is always 0(namely cpu=0),
> >however the apicid related to each processor keeps the same as in 1st
> >kernel. In my HP Z420 machine, the apicid for BSP is 0, so I just make a
> >test patch which depends on the fact that apicid for BSP is 0. Maybe
> >generally the apicid for BSP can't be guaranteed, then passing it from
> >1st kernel to 2nd kernel in cmdline is very helpful, just as you have
> >done for disable_cpu_apic.
> >
> >On my HP z420, I add nr_cpus=4 in /etc/sysconfig/kdump, and then execute
> >below command, then 3 APs (1 boot cpu and 2 AP) can be waken up
> >correctly, but BSP failed because NMI received for unknown reason 21 on
> >CPU0. I think I need further check why BSP failed to wake up by nmi. But
> >3 processors are brought up successfully and kdump is successful too.
> >
> >sudo taskset -c 1 sh -c "echo c >/proc/sysrq-trigger"
> >
> >[    0.296831] smpboot: Booting Node   0, Processors  #   1
> >[    0.302095]
> >*****************************************************cpu=1, apicid=0, wakeup_cpu_via_init_nmi
> >[    0.311942] cpu=1, apicid=0, register_nmi_handlercpu=1, apicid=0, wakeup_secondary_cpu_via_nmi
> >[    0.320826] Uhhuh. NMI received for unknown reason 21 on CPU 0.
> >[    0.327129] Do you have a strange power saving mode enabled?
> >[    0.333858] Dazed and confused, but trying to continue
> >[    0.339290] cpu=1, apicid=0, wakeup_cpu_via_init_nmi
> >[    2.409099] Uhhuh. NMI received for unknown reason 21 on CPU 0.
> >[    2.415393] Do you have a strange power saving mode enabled?
> >[    2.421142] Dazed and confused, but trying to continue
> >[    5.379519] smpboot: CPU1: Not responding
> >[    5.383692] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> >
> 
> We've already discussed this approach and concluded this is not applicable
> to our issue.
> Follow http://lists.infradead.org/pipermail/kexec/2012-October/006905.html.
> 
> The reason is:
> 
> - The cpu0-hotplugging approach assumes BSP to be halting before initiating
>   NMI to it while in our case, BSP is halting in the 1st kernel or is
>   running in arbitrary position of the 1st kernel in catastrophic state.
> 
> - In general, NMI modifies stack, which means if throwing NMI to the BSP
>   in the 1st kernel, stack on the 1st kernel is modified. It's unpermissible
>   from kdump's perspective.

Hi HATAYAMA,

All right. I didn't think of the stack issues NMI will bring. In fact
without this NMI stack problem, using CPU0 Hotplug as a base and sending
nmi to bsp will be good, because BSP failure can be acceptable, then
(N-1)cpus can be used. Later on if possible the contexts modifying can
be changed to let BSP wake up correctly. After all, from the user's
point of view, multiple cpus can be waken up, why not waking up all cpus
including BSP.  

Anyway, this issue has been discussed so long time, and it will be great
to run multiple cpus in 2nd kernel. This evolution may be like CPU0 Hotplug,
they let cpus except of BSP hot plug available, then hanle the last cpu -
the BSP finally. From this perspective, I like your patch and hope it
can be merged asap.

Baoquan
Thanks a lot

> 
> - On x86_64, there are two cases where stack is changed to another one
>   when receiving interrupts. One is when receiving interrupt in user mode.
>   The other is when using Interrupt Stack Table (IST), which is already
>   used in the current x86_64 implementation.
> 
>   By using either, it would be possible to wake up BSP in the 1st kernel
>   by modifying the contexts on the 2nd kernel's NMI stack pushed on when NMI
>   to the 1st kernel is initiated.
> 
>   However, this approach depends on the logic in the 1st kernel, there's
>   no guarantee that it works well. Consider severely buggy situation again.
> 
> - To do this approach rigorously, we need to check if states of BSP and APs
>   are kept in just what we assume in the place where logic is guaranteed to be
>   sane, i.e., at least after purgatory. However, adding new logic in the
>   purgatory means we are forced to introduce additional dependency between
>   kernel and kexec. The process performed in purgatory itself is not so
>   simple.I don't like this complication.
> 
> To sum up, I think the current idea is simple enough approach.
> 
> -- 
> Thanks.
> HATAYAMA, Daisuke
> 
> 
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