Re: [PATCH 2/2] x86/numa: instance all parsed numa node

[Andy Lutomirski]

> On Jul 8, 2019, at 3:35 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Mon, 8 Jul 2019, Pingfan Liu wrote:
>>> On Mon, Jul 8, 2019 at 3:44 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>> 
>>>> On Fri, 5 Jul 2019, Pingfan Liu wrote:
>>>> 
>>>> I hit a bug on an AMD machine, with kexec -l nr_cpus=4 option. nr_cpus option
>>>> is used to speed up kdump process, so it is not a rare case.
>>> 
>>> But fundamentally wrong, really.
>>> 
>>> The rest of the CPUs are in a half baken state and any broadcast event,
>>> e.g. MCE or a stray IPI, will result in a undiagnosable crash.
>> Very appreciate if you can pay more word on it? I tried to figure out
>> your point, but fail.
>> 
>> For "a half baked state", I think you concern about LAPIC state, and I
>> expand this point like the following:
> 
> It's not only the APIC state. It's the state of the CPUs in general.
> 
>> For IPI: when capture kernel BSP is up, the rest cpus are still loop
>> inside crash_nmi_callback(), so there is no way to eject new IPI from
>> these cpu. Also we disable_local_APIC(), which effectively prevent the
>> LAPIC from responding to IPI, except NMI/INIT/SIPI, which will not
>> occur in crash case.
> 
> Fair enough for the IPI case.
> 
>> For MCE, I am not sure whether it can broadcast or not between cpus,
>> but as my understanding, it can not. Then is it a problem?
> 
> It can and it does.
> 
> That's the whole point why we bring up all CPUs in the 'nosmt' case and
> shut the siblings down again after setting CR4.MCE. Actually that's in fact
> a 'let's hope no MCE hits before that happened' approach, but that's all we
> can do.
> 
> If we don't do that then the MCE broadcast can hit a CPU which has some
> firmware initialized state. The result can be a full system lockup, triple
> fault etc.
> 
> So when the MCE hits a CPU which is still in the crashed kernel lala state,
> then all hell breaks lose.
> 
>> From another view point, is there any difference between nr_cpus=1 and
>> nr_cpus> 1 in crashing case? If stray IPI raises issue to nr_cpus>1,
>> it does for nr_cpus=1.
> 
> Anything less than the actual number of present CPUs is problematic except
> you use the 'let's hope nothing happens' approach. We could add an option
> to stop the bringup at the early online state similar to what we do for
> 'nosmt'.
> 
> 

How about we change nr_cpus to do that instead so we never have to have this conversation again?