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

[Pingfan Liu <kernelfans@gmail.com>]

On Mon, Jul 8, 2019 at 3:44 AM Thomas Gleixner <tglx@linutronix.de> 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:

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.

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?

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.

Thanks,
  Pingfan