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

[Pingfan Liu <kernelfans@gmail.com>]

On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Mon, 8 Jul 2019, Pingfan Liu wrote:
> > 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:
>
> It's not only the APIC state. It's the state of the CPUs in general.
For other states, "kexec -l " is a kind of boot loader and the boot
cpu complies with the kernel boot up provision. As for the rest AP,
they are pinged at loop before receiving #INIT IPI. Then the left
things is the same as SMP boot up.

>
> > 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.
Thank you for the comprehensive explain. With your guide, now, I have
a full understanding of the issue.

But when I tried to add something to enable CR4.MCE in
crash_nmi_callback(), I realized that it is undo-able in some case (if
crashed, we will not ask an offline smt cpu to online), also it is
needless. "kexec -l/-p" takes the advantage of the cpu state in the
first kernel, where all logical cpu has CR4.MCE=1.

So kexec is exempt from this bug if the first kernel already do it.
>
> > 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'.
Yes, we should do something about nr_cpus param for the first kernel.

Thanks,
  Pingfan