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

From: Pingfan Liu
Date: Tue Jul 09 2019 - 03:24:44 EST

On Tue, Jul 9, 2019 at 2:12 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, 9 Jul 2019, Pingfan Liu wrote:
> > On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > > 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.
> No. If the MCE broadcast is handled by a CPU which is stuck in the old
> kernel stop loop, then it will execute on the old kernel and eventually run
> into the memory corruption which crashed the old one.
Yes, you are right. Stuck cpu may execute the old do_machine_check()
code. But I just found out that we have
do_machine_check()->__mc_check_crashing_cpu() to against this case.

And I think the MCE issue with nr_cpus is not closely related with
this series, can
be a separated issue. I had question whether Andy will take it, if
not, I am glad to do it.

Thanks and regards,