Re: [PATCH v7 8/9] powerpc/mce: Add sysctl control for recovery action on MCE.

[Nicholas Piggin <npiggin@gmail.com>]

On Thu, 09 Aug 2018 00:56:00 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com> writes:
> > From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
> >
> > Introduce recovery action for recovered memory errors (MCEs). There are
> > soft memory errors like SLB Multihit, which can be a result of a bad
> > hardware OR software BUG. Kernel can easily recover from these soft errors
> > by flushing SLB contents. After the recovery kernel can still continue to
> > function without any issue. But in some scenario's we may keep getting
> > these soft errors until the root cause is fixed. To be able to analyze and
> > find the root cause, best way is to gather enough data and system state at
> > the time of MCE. Hence this patch introduces a sysctl knob where user can
> > decide either to continue after recovery or panic the kernel to capture the
> > dump.  
> 
> I'm not convinced we want this.
> 
> As we've discovered it's often not possible to reconstruct what happened
> based on a dump anyway.
> 
> The key thing you need is the content of the SLB and that's not included
> in a dump.
> 
> So I think we should dump the SLB content when we get the MCE (which
> this series does) and any other useful info, and then if we can recover
> we should.

Yeah it's a lot of knobs that administrators can hardly be expected to
tune. Hypervisor or firmware should really eventually make the MCE
unrecoverable if we aren't making progress.

That said, x86 has a bunch of options, and for debugging a rare crash
or specialised installations it might be useful. But we should follow
the normal format, /proc/sys/kernel/panic_on_mce.

Thanks,
Nick