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

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, 9 Aug 2018 18:33:33 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> On Thu, 9 Aug 2018 13:39:45 +0530
> Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com> wrote:
> 
> > On Thu, Aug 09, 2018 at 06:02:53PM +1000, Nicholas Piggin wrote:  
> > > On Thu, 09 Aug 2018 16:34:07 +1000
> > > Michael Ellerman <mpe@ellerman.id.au> wrote:
> > >     
> > > > "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:    
> > > > > On 08/08/2018 08:26 PM, Michael Ellerman 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.      
> > > > >
> > > > > The reasoning there is what if we got multi-hit due to some
> > > > > corruption in slb_cache_ptr. ie. some part of kernel is
> > > > > wrongly updating the paca data structure due to wrong
> > > > > pointer. Now that is far fetched, but then possible right?.
> > > > > Hence the idea that, if we don't have much insight into why a
> > > > > slb multi-hit occur from the dmesg which include slb content,
> > > > > slb_cache contents etc, there should be an easy way to force
> > > > > a dump that might assist in further debug.      
> > > > 
> > > > If you're debugging something complex that you can't determine
> > > > from the SLB dump then you should be running a debug kernel
> > > > anyway. And if anything you want to drop into xmon and sit
> > > > there, preserving the most state, rather than taking a dump.    
> > > 
> > > I'm not saying for a dump specifically, just some form of crash.
> > > And we really should have an option to xmon on panic, but that's
> > > another story.    
> > 
> > That's fine during development or in a lab, not something we could
> > enforce in a customer environment, could we?  
> 
> xmon on panic? Not something to enforce but IMO (without thinking
> about it too much but having encountered it several times) it should
> probably be tied xmon on BUG option.

You should get that with this patch and xmon=on or am I missing
something?

Thanks

Michal