On Thu, 09 Aug 2018 16:34:07 +1000 Michael Ellerman <m...@ellerman.id.au> wrote:
> "Aneesh Kumar K.V" <aneesh.ku...@linux.ibm.com> writes: > > On 08/08/2018 08:26 PM, Michael Ellerman wrote: > >> Mahesh J Salgaonkar <mah...@linux.vnet.ibm.com> writes: > >>> From: Mahesh Salgaonkar <mah...@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. I think HA/failover kind of environments use options like this too. If anything starts going bad they don't want to try limping along but stop ASAP. Thanks, Nick
