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

[Michael Ellerman]

"Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com> writes:
> On 08/08/2018 08:26 PM, Michael Ellerman wrote:
>> Mahesh J Salgaonkar <mahesh at linux.vnet.ibm.com> writes:
>>> From: Mahesh Salgaonkar <mahesh at 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.

The last SLB multi-hit I debugged was this:

  https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=db7130d63fd8


Which took quite a while to track down, including a bunch of tracing and
so on. A dump would not have helped in the slightest.

cheers