On 24/03/22 00:02, Eric DeVolder wrote:

On 3/21/22 03:04, Sourabh Jain wrote:

quoted

This patch series implements the crash hotplug handler on PowerPC 
introduced
by https://lkml.org/lkml/2022/3/3/674 patch series.


The Problem:
============
Post hotplug/DLPAR events the capture kernel holds stale information 
about the
system. Dump collection with stale capture kernel might end up in 
dump capture
failure or an inaccurate dump collection.


Existing solution:
==================
The existing solution to keep the capture kernel up-to-date is 
observe the
hotplug event via udev rule and trigger a full capture kernel reload 
post
hotplug event.

Shortcomings:
------------------------------------------------
- Leaves a window where kernel crash might not lead to successful dump
   collection.
- Reloading all kexec components for each hotplug is inefficient. 
Since only
   one or two kexec components need to be updated due to hotplug 
event reloading
   all kexec component is redundant.
- udev rules are prone to races if hotplug events are frequent.

More about issues with an existing solution is posted here:
  - https://lkml.org/lkml/2020/12/14/532
  - 
https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-February/240254.html

Proposed Solution:
==================
Instead of reloading all kexec segments on hotplug event, this patch 
series
focuses on updating only the relevant kexec segment. Once the kexec
segments are loaded in the kernel reserved area then an arch-specific 
hotplug handler
will update the relevant kexec segment based on hotplug event type.

As mentioned above this patch series implemented a PowerPC crash hotplug
handler for the CPU. The crash hotplug handler memory is in our TODO 
list.


A couple of minor changes are required to realize the benefit of the 
patch
series:

- disalble the udev rule:

   comment out the below line in kdump udev rule file:

fwiw, this will need to be conditionalized on arch, ie to skip for 
ppc64. I'm doing the same for x86_64.

I think kexec-tools maintains a separate udev rule file for PowerPC. I 
will ensure CPU rules gets removed from kdump udev rules once this 
feature is upstream.

Thanks,
- Sourabh Jain