[PATCH v10 0/5] PowerPC: In-kernel handling of CPU/Memory hotplug/online/offline events for kdump kernel

[Sourabh Jain]

The Problem:
============
Post CPU/Memory hot plug/unplug and online/offline events the  kernel
holds stale information about the system. Dump collection with stale
kdump kernel might end up in dump capture failure or an inaccurate dump
collection.

Existing solution:
==================
The existing solution to keep the kdump kernel up-to-date by monitoring
CPU/Memory hotplug/online/offline events via udev rule and trigger a full
kdump kernel reload for every hotplug event.

Shortcomings:
------------------------------------------------
- Leaves a window where kernel crash might not lead to a successful dump
  collection.
- Reloading all kexec components for each hotplug is inefficient.
- 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 CPU/Memory hotplug/online/offline
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.

Series Dependencies
====================
This patch series implements the crash hotplug handler on PowerPC. The generic
crash hotplug handler is introduced by https://lkml.org/lkml/2023/4/4/1136 patch
series.

Git tree for testing:
=====================
The below git tree has this patch series applied on top of dependent patch
series.
https://github.com/sourabhjains/linux/tree/e21-s10

To realise the feature the kdump udev rule must updated to avoid
reloading of kdump reload on CPU/Memory hotplug/online/offline events.

  RHEL: /usr/lib/udev/rules.d/98-kexec.rules

	-SUBSYSTEM=="cpu", ACTION=="online", GOTO="kdump_reload_cpu"
	-SUBSYSTEM=="memory", ACTION=="online", GOTO="kdump_reload_mem"
	-SUBSYSTEM=="memory", ACTION=="offline", GOTO="kdump_reload_mem"
	+SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
	+SUBSYSTEM=="memory", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"

Note: only kexec_file_load syscall will work. For kexec_load minor changes are
required in kexec tool.

---
Changelog:

v10:
  - Drop the patch that adds fdt_index attribute to struct kimage_arch
    Find the fdt segment index when needed.
  - Added more details into commits messages.
  - Rebased onto 6.3.0-rc5

v9:
  - Removed patch to prepare elfcorehdr crash notes for possible CPUs.
    The patch is moved to generic patch series that introduces generic
    infrastructure for in kernel crash update.
  - Removed patch to pass the hotplug action type to the arch crash
    hotplug handler function. The generic patch series has introduced
    the hotplug action type in kimage struct.
  - Add detail commit message for better understanding.

v8:
  - Restrict fdt_index initialization to machine_kexec_post_load
    it work for both kexec_load and kexec_file_load.[3/8] Laurent Dufour

  - Updated the logic to find the number of offline core. [6/8]

  - Changed the logic to find the elfcore program header to accommodate
    future memory ranges due memory hotplug events. [8/8]

v7
  - added a new config to configure this feature
  - pass hotplug action type to arch specific handler

v6
  - Added crash memory hotplug support

v5:
  - Replace COFNIG_CRASH_HOTPLUG with CONFIG_HOTPLUG_CPU.
  - Move fdt segment identification for kexec_load case to load path
    instead of crash hotplug handler
  - Keep new attribute defined under kimage_arch to track FDT segment
    under CONFIG_HOTPLUG_CPU config.

v4:
  - Update the logic to find the additional space needed for hotadd CPUs post
    kexec load. Refer "[RFC v4 PATCH 4/5] powerpc/crash hp: add crash hotplug
    support for kexec_file_load" patch to know more about the change.
  - Fix a couple of typo.
  - Replace pr_err to pr_info_once to warn user about memory hotplug
    support.
  - In crash hotplug handle exit the for loop if FDT segment is found.

v3
  - Move fdt_index and fdt_index_vaild variables to kimage_arch struct.
  - Rebase patche on top of https://lkml.org/lkml/2022/3/3/674 [v5]
  - Fixed warning reported by checpatch script

v2:
  - Use generic hotplug handler introduced by https://lkml.org/lkml/2022/2/9/1406, a
    significant change from v1.

Sourabh Jain (5):
  powerpc/kexec: turn some static helper functions public
  powerpc/crash: introduce a new config option CRASH_HOTPLUG
  powerpc/crash: add crash CPU hotplug support
  crash: forward memory_notify args to arch crash hotplug handler
  powerpc/kexec: add crash memory hotplug support

 arch/powerpc/Kconfig                    |  12 +
 arch/powerpc/include/asm/kexec.h        |  10 +
 arch/powerpc/include/asm/kexec_ranges.h |   1 +
 arch/powerpc/kexec/core_64.c            | 301 ++++++++++++++++++++++++
 arch/powerpc/kexec/elf_64.c             |  12 +-
 arch/powerpc/kexec/file_load_64.c       | 212 ++++-------------
 arch/powerpc/kexec/ranges.c             |  85 +++++++
 arch/x86/include/asm/kexec.h            |   2 +-
 arch/x86/kernel/crash.c                 |   3 +-
 include/linux/kexec.h                   |   2 +-
 kernel/crash_core.c                     |  14 +-
 11 files changed, 479 insertions(+), 175 deletions(-)

-- 
2.39.2