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

[Eric DeVolder <eric.devolder@oracle.com>]

On 4/23/23 05:52, Sourabh Jain wrote:
> 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"
> 

I didn't see in the patch series where you would have the equivalent to the following (needed for 
the sysfs crash_hotplug entries):

#ifdef CONFIG_HOTPLUG_CPU
static inline int crash_hotplug_cpu_support(void) { return 1; }
#define crash_hotplug_cpu_support crash_hotplug_cpu_support
#endif

#ifdef CONFIG_MEMORY_HOTPLUG
static inline int crash_hotplug_memory_support(void) { return 1; }
#define crash_hotplug_memory_support crash_hotplug_memory_support
#endif

> 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(-)
>