Re: [PATCH v9 2/6] powerpc/crash: introduce a new config option CRASH_HOTPLUG

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

On 3/12/23 13:11, Sourabh Jain wrote:
> Due to CPU/Memory hotplug events the system resources changes. A similar
> change should reflect in the loaded kdump kernel image that describes
> the state of the CPU and memory of the running kernel.
> 
> If the kdump kernel image is not updated after the CPU or Memory hotplug
> events and it tries to collect the dump with the stale system resource
> data this might lead to dump collection failure or an inaccurate dump
> collection.
> 
> The current method to keep the kdump kernel up to date is by triggering
> reload (i.e unload and load) the entire kdump kernel image whenever a
> CPU or Memory hotplug event is observed by udev in the userspace.
> Reloading the complete kdump kernel image is an expensive task. It can
> be easily avoided by doing the in-kernel updates to specific kdump
> kernel image components which are responsible for describing CPU and
> Memory resources of the running kernel to the kdump kernel.
> 
> The kernel changes related to in-kernel update to the kdump kernel image
> on CPU/Memory hotplug events are kept under the CRASH_HOTPLUG config
> option.
> 
> Later in the series, a powerpc crash hotplug handler is introduced to
> update the kdump kernel image on CPU/Memory hotplug events. This arch
> specific handler is trigger from a generic crash handler that registers
> with the CPU and memory notifiers.
> 
> The CRASH_HOTPLUG config option is enabled by default.
> 
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>   arch/powerpc/Kconfig | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index a6c4407d3ec83..2f45b3f5175cb 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -681,6 +681,18 @@ config CRASH_DUMP
>   	  The same kernel binary can be used as production kernel and dump
>   	  capture kernel.
>   
> +config CRASH_HOTPLUG
> +	bool "Update crash capture system on CPU/Memory hotplug event"
Fwiw, online/offline changes also flow through this infrastructure...
eric

> +	default y
> +	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> +	help
> +	  In kernel update to relevant kexec segments due to change
> +	  in the system configuration, rather reloading all the kexec
> +	  segments again from userspace by monitoring CPU/Memory
> +	  hotplug events in the userspace using udev.
> +
> +	  If unsure, say N.
> +
>   config FA_DUMP
>   	bool "Firmware-assisted dump"
>   	depends on PPC64 && (PPC_RTAS || PPC_POWERNV)