Re: [PATCH v19 7/7] x86/crash: optimize cpu changes

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 06/03/23 21:52, Eric DeVolder wrote:
> This patch is dependent upon the patch 'crash: change
> crash_prepare_elf64_headers() to for_each_possible_cpu()'. With that
> patch, crash_prepare_elf64_headers() writes out an ELF cpu PT_NOTE
> for all possible cpus, thus further cpu changes to the elfcorehdr
> are not needed.
>
> This change works for kexec_file_load() and kexec_load() syscalls.
> For kexec_file_load(), crash_prepare_elf64_headers() is utilized
> directly and thus all ELF cpu PT_NOTEs are in the elfcorehdr already.
> This is the kimage->file_mode term.
> For kexec_load() syscall, one cpu or memory change will cause the
> elfcorehdr to be updated via crash_prepare_elf64_headers() and at
> that point all ELF cpu PT_NOTEs are in the elfcorehdr. This is the
> kimage->elfcorehdr_updated term.
>
> This code is intentionally *NOT* hoisted into
> crash_handle_hotplug_event() as it would prevent the arch-specific
> handler from running for cpu changes. This would break PPC, for
> example, which needs to update other information besides the
> elfcorehdr, on cpu changes.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>   arch/x86/kernel/crash.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index c9aaec9de775..82ea2b1bdc61 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -471,6 +471,16 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
>   	unsigned long mem, memsz;
>   	unsigned long elfsz = 0;
>   
> +	/* As crash_prepare_elf64_headers() has already described all
> +	 * possible cpus, there is no need to update the elfcorehdr
> +	 * for additional cpu changes. This works for both kexec_load()
> +	 * and kexec_file_load() syscalls.
> +	 */
> +	if ((image->file_mode || image->elfcorehdr_updated) &&
> +		((image->hp_action == KEXEC_CRASH_HP_ADD_CPU) ||
> +		(image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)))
> +		return;

I like the idea of having elfcorehdr_updated to track elfcoredhr segment 
updates.
It handles the possible CPU thing for kexec_load syscall nicely.

Thanks for incorporating the possible CPU change.

The patch series looks good to me.
Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>

- Sourabh Jain