Re: [PATCH v8 4/8] crash: add phdr for possible CPUs in elfcorehdr

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

On 2/1/23 00:38, Sourabh Jain wrote:
> On architectures like PowerPC the crash notes are available for all
> possible CPUs. So let's populate the elfcorehdr for all possible
> CPUs having crash notes to avoid updating elfcorehdr during in-kernel
> crash update on CPU hotplug events.
> 
> The similar technique is used in kexec-tool for kexec_load case.
> 
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>   kernel/crash_core.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 37c594858fd51..898d8d2fe2e2e 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -364,8 +364,8 @@ int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
>   	ehdr->e_ehsize = sizeof(Elf64_Ehdr);
>   	ehdr->e_phentsize = sizeof(Elf64_Phdr);
>   
> -	/* Prepare one phdr of type PT_NOTE for each present CPU */
> -	for_each_present_cpu(cpu) {
> +	/* Prepare one phdr of type PT_NOTE for possible CPU with crash note. */
> +	for_each_possible_cpu(cpu) {

Sourabh,
Thomas Gleixner is suggesting moving away from for_each_present_cpu() to for_each_online_cpu(). 
Using for_each_online_cpu() is going to the minimum number of needed, whereas your approach of 
for_each_possible_cpu() would be to the maximum number needed.

What would be the ramifications to ppc for moving towards for_each_online_cpu()?

In my next patch series, I have finally figured out how to use cpuhp framework to where it is 
possible to use for_each_online_cpu() here, but that is at odds with your changes here.

Thanks,
eric



>   #ifdef CONFIG_CRASH_HOTPLUG
>   		if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
>   			/* Skip the soon-to-be offlined cpu */
> @@ -373,8 +373,11 @@ int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
>   				continue;
>   		}
>   #endif
> -		phdr->p_type = PT_NOTE;
>   		notes_addr = per_cpu_ptr_to_phys(per_cpu_ptr(crash_notes, cpu));
> +		if (!notes_addr)
> +			continue;
> +
> +		phdr->p_type = PT_NOTE;
>   		phdr->p_offset = phdr->p_paddr = notes_addr;
>   		phdr->p_filesz = phdr->p_memsz = sizeof(note_buf_t);
>   		(ehdr->e_phnum)++;