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

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

On 02/02/23 21:07, Eric DeVolder wrote:
>
>
> 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()?

I was in the impression that if CPU notes are missing for offline CPUs 
in the elfcorehdr then makedumpfile will mess up the
CPU IDs.

But somehow replacing for_each_present_cpu with for_each_online_cpu 
worked on PowerPC, even after disabling a couple of CPUs.

So things are fine if we pack PT_LOAD for online CPUs instead of present 
CPUs but,
I need to investigate how makedumpfile is able to map PT_LOAD to online 
CPUs.

- Sourabh Jain