Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr notes

[Thomas Gleixner]

On Mon, Feb 06 2023 at 13:42, Sourabh Jain wrote:
> On 01/02/23 17:03, Thomas Gleixner wrote:
>> Also in case of loading the crash kernel in the situation where not all
>> present CPUs are online (think boot time SMT disable) then your
>> resulting crash image will contain all present CPUs and none of the
>> offline CPUs are excluded.
>>
>> How does that make any sense at all?
>>
>> This image->hp_action and image->offlinecpu dance is engineering
>> voodoo. You just can do:
>>
>>          for_each_present_cpu(cpu) {
>>              if (!cpu_online(cpu))
>>              	continue;
>>              do_stuff(cpu);
>>
>> which does the right thing in all situations and can be further
>> simplified to:
>>
>>          for_each_online_cpu(cpu) {
>>              do_stuff(cpu);
>
> What will be the implication on x86 if we pack PT_NOTE for possible
> CPUs?

I don't know.

> IIUC, on boot the crash notes are create for possible CPUs using pcpu_alloc
> and when the system is on crash path the crash notes for online CPUs is
> populated with the required data and rest crash notes are untouched.

Which should be fine. That's a problem of postprocessing and it's
unclear to me from the changelogs what the actual problem is which is
trying to be solved here.

Thanks,

        tglx