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

[Eric DeVolder]

On 2/8/23 07:44, Thomas Gleixner wrote:
> Eric!
> 
> On Tue, Feb 07 2023 at 11:23, Eric DeVolder wrote:
>> On 2/1/23 05:33, Thomas Gleixner wrote:
>>
>> So my latest solution is introduce two new CPUHP states, CPUHP_AP_ELFCOREHDR_ONLINE
>> for onlining and CPUHP_BP_ELFCOREHDR_OFFLINE for offlining. I'm open to better names.
>>
>> The CPUHP_AP_ELFCOREHDR_ONLINE needs to be placed after CPUHP_BRINGUP_CPU. My
>> attempts at locating this state failed when inside the STARTING section, so I located
>> this just inside the ONLINE sectoin. The crash hotplug handler is registered on
>> this state as the callback for the .startup method.
>>
>> The CPUHP_BP_ELFCOREHDR_OFFLINE needs to be placed before CPUHP_TEARDOWN_CPU, and I
>> placed it at the end of the PREPARE section. This crash hotplug handler is also
>> registered on this state as the callback for the .teardown method.
> 
> TBH, that's still overengineered. Something like this:
> 
> bool cpu_is_alive(unsigned int cpu)
> {
> 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
> 
> 	return data_race(st->state) <= CPUHP_AP_IDLE_DEAD;
> }
> 
> and use this to query the actual state at crash time. That spares all
> those callback heuristics.
> 
>> I'm making my way though percpu crash_notes, elfcorehdr, vmcoreinfo,
>> makedumpfile and (the consumer of it all) the userspace crash utility,
>> in order to understand the impact of moving from for_each_present_cpu()
>> to for_each_online_cpu().
> 
> Is the packing actually worth the trouble? What's the actual win?
> 
> Thanks,
> 
>          tglx
> 
> 

Thomas,
I've investigated the passing of crash notes through the vmcore. What I've learned is that:

- linux/fs/proc/vmcore.c (which makedumpfile references to do its job) does
   not care what the contents of cpu PT_NOTES are, but it does coalesce them together.

- makedumpfile will count the number of cpu PT_NOTES in order to determine its
   nr_cpus variable, which is reported in a header, but otherwise unused (except
   for sadump method).

- the crash utility, for the purposes of determining the cpus, does not appear to
   reference the elfcorehdr PT_NOTEs. Instead it locates the various
   cpu_[possible|present|online]_mask and computes nr_cpus from that, and also of
   course which are online. In addition, when crash does reference the cpu PT_NOTE,
   to get its prstatus, it does so by using a percpu technique directly in the vmcore
   image memory, not via the ELF structure. Said differently, it appears to me that
   crash utility doesn't rely on the ELF PT_NOTEs for cpus; rather it obtains them
   via kernel cpumasks and the memory within the vmcore.

With this understanding, I did some testing. Perhaps the most telling test was that I
changed the number of cpu PT_NOTEs emitted in the crash_prepare_elf64_headers() to just 1,
hot plugged some cpus, then also took a few offline sparsely via chcpu, then generated a
vmcore. The crash utility had no problem loading the vmcore, it reported the proper number
of cpus and the number offline (despite only one cpu PT_NOTE), and changing to a different
cpu via 'set -c 30' and the backtrace was completely valid.

My take away is that crash utility does not rely upon ELF cpu PT_NOTEs, it obtains the
cpu information directly from kernel data structures. Perhaps at one time crash relied
upon the ELF information, but no more. (Perhaps there are other crash dump analyzers
that might rely on the ELF info?)

So, all this to say that I see no need to change crash_prepare_elf64_headers(). There
is no compelling reason to move away from for_each_present_cpu(), or modify the list for
online/offline.

Which then leaves the topic of the cpuhp state on which to register. Perhaps reverting
back to the use of CPUHP_BP_PREPARE_DYN is the right answer. There does not appear to
be a compelling need to accurately track whether the cpu went online/offline for the
purposes of creating the elfcorehdr, as ultimately the crash utility pulls that from
kernel data structures, not the elfcorehdr.

I think this is what Sourabh has known and has been advocating for an optimization
path that allows not regenerating the elfcorehdr on cpu changes (because all the percpu
structs are all laid out). I do think it best to leave that as an arch choice.

Comments?

Thanks!
eric