Date:   Fri, 10 Feb 2023 11:59:03 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Eric DeVolder <eric.devolder@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr
 notes


On 10/02/23 01:09, Eric DeVolder wrote:
>
>
> On 2/9/23 12:43, Sourabh Jain wrote:
>> Hello Eric,
>>
>> On 09/02/23 23:01, Eric DeVolder wrote:
>>>
>>>
>>> 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.
>>
>> Since things are clear on how the PT_NOTES are consumed in kdump 
>> kernel [fs/proc/vmcore.c],
>> makedumpfile, and crash tool I need your opinion on this:
>>
>> Do we really need to regenerate elfcorehdr for CPU hotplug events?
>> If yes, can you please list the elfcorehdr components that changes 
>> due to CPU hotplug.
> Due to the use of for_each_present_cpu(), it is possible for the 
> number of cpu PT_NOTEs
> to fluctuate as cpus are un/plugged. Onlining/offlining of cpus does 
> not impact the
> number of cpu PT_NOTEs (as the cpus are still present).
>
>>
>>  From what I understood, crash notes are prepared for possible CPUs 
>> as system boots and
>> could be used to create a PT_NOTE section for each possible CPU while 
>> generating the elfcorehdr
>> during the kdump kernel load.
>>
>> Now once the elfcorehdr is loaded with PT_NOTEs for every possible 
>> CPU there is no need to
>> regenerate it for CPU hotplug events. Or do we?
>
> For onlining/offlining of cpus, there is no need to regenerate the 
> elfcorehdr. However,
> for actual hot un/plug of cpus, the answer is yes due to 
> for_each_present_cpu(). The
> caveat here of course is that if crash utility is the only coredump 
> analyzer of concern,
> then it doesn't care about these cpu PT_NOTEs and there would be no 
> need to re-generate them.
>
> Also, I'm not sure if ARM cpu hotplug, which is just now coming into 
> mainstream, impacts
> any of this.
>
> Perhaps the one item that might help here is to distinguish between 
> actual hot un/plug of
> cpus, versus onlining/offlining. At the moment, I can not distinguish 
> between a hot plug
> event and an online event (and unplug/offline). If those were 
> distinguishable, then we
> could only regenerate on un/plug events.
>
> Or perhaps moving to for_each_possible_cpu() is the better choice?

Yes, because once elfcorehdr is built with possible CPUs we don't have 
to worry about
hot[un]plug case.

Here is my view on how things should be handled if a core-dump analyzer 
is dependent on
elfcorehdr PT_NOTEs to find online/offline CPUs.

A PT_NOTE in elfcorehdr holds the address of the corresponding crash 
notes (kernel has
one crash note per CPU for every possible CPU). Though the crash notes 
are allocated
during the boot time they are populated when the system is on the crash 
path.

This is how crash notes are populated on PowerPC and I am expecting it 
would be something
similar on other architectures too.

The crashing CPU sends IPI to every other online CPU with a callback 
function that updates the
crash notes of that specific CPU. Once the IPI completes the crashing 
CPU updates its own crash
note and proceeds further.

The crash notes of CPUs remain uninitialized if the CPUs were offline or 
hot unplugged at the time
system crash. The core-dump analyzer should be able to identify 
[un]/initialized crash notes
and display the information accordingly.

Thoughts?

- Sourabh

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