Date:   Mon, 13 Feb 2023 10:10:24 +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 11/02/23 06:05, Eric DeVolder wrote:
>
>
> On 2/10/23 00:29, Sourabh Jain wrote:
>>
>> 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
>
> In general, I agree with your points. You've presented a strong case 
> to go with for_each_possible_cpu() in crash_prepare_elf64_headers() 
> and those crash notes would always be present, and we can ignore 
> changes to cpus wrt/ elfcorehdr updates.
>
> But what do we do about kexec_load() syscall? The way the userspace 
> utility works is it determines cpus by:
>  nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
> which is not the equivalent of possible_cpus. So the complete list of 
> cpu PT_NOTEs is not generated up front. We would need a solution for 
> that?
Hello Eric,

The sysconf document says _SC_NPROCESSORS_CONF is processors configured, 
isn't that equivalent to possible CPUs?

What exactly sysconf(_SC_NPROCESSORS_CONF) returns on x86? IIUC, on 
powerPC it is possible CPUs.

In case sysconf(_SC_NPROCESSORS_CONF) is not consistent then we can go with:
/sys/devices/system/cpu/possible for kexec_load case.

Thoughts?

- Sourabh Jain

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