Messages in this thread | | | Date | Wed, 1 Mar 2023 08:16:24 -0600 | Subject | Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr notes | From | Eric DeVolder <> |
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On 3/1/23 00:22, Sourabh Jain wrote: > > On 01/03/23 03:20, Eric DeVolder wrote: >> >> >> On 2/27/23 00:11, Sourabh Jain wrote: >>> >>> On 25/02/23 01:46, Eric DeVolder wrote: >>>> >>>> >>>> On 2/24/23 02:34, Sourabh Jain wrote: >>>>> >>>>> On 24/02/23 02:04, 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 >>>>>> >>>>>> I've been examining what it would mean to move to for_each_possible_cpu() in >>>>>> crash_prepare_elf64_headers(). I think it means: >>>>>> >>>>>> - Changing for_each_present_cpu() to for_each_possible_cpu() in crash_prepare_elf64_headers(). >>>>>> - For kexec_load() syscall path, rewrite the incoming/supplied elfcorehdr immediately on the >>>>>> load with the elfcorehdr generated by crash_prepare_elf64_headers(). >>>>>> - Eliminate/remove the cpuhp machinery for handling crash hotplug events. >>>>> >>>>> If for_each_present_cpu is replaced with for_each_possible_cpu I still need cpuhp machinery >>>>> to update FDT kexec segment for CPU hot add case. >>>> >>>> Ah, ok, that's important! So the cpuhp callbacks are still needed. >>>>> >>>>> >>>>>> >>>>>> This would then setup PT_NOTEs for all possible cpus, which should in theory accommodate crash >>>>>> analyzers that rely on ELF PT_NOTEs for crash_notes. >>>>>> >>>>>> If staying with for_each_present_cpu() is ultimately decided, then I think leaving the cpuhp >>>>>> machinery in place and each arch could decide how to handle crash cpu hotplug events. The >>>>>> overhead for doing this is very minimal, and the events are likely very infrequent. >>>>> >>>>> I agree. Some architectures may need cpuhp machinery to update kexec segment[s] other then >>>>> elfcorehdr. For example FDT on PowerPC. >>>>> >>>>> - Sourabh Jain >>>> >>>> OK, I was thinking that the desire was to eliminate the cpuhp callbacks. In reality, the desire >>>> is to change to for_each_possible_cpu(). Given that the kernel creates crash_notes for all >>>> possible cpus upon kernel boot, there seems to be no reason to not do this? >>>> >>>> HOWEVER... >>>> >>>> It's not clear to me that this particular change needs to be part of this series. It's inclusion >>>> would facilitate PPC support, but doesn't "solve" anything in general. In fact it causes >>>> kexec_load and kexec_file_load to deviate (kexec_load via userspace kexec does the equivalent of >>>> for_each_present_cpu() where as with this change kexec_file_load would do >>>> for_each_possible_cpu(); until a hot plug event then both would do for_each_possible_cpu()). And >>>> if this change were to arrive as part of Sourabh's PPC support, then it does not appear to >>>> impact x86 (not sure about other arches). And the 'crash' dump analyzer doesn't care either way. >>>> >>>> Including this change would enable an optimization path (for x86 at least) that short-circuits >>>> cpu hotplug changes in the arch crash handler, for example: >>>> >>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c >>>> index aca3f1817674..0883f6b11de4 100644 >>>> --- a/arch/x86/kernel/crash.c >>>> +++ b/arch/x86/kernel/crash.c >>>> @@ -473,6 +473,11 @@ void arch_crash_handle_hotplug_event(struct kimage *image) >>>> unsigned long mem, memsz; >>>> unsigned long elfsz = 0; >>>> >>>> + if (image->file_mode && ( >>>> + image->hp_action == KEXEC_CRASH_HP_ADD_CPU || >>>> + image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)) >>>> + return; >>>> + >>>> /* >>>> * Create the new elfcorehdr reflecting the changes to CPU and/or >>>> * memory resources. >>>> >>>> I'm not sure that is compelling given the infrequent nature of cpu hotplug events. >>> It certainly closes/reduces the window where kdump is not active due kexec segment update.| >> >> Fair enough. I plan to include this change in v19. >> >>> >>>> >>>> In my mind I still have a question about kexec_load() path. The userspace kexec can not do the >>>> equivalent of for_each_possible_cpu(). It can obtain max possible cpus from >>>> /sys/devices/system/cpu/possible, but for those cpus not present the >>>> /sys/devices/system/cpu/cpuXX is not available and so the crash_notes entries is not available. >>>> My attempts to expose all cpuXX lead to odd behavior that was requiring changes in ACPI and arch >>>> code that looked untenable. >>>> >>>> There seem to be these options available for kexec_load() path: >>>> - immediately rewrite the elfcorehdr upon load via a call to crash_prepare_elf64_headers(). I've >>>> made this work with the following, as proof of concept: >>> Yes regenerating/patching the elfcorehdr could be an option for kexec_load syscall. >> So this is not needed by x86, but more so by ppc. Should this change be in the ppc set or this set? > Since /sys/devices/system/cpu/cpuXX represents possible CPUs on PowerPC, there is no need for > elfcorehdr regeneration on PowerPC for kexec_load case > for CPU hotplug events. > > My ask is, keep the cpuhp machinery so that architectures can update other kexec segments if needed > of CPU add/remove case. > > In case x86 has nothing to update on CPU hotplug events and you want remove the CPU hp machinery I > can add the same > in ppc patch series.
I'll keep the cpuhp machinery; in general it is needed for kexec_load usage in particular since we are changing crash_prepare_elf64_headers() to for_each_possible_cpu(). eric
> > Thanks, > Sourabh Jain
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