[PATCH v21 0/7] crash: Kernel handling of CPU and memory hot un/plug

[Eric DeVolder]

On 4/27/23 02:08, Hari Bathini wrote:
> Hi Eric,
> 
> On 04/04/23 11:33 pm, Eric DeVolder wrote:
>> Once the kdump service is loaded, if changes to CPUs or memory occur,
>> either by hot un/plug or off/onlining, the crash elfcorehdr must also
>> be updated.
>>
>> The elfcorehdr describes to kdump the CPUs and memory in the system,
>> and any inaccuracies can result in a vmcore with missing CPU context
>> or memory regions.
>>
>> The current solution utilizes udev to initiate an unload-then-reload
>> of the kdump image (eg. kernel, initrd, boot_params, purgatory and
>> elfcorehdr) by the userspace kexec utility. In the original post I
>> outlined the significant performance problems related to offloading
>> this activity to userspace.
>>
>> This patchset introduces a generic crash handler that registers with
>> the CPU and memory notifiers. Upon CPU or memory changes, from either
>> hot un/plug or off/onlining, this generic handler is invoked and
>> performs important housekeeping, for example obtaining the appropriate
>> lock, and then invokes an architecture specific handler to do the
>> appropriate elfcorehdr update.
>>
>> Note the description in patch 'crash: change crash_prepare_elf64_headers()
>> to for_each_possible_cpu()' and 'x86/crash: optimize CPU changes' that
>> enables further optimizations related to CPU plug/unplug/online/offline
>> performance of elfcorehdr updates.
>>
>> In the case of x86_64, the arch specific handler generates a new
>> elfcorehdr, and overwrites the old one in memory; thus no involvement
>> with userspace needed.
>>
>> To realize the benefits/test this patchset, one must make a couple
>> of minor changes to userspace:
>>
>>   - Prevent udev from updating kdump crash kernel on hot un/plug changes.
>>     Add the following as the first lines to the RHEL udev rule file
>>     /usr/lib/udev/rules.d/98-kexec.rules:
>>
>>     # The kernel updates the crash elfcorehdr for CPU and memory changes
>>     SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>     SUBSYSTEM=="memory", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>
>>     With this changeset applied, the two rules evaluate to false for
>>     CPU and memory change events and thus skip the userspace
>>     unload-then-reload of kdump.
>>
>>   - Change to the kexec_file_load for loading the kdump kernel:
>>     Eg. on RHEL: in /usr/bin/kdumpctl, change to:
>>      standard_kexec_args="-p -d -s"
>>     which adds the -s to select kexec_file_load() syscall.
>>
>> This kernel patchset also supports kexec_load() with a modified kexec
>> userspace utility. A working changeset to the kexec userspace utility
>> is posted to the kexec-tools mailing list here:
>>
>>   http://lists.infradead.org/pipermail/kexec/2022-October/026032.html
> 
> With the in-kernel update, the size is anyway getting calculated in
> kernel for kexec_file_load case, how about passing the recommended size
> for elfcorehdr segment and any other segment that needs an in-kernel update as a sysfs attribute 
> instead of kexec-tools having to do the
> calculation again in the userspace  (get_elfcorehdrsz()) for kexec_load
> case. That makes segment size calculation less error prone.
> 
> Thanks
> Hari
Makes sense, I'll add that to v22.
Thanks!
eric