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

[Hari Bathini]

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