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

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On 4/6/23 06:06, Baoquan He wrote:
On 04/04/23 at 02:03pm, 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

To use the kexec-tools patch, apply, build and install kexec-tools,
then change the kdumpctl's standard_kexec_args to replace the -s with
--hotplug. The removal of -s reverts to the kexec_load syscall and
the addition of --hotplug invokes the changes put forth in the
kexec-tools patch.

Other than the nitpick in patch 2, this series looks good to me.

Acked-by: Baoquan He <bhe@xxxxxxxxxx>


Thank you! I'll await your preference on memory range handling.
eric

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