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

[Eric DeVolder]

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