linux-kernel - Re: [PATCH v14 0/7] crash: Kernel handling of CPU and memory hot un/plug

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Date:   Wed, 23 Nov 2022 21:01:59 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v14 0/7] crash: Kernel handling of CPU and memory hot
 un/plug


On 17/11/22 03:16, Eric DeVolder wrote:
> When the kdump service is loaded, if a CPU or memory is hot
> un/plugged, the crash elfcorehdr, which describes the CPUs
> and memory in the system, must also be updated, else the resulting
> vmcore is inaccurate (eg. missing either 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, puratory and
> elfcorehdr) by the userspace kexec utility. In previous posts I have
> outlined the significant performance problems related to offloading
> this activity to userspace.
>
> This patchset introduces a generic crash hot un/plug handler that
> registers with the CPU and memory notifiers. Upon CPU or memory
> changes, 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
> 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 handles updates to 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"
>
>     These lines will cause cpu and memory hot un/plug events to be
>     skipped within this rule file, with this changset applied.
>
>   - 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.
>
> Regards,
> eric
> ---
> v14: 16nov2022
>   - Rebased onto 6.1.0-rc5
>   - Introduced CRASH_HOTPLUG Kconfig item to better fine tune
>     compilation of feature components, per Boris.
>   - Removed hp_action parameter to arch_crash_handle_hotplug_event()
>     as it is unused.

But what if an architecture has different actions based on hp type?

- Sourabh Jain