Re: [PATCH v8 0/7] crash: Kernel handling of CPU and memory hot un/plug

[Sourabh Jain]

Hello Eric,

On 26/05/22 18:46, Eric DeVolder wrote:
> On 5/25/22 10:13, Sourabh Jain wrote:
> > Hello Eric,
> >
> > On 06/05/22 00:15, Eric DeVolder wrote:
> > > When the kdump service is loaded, if a CPU or memory is hot
> > > un/plugged, the crash elfcorehdr (for x86), 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 (e. 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. No involvement
> > > with userspace needed.
> > >
> > > To realize the benefits/test this patchset, one must make a couple
> > > of minor changes to userspace:
> > >
> > >   - Disable the udev rule for updating kdump on hot un/plug changes.
> > >     Add the following as the first two lines to the udev rule file
> > >     /usr/lib/udev/rules.d/98-kexec.rules:
> >
> > If we can have a sysfs attribute to advertise this feature then 
> > userspace
> > utilities (kexec tool/udev rules) can take action accordingly. In 
> > short, it will
> > help us maintain backward compatibility.
> >
> > kexec tool can use the new sysfs attribute and allocate additional 
> > buffer space
> > for elfcorehdr accordingly. Similarly, the checksum-related changes 
> > can come
> > under this check.
> >
> > Udev rule can use this sysfs file to decide kdump service reload is 
> > required or not.
>
> Great idea. I've been working on the corresponding udev and 
> kexec-tools changes and your input/idea here is quite timely.
>
> I have boolean "crash_hotplug" as a core_param(), so it will show up as:
>
> # cat /sys/module/kernel/parameters/crash_hotplug
> N

How about using 0-1 instead Y/N?
0 = crash hotplug not supported
1 = crash hotplug supported

Also how about keeping sysfs here instead?
/sys/kernel/kexec_crash_hotplug

Thanks,
Souabh Jain