Re: [PATCH 2/2] powerpc/cpu: post the event cpux add/remove instead of online/offline during hotplug

On Sat, Aug 4, 2018 at 10:48 AM Michael Ellerman [off-list ref] wrote:

Hi Pingfan,

Pingfan Liu [off-list ref] writes:

quoted

Technically speaking, echo 1/0 > cpuX/online is only a subset of cpu
hotplug/unplug, i.e. add/remove. The latter one includes the physical
adding/removing of a cpu device. Some user space tools such as kexec-tools
resort to the event add/remove to automatically rebuild dtb.
If the dtb is not rebuilt correctly, we may hang on 2nd kernel due to
lack the info of boot-cpu-hwid in dtb.

I notice you also sent a patch for ppc64_cpu to deal with CPUs being
removed rather than just offlined.

If I apply this patch then existing ppc64_cpu (without your other patch)
will break. Is that right?

Yes. Since removing cpu will make a hole in cpu map, but ppc64_cpu
code takes the assumption that the cpu map is contiguous.

Thanks,
Pingfan