Re: [Bug fix PATCH v2] numa, cpu hotplug: Change links of CPU andnode when changing node number by onlining CPU

[Yasuaki Ishimatsu]

2013/04/23 7:35, Andrew Morton wrote:
> On Fri, 19 Apr 2013 14:23:23 +0900 Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx> wrote:
>
> > When booting x86 system contains memoryless node, node numbers of CPUs
> > on memoryless node were changed to nearest online node number by
> > init_cpu_to_node() because the node is not online.
> >
> > ...
> >
> > If we hot add memory to memoryless node and offine/online all CPUs on
> > the node, node numbers of these CPUs are changed to correct node numbers
> > by srat_detect_node() because the node become online.
>
> OK, here's a dumb question.
>
> At boot time the CPUs are assigned to the "nearest online node" rather
> than to their real memoryless node.  The patch arranges for those CPUs
> to still be assigned to the "nearest online node" _after_ some memory
> is hot-added to their real node.  Correct?

Yes. For changing node number of CPUs safely, we should offline CPUs.

> Would it not be better to fix this by assigning those CPUs to their real,
> memoryless node right at the initial boot?  Or is there something in
> the kernel which makes cpus-on-a-memoryless-node not work correctly?

I think assigning CPUs to real node is better. But current Linux's node
strongly depend on memory. Thus if we just create cpus-on-a-memoryless-node,
the kernel cannot work correctly.

Thanks,
Yasuaki Ishimatsu

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