On 23/08/2017 13:00, Michael Ellerman wrote:

Hi Tejun,

Tejun Heo [off-list ref] writes:

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Hello, Michael.

On Tue, Aug 22, 2017 at 11:41:41AM +1000, Michael Ellerman wrote:

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This is something powerpc needs to fix.

There is no way for us to fix it.

I don't think that's true.  The CPU id used in kernel doesn't have to
match the physical one and arch code should be able to pre-map CPU IDs
to nodes and use the matching one when hotplugging CPUs.  I'm not
saying that's the best way to solve the problem tho.

We already virtualise the CPU numbers, but not the node IDs. And it's
the node IDs that are really the problem.

So yeah I guess we might be able to make that work, but I'd have to
think about it a bit more.

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It could be that the best way forward is making cpu <-> node mapping
dynamic and properly synchronized.

We don't need it to be dynamic (at least for this bug).

Laurent is booting Qemu with a fixed CPU <-> Node mapping, it's just
that because some CPUs aren't present at boot we don't know what the
node mapping is. (Correct me if I'm wrong Laurent).

You're correct.

Qemu is started with:

    -numa node,cpus=0-1 -numa node,cpus=2-3 \
    -smp 2,maxcpus=4,cores=4,threads=1,sockets=1

Which means we have 2 nodes with cpu ids 0 and 1 on node 0, and cpu ids
2 and 3 on node 1, but at boot only 2 CPUs are present.

The problem I try to fix with this series is when we hotplug a third
CPU, to node 1 with id 2.

Thanks,
Laurent