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SubjectRe: [RFC v3] sched/topology: fix kernel crash when a CPU is hotplugged in a memoryless node
> > node 0 (because firmware doesn't provide the distance information for
> > memoryless/cpuless nodes):
> >
> > node 0 1 2 3
> > 0: 10 40 10 10
> > 1: 40 10 40 40
> > 2: 10 40 10 10
> > 3: 10 40 10 10
>
> *groan*... what does it do for things like percpu memory? ISTR the
> per-cpu chunks are all allocated early too. Having them all use memory
> out of node-0 would seem sub-optimal.

In the specific failing case, there is only one node with memory; all other
nodes are cpu only nodes.

However in the generic case since its just a cpu hotplug ops, the memory
allocated for per-cpu chunks allocated early would remain.

May be Michael Ellerman can correct me here.

>
> > We should have:
> >
> > node 0 1 2 3
> > 0: 10 40 40 40
> > 1: 40 10 40 40
> > 2: 40 40 10 40
> > 3: 40 40 40 10
>
> Can it happen that it introduces a new distance in the table? One that
> hasn't been seen before? This example only has 10 and 40, but suppose
> the new node lands at distance 20 (or 80); can such a thing happen?
>
> If not; why not?

Yes distances can be 20, 40 or 80. There is nothing that makes the node
distance to be 40 always.

> So you're relying on sched_domain_numa_masks_set/clear() to fix this up,
> but that in turn relies on the sched_domain_numa_levels thing to stay
> accurate.
>
> This all seems very fragile and unfortunate.
>

Any reasons why this is fragile?

--
Thanks and Regards
Srikar Dronamraju

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