[regression] 3.0-rc boot failure -- bisected to cd4ea6ae3982

[Anton Blanchard]

Hi,

> Urgh.. so those spans are generated by sched_domain_node_span(), and
> it looks like that simply picks the 15 nearest nodes to the one we've
> got without consideration for overlap with previously generated spans.

I do wonder if we need this extra level at all on ppc64. From memory
SGI added it for their massive setups, but our largest setup is 32 nodes
and breaking that down into 16 node chunks seems overkill.

I just realised we were setting NEWIDLE on our node definition and that
was causing large amounts of rebalance work even with
SD_NODES_PER_DOMAIN=16.

After removing it and bumping SD_NODES_PER_DOMAIN to 32, things look
pretty good.

Perhaps we should allow an arch to override SD_NODES_PER_DOMAIN so this
extra level is only used by SGI boxes.

Anton