Re: [RFC,1/2] powerpc/numa: fix cpu_to_node() usage during boot

On Thu, 2015-02-07 at 23:02:02 UTC, Nishanth Aravamudan wrote:

Much like on x86, now that powerpc is using USE_PERCPU_NUMA_NODE_ID, we
have an ordering issue during boot with early calls to cpu_to_node().

"now that .." implies we changed something and broke this. What commit was
it that changed the behaviour?

The value returned by those calls now depend on the per-cpu area being
setup, but that is not guaranteed to be the case during boot. Instead,
we need to add an early_cpu_to_node() which doesn't use the per-CPU area
and call that from certain spots that are known to invoke cpu_to_node()
before the per-CPU areas are not configured.

On an example 2-node NUMA system with the following topology:

available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3
node 0 size: 2029 MB
node 0 free: 1753 MB
node 1 cpus: 4 5 6 7
node 1 size: 2045 MB
node 1 free: 1945 MB
node distances:
node   0   1 
  0:  10  40 
  1:  40  10 

we currently emit at boot:

[    0.000000] pcpu-alloc: [0] 0 1 2 3 [0] 4 5 6 7 

After this commit, we correctly emit:

[    0.000000] pcpu-alloc: [0] 0 1 2 3 [1] 4 5 6 7 

So it looks fairly sane, and I guess it's a bug fix.

But I'm a bit reluctant to put it in straight away without some time in next.

It looks like the symptom is that the per-cpu areas are all allocated on node
0, is that all that goes wrong?

cheers