Node 0 not necessary for powerpc?

[David Rientjes]

On Tue, 11 Mar 2014, Nishanth Aravamudan wrote:

> I have a P7 system that has no node0, but a node0 shows up in numactl
> --hardware, which has no cpus and no memory (and no PCI devices):
> 
> numactl --hardware
> available: 4 nodes (0-3)
> node 0 cpus:
> node 0 size: 0 MB
> node 0 free: 0 MB
> node 1 cpus: 0 1 2 3 4 5 6 7 8 9 10 11
> node 1 size: 0 MB
> node 1 free: 0 MB
> node 2 cpus:
> node 2 size: 7935 MB
> node 2 free: 7716 MB
> node 3 cpus:
> node 3 size: 8395 MB
> node 3 free: 8015 MB
> node distances:
> node   0   1   2   3 
>   0:  10  20  10  20 
>   1:  20  10  20  20 
>   2:  10  20  10  20 
>   3:  20  20  20  10 
> 
> This is because we statically initialize N_ONLINE to be [0] in
> mm/page_alloc.c:
> 
>         [N_ONLINE] = { { [0] = 1UL } },
> 
> I'm not sure what the architectural requirements are here, but at least
> on this test system, removing this initialization, it boots fine and is
> running. I've not yet tried stress tests, but it's survived the
> beginnings of kernbench so far.
> 
> numactl --hardware
> available: 3 nodes (1-3)
> node 1 cpus: 0 1 2 3 4 5 6 7 8 9 10 11
> node 1 size: 0 MB
> node 1 free: 0 MB
> node 2 cpus:
> node 2 size: 7935 MB
> node 2 free: 7479 MB
> node 3 cpus:
> node 3 size: 8396 MB
> node 3 free: 8375 MB
> node distances:
> node   1   2   3 
>   1:  10  20  20 
>   2:  20  10  20 
>   3:  20  20  10
> 
> Perhaps we could put in a ARCH_DOES_NOT_NEED_NODE0 and only define it on
> powerpc for now, conditionalizing the above initialization on that?
> 

I don't know if anything has recently changed in the past year or so, but 
I've booted x86 machines with a hacked BIOS so that all memory on node 0 
is hotpluggable and offline, so I believe this is possible on x86 as well.