[RFC PATCH 2/3] topology: support node_numa_mem() for determining the fallback node

[Nishanth Aravamudan]

On 18.02.2014 [13:58:20 -0600], Christoph Lameter wrote:
> On Tue, 18 Feb 2014, Nishanth Aravamudan wrote:
> 
> >
> > Well, on powerpc, with the hypervisor providing the resources and the
> > topology, you can have cpuless and memoryless nodes. I'm not sure how
> > "fake" the NUMA is -- as I think since the resources are virtualized to
> > be one system, it's logically possible that the actual topology of the
> > resources can be CPUs from physical node 0 and memory from physical node
> > 2. I would think with KVM on a sufficiently large (physically NUMA
> > x86_64) and loaded system, one could cause the same sort of
> > configuration to occur for a guest?
> 
> Ok but since you have a virtualized environment: Why not provide a fake
> home node with fake memory that could be anywhere? This would avoid the
> whole problem of supporting such a config at the kernel level.

We use the topology provided by the hypervisor, it does actually reflect
where CPUs and memory are, and their corresponding performance/NUMA
characteristics.

> Do not have a fake node that has no memory.
> 
> > In any case, these configurations happen fairly often on long-running
> > (not rebooted) systems as LPARs are created/destroyed, resources are
> > DLPAR'd in and out of LPARs, etc.
> 
> Ok then also move the memory of the local node somewhere?

This happens below the OS, we don't control the hypervisor's decisions.
I'm not sure if that's what you are suggesting.

Thanks,
Nish