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

[Nishanth Aravamudan]

On 18.02.2014 [10:57:09 -0600], Christoph Lameter wrote:
> On Mon, 17 Feb 2014, Joonsoo Kim wrote:
> 
> > On Wed, Feb 12, 2014 at 10:51:37PM -0800, Nishanth Aravamudan wrote:
> > > Hi Joonsoo,
> > > Also, given that only ia64 and (hopefuly soon) ppc64 can set
> > > CONFIG_HAVE_MEMORYLESS_NODES, does that mean x86_64 can't have
> > > memoryless nodes present? Even with fakenuma? Just curious.
> 
> x86_64 currently does not support memoryless nodes otherwise it would
> have set CONFIG_HAVE_MEMORYLESS_NODES in the kconfig. Memoryless nodes are
> a bit strange given that the NUMA paradigm is to have NUMA nodes (meaning
> memory) with processors. MEMORYLESS nodes means that we have a fake NUMA
> node without memory but just processors. Not very efficient. Not sure why
> people use these configurations.

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?

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.

> > I don't know, because I'm not expert on NUMA system :)
> > At first glance, fakenuma can't be used for testing
> > CONFIG_HAVE_MEMORYLESS_NODES. Maybe some modification is needed.
> 
> Well yeah. You'd have to do some mods to enable that testing.

I might look into it, as it might have sped up testing these changes.

Thanks,
Nish