Re: [Qemu-devel] qemu-ppc and NUMA topology

[Nishanth Aravamudan]

On 27.05.2014 [14:59:03 -0700], Nishanth Aravamudan wrote:
> On 20.05.2014 [12:44:15 +1000], Alexey Kardashevskiy wrote:
> > On 05/20/2014 10:06 AM, Nishanth Aravamudan wrote:
> > > On 19.05.2014 [15:37:52 -0700], Nishanth Aravamudan wrote:
> > >> Hi Alexey,
> > >>
> > >> I've been looking at hw/ppc/spapr.c::spapr_populate_memory() and ran
> > >> into a few questions:
> > >>
> > >> 1) The values from 1 to nb_numa_nodes are used as indices into the
> > >> node_mem array, but that is not populated, necessarily, linearly.
> > >> vl.c::add_node() uses the nodeid parameter as the index into node_mem,
> > >> if it is specified.
> > >>
> > >> 2) The node ID is based upon the index into the array, but it seems like
> > >> it should actually be based upon the nodeid specified, if any. That is,
> > >> we set the value at index 4 (which is statically the reference point in
> > >> 'ibm,associativity-reference-points') of 'ibm,associativty' for each
> > >> 'ibm,address@hidden' node to the index we are currently at. But as
> > >> mentioned in 1) above that index isn't necessarily currently the nodeid
> > >> specified on the command-line.
> > >>
> > >> What this all means, is that if I specify something like:
> > >>
> > >> -numa node,nodeid=1,cpus=0-7,mem=2048 -numa
> > >> node,nodeid=5,cpus=8-15,mem=0 -numa node,nodeid=9,mem=2048
> > >>
> > >> Linux sees:
> > >>
> > >> numactl --hardware
> > >> available: 3 nodes (0-2)
> > >> node 0 cpus: 8 9 10 11 12 13 14 15
> > >> node 0 size: 0 MB
> > >> node 0 free: 0 MB
> > >> node 1 cpus: 0 1 2 3 4 5 6 7
> > >> node 1 size: 2024 MB
> > >> node 1 free: 1560 MB
> > >> node 2 cpus:
> > >> node 2 size: 0 MB
> > >> node 2 free: 0 MB
> > >>
> > >> Maybe we don't really care about this, but I just noticed it when trying
> > >> to reproduce some really weird topologies from PowerVM.
> > > 
> > > Upon further investigation into node_mem, it seems like this assumption
> > > is present throughout the qemu code, e.g, the qemu monitor 'info numa'
> > > command. Will just document it for myself as a weird way to make
> > > memoryless nodes show up :)
> > 
> > I never looked closely at this NUMA business so I know as much as you do :)
> > You seem to be right, vl.c seems to get things right (it uses nodeid as an
> > index) but spapr.c is broken and we probably should fix it but it does not
> > sound very urgent to me...
> 
> Well, and looking at it more, it feels like perhaps that none of the
> qemu code is particularly careful about this -- and since you can
> explicitly assign 0 memory to a node, you can't simply check for 0 in
> node_mem for an unassigned node (and node_mem is an unsigned array).
> 
> I'll look at the behavior on x86 and get back to you.

Well, it looks like ppc is no worse off than x86 here -- passing a
similar command-line to qemu-system-x86_64, I get the same result in the
VM (nodes numbered starting at 0, etc).

Perhaps it makes sense to not allow non-sequential NUMA node ordering,
since it isn't really supported anyways? I'm not entirely sure I see why
it'd be necessary for a guest in any case.

Thanks,
Nish