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,memory@....' 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. -Nish
