[PATCH 0/1] powerpc/numa: do not skip node 0 in lookup table

[Daniel Henrique Barboza]

Hi,

This is a simple fix that I made while testing NUMA changes
I'm making in QEMU [1]. Setting any non-zero value to the
associativity of NUMA node 0 has no impact in the output
of 'numactl' because the distance_lookup_table is never
initialized for node 0.

Seeing through the LOPAPR spec and git history I found no
technical reason to skip node 0, which makes me believe this is
a bug that got under the radar up until now because no one
attempted to set node 0 associativity like I'm doing now.

For anyone wishing to give it a spin, using the QEMU build
in [1] and experimenting with NUMA distances, such as:

sudo ./qemu-system-ppc64 -machine pseries-5.2,accel=kvm,usb=off,dump-guest-core=off -m 65536 -overcommit mem-lock=off -smp 4,sockets=4,cores=1,threads=1 -rtc base=utc -display none -vga none -nographic -boot menu=on -device spapr-pci-host-bridge,index=1,id=pci.1 -device spapr-pci-host-bridge,index=2,id=pci.2 -device spapr-pci-host-bridge,index=3,id=pci.3 -device spapr-pci-host-bridge,index=4,id=pci.4 -device qemu-xhci,id=usb,bus=pci.0,addr=0x2 -drive file=/home/danielhb/f32.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -device usb-kbd,id=input0,bus=usb.0,port=1 -device usb-mouse,id=input1,bus=usb.0,port=2 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -msg timestamp=on \
-numa node,nodeid=0,cpus=0 -numa node,nodeid=1,cpus=1 \
-numa node,nodeid=2,cpus=2 -numa node,nodeid=3,cpus=3 \
-numa dist,src=0,dst=1,val=80 -numa dist,src=0,dst=2,val=80 \
-numa dist,src=0,dst=3,val=80 -numa dist,src=1,dst=2,val=80 \
-numa dist,src=1,dst=3,val=80 -numa dist,src=2,dst=3,val=80

The current kernel code will ignore the associativity of
node 0, and numactl will output this:

node distances:
node   0   1   2   3 
  0:  10  160  160  160 
  1:  160  10  80  80 
  2:  160  80  10  80 
  3:  160  80  80  10 

With this patch:

node distances:
node   0   1   2   3 
  0:  10  160  160  160 
  1:  160  10  80  40 
  2:  160  80  10  20 
  3:  160  40  20  10 


If anyone wonders, this patch has no conflict with the proposed
NUMA changes in [2] because Aneesh isn't changing this line.


[1] https://github.com/danielhb/qemu/tree/spapr_numa_v1
[2] https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200731111916.243569-1-aneesh.kumar@linux.ibm.com/


Daniel Henrique Barboza (1):
  powerpc/numa: do not skip node 0 when init lookup table

 arch/powerpc/mm/numa.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.26.2