[PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality

Yury Norov
2022-11-12T19:09:56Z

cpumask_local_spread() currently checks local node for presence of i'th
CPU, and then if it finds nothing makes a flat search among all non-local
CPUs. We can do it better by checking CPUs per NUMA hops.

This series is inspired by Tariq Toukan and Valentin Schneider's "net/mlx5e:
Improve remote NUMA preferences used for the IRQ affinity hints"

https://patchwork.kernel.org/project/netdevbpf/patch/20220728191203.4055-3-tariqt@nvidia.com/

According to their measurements, for mlx5e:

Bottleneck in RX side is released, reached linerate (~1.8x speedup).
~30% less cpu util on TX.

This patch makes cpumask_local_spread() traversing CPUs based on NUMA
distance, just as well, and I expect comparabale improvement for its
users, as in case of mlx5e.

I tested new behavior on my VM with the following NUMA configuration:

root@debian:~# numactl -H
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3
node 0 size: 3869 MB
node 0 free: 3740 MB
node 1 cpus: 4 5
node 1 size: 1969 MB
node 1 free: 1937 MB
node 2 cpus: 6 7
node 2 size: 1967 MB
node 2 free: 1873 MB
node 3 cpus: 8 9 10 11 12 13 14 15
node 3 size: 7842 MB
node 3 free: 7723 MB
node distances:
node 0 1 2 3
0: 10 50 30 70
1: 50 10 70 30
2: 30 70 10 50
3: 70 30 50 10

And the cpumask_local_spread() for each node and offset traversing looks
like this:

node 0: 0 1 2 3 6 7 4 5 8 9 10 11 12 13 14 15
node 1: 4 5 8 9 10 11 12 13 14 15 0 1 2 3 6 7
node 2: 6 7 0 1 2 3 8 9 10 11 12 13 14 15 4 5
node 3: 8 9 10 11 12 13 14 15 4 5 6 7 0 1 2 3

v1: https://lore.kernel.org/lkml/20221111040027.621646-5-yury.norov@gmail.com/T/
v2:
- use bsearch() in sched_numa_find_nth_cpu();
- fix missing 'static inline' in 3rd patch.

Yury Norov (4):
lib/find: introduce find_nth_and_andnot_bit
cpumask: introduce cpumask_nth_and_andnot
sched: add sched_numa_find_nth_cpu()
cpumask: improve on cpumask_local_spread() locality

include/linux/cpumask.h | 20 +++++++++++++++
include/linux/find.h | 33 ++++++++++++++++++++++++
include/linux/topology.h | 8 ++++++
kernel/sched/topology.c | 55 ++++++++++++++++++++++++++++++++++++++++
lib/cpumask.c | 12 ++-------
lib/find_bit.c | 9 +++++++
6 files changed, 127 insertions(+), 10 deletions(-)

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2.34.1