[PATCH v3 0/5] cpumask: improve on cpumask_local_spread() locality

[Yury Norov]

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@xxxxxxxxxx/

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 comparable 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@xxxxxxxxx/T/
v2: https://lore.kernel.org/all/20221112190946.728270-3-yury.norov@xxxxxxxxx/T/
v3:
 - fix typo in find_nth_and_andnot_bit();
 - add 5th patch that simplifies cpumask_local_spread();
 - address various coding style nits.

Yury Norov (5):
  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
  lib/cpumask: reorganize cpumask_local_spread() logic

 include/linux/cpumask.h  | 20 ++++++++++++++
 include/linux/find.h     | 33 +++++++++++++++++++++++
 include/linux/topology.h |  8 ++++++
 kernel/sched/topology.c  | 57 ++++++++++++++++++++++++++++++++++++++++
 lib/cpumask.c            | 26 +++++-------------
 lib/find_bit.c           |  9 +++++++
 6 files changed, 134 insertions(+), 19 deletions(-)

-- 
2.34.1