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

Valentin Schneider
2022-11-15T17:26:14Z

Hi,

On 12/11/22 11:09, Yury Norov wrote:
> 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
>

Is this meant as a replacement for [1]?

I like that this is changing an existing interface so that all current
users directly benefit from the change. Now, about half of the users of
cpumask_local_spread() use it in a loop with incremental @i parameter,
which makes the repeated bsearch a bit of a shame, but then I'm tempted to
say the first point makes it worth it.

[1]: https://lore.kernel.org/all/20221028164959.1367250-1-vschneid@redhat.com/

> 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(-)
>
> --
> 2.34.1