On Tue, Nov 15, 2022 at 05:24:56PM +0000, Valentin Schneider wrote: > 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@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 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]? No. Your series adds an iterator, and in my experience the code that uses iterators of that sort is almost always better and easier to understand than cpumask_nth() or cpumask_next()-like users. My series has the only advantage that it allows keep existing codebase untouched. > 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@xxxxxxxxxx/ In terms of very common case of sequential invocation of local_spread() for cpus from 0 to nr_cpu_ids, the complexity of my approach is n * log n, and your approach is amortized O(n), which is better. Not a big deal _now_, as you mentioned in the other email. But we never know how things will evolve, right? So, I would take both and maybe in comment to cpumask_local_spread() mention that there's a better alternative for those who call the function for all CPUs incrementally. Thanks, Yury
