netdev - Re: [PATCH v2 0/4] cpumask: improve on cpumask_local

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Date:   Tue, 15 Nov 2022 17:24:56 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Barry Song <baohua@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        haniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gal Pressman <gal@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Saeed Mahameed <saeedm@...dia.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Yury Norov <yury.norov@...il.com>, linux-crypto@...r.kernel.org,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality

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@...ian:~# 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