Re: [PATCH v2 0/8] sched/topology: add for_each_numa_cpu() macro

[Valentin Schneider]

On 30/04/23 10:18, Yury Norov wrote:
> for_each_cpu() is widely used in kernel, and it's beneficial to create
> a NUMA-aware version of the macro.
>
> Recently added for_each_numa_hop_mask() works, but switching existing
> codebase to it is not an easy process.
>
> This series adds for_each_numa_cpu(), which is designed to be similar to
> the for_each_cpu(). It allows to convert existing code to NUMA-aware as
> simple as adding a hop iterator variable and passing it inside new macro.
> for_each_numa_cpu() takes care of the rest.
>
> At the moment, we have 2 users of NUMA-aware enumerators. One is
> Melanox's in-tree driver, and another is Intel's in-review driver:
>
> https://lore.kernel.org/lkml/20230216145455.661709-1-pawel.chmielewski@xxxxxxxxx/
>
> Both real-life examples follow the same pattern:
>
>         for_each_numa_hop_mask(cpus, prev, node) {
>                 for_each_cpu_andnot(cpu, cpus, prev) {
>                         if (cnt++ == max_num)
>                                 goto out;
>                         do_something(cpu);
>                 }
>                 prev = cpus;
>         }
>
> With the new macro, it has a more standard look, like this:
>
>         for_each_numa_cpu(cpu, hop, node, cpu_possible_mask) {
>                 if (cnt++ == max_num)
>                         break;
>                 do_something(cpu);
>         }
>
> Straight conversion of existing for_each_cpu() codebase to NUMA-aware
> version with for_each_numa_hop_mask() is difficult because it doesn't
> take a user-provided cpu mask, and eventually ends up with open-coded
> double loop. With for_each_numa_cpu() it shouldn't be a brainteaser.
> Consider the NUMA-ignorant example:
>
>         cpumask_t cpus = get_mask();
>         int cnt = 0, cpu;
>
>         for_each_cpu(cpu, cpus) {
>                 if (cnt++ == max_num)
>                         break;
>                 do_something(cpu);
>         }
>
> Converting it to NUMA-aware version would be as simple as:
>
>         cpumask_t cpus = get_mask();
>         int node = get_node();
>         int cnt = 0, hop, cpu;
>
>         for_each_numa_cpu(cpu, hop, node, cpus) {
>                 if (cnt++ == max_num)
>                         break;
>                 do_something(cpu);
>         }
>
> The latter looks more verbose and avoids from open-coding that annoying
> double loop. Another advantage is that it works with a 'hop' parameter with
> the clear meaning of NUMA distance, and doesn't make people not familiar
> to enumerator internals bothering with current and previous masks machinery.
>

LGTM, I ran the tests on a few NUMA topologies and that all seems to behave
as expected. Thanks for working on this! 

Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>