Re: [PATCH v3 2/7] lib/group_cpus: optimize inner loop in grp_spread_init_one()

[Ming Lei]

On Mon, Dec 11, 2023 at 08:21:02PM -0800, Yury Norov wrote:
> The loop starts from the beginning every time we switch to the next
> sibling mask. This is the Schlemiel the Painter's style of coding
> because we know for sure that nmsk is clear up to current CPU, and we
> can just continue from the next CPU.
> 
> Also, we can do it nicer if leverage the dedicated for_each() iterator,
> and simplify the logic of clearing a bit in nmsk.
> 
> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> ---
>  lib/group_cpus.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index ee272c4cefcc..10dead3ab0e0 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -30,14 +30,13 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  
>  		/* If the cpu has siblings, use them first */
>  		siblmsk = topology_sibling_cpumask(cpu);
> -		for (sibl = -1; cpus_per_grp > 0; ) {
> -			sibl = cpumask_next(sibl, siblmsk);
> -			if (sibl >= nr_cpu_ids)
> -				break;
> -			if (!cpumask_test_and_clear_cpu(sibl, nmsk))
> -				continue;
> +		sibl = cpu + 1;

It doesn't have to 'cpu + 1', see below comment.

> +
> +		for_each_cpu_and_from(sibl, siblmsk, nmsk) {
> +			cpumask_clear_cpu(sibl, nmsk);
>  			cpumask_set_cpu(sibl, irqmsk);
> -			cpus_per_grp--;
> +			if (cpus_per_grp-- == 0)

			if (--cpus_per_grp == 0)

> +				return;

I think for_each_cpu_and() should work just fine, cause cpu has been cleared
from nmsk, so the change can be something like, then patch 1 isn't
necessary.


		for_each_cpu_and(sibl, siblmsk, nmsk) {
			cpumask_clear_cpu(sibl, nmsk);
  			cpumask_set_cpu(sibl, irqmsk);
			if (--cpus_per_grp == 0)
				return;
		}


Thanks,
Ming