Re: [PATCH] sched_ext: idle: small CPU iteration refactoring

[Yury Norov <yury.norov@gmail.com>]

On Sat, Jan 04, 2025 at 10:00:09AM +0100, Andrea Righi wrote:
> loop to check if all SMT CPUs are idle with
> cpumask_subset(). This simplifies the code and slightly improves
> efficiency, while preserving the original behavior.

In my experience, replacing iterator with a dedicated function makes
you 400-500 times faster. So 'slightly' sounds like an understatement
here.

> Note that idle_masks.smt handling remains racy, which is acceptable as
> it serves as an optimization and is self-correcting.
> 
> Suggested-by: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

Reviewed-by: Yury Norov <yury.norov@gmail.com>

> ---
>  kernel/sched/ext.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 926579624c41..0ce116e0f67c 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3671,10 +3671,8 @@ void __scx_update_idle(struct rq *rq, bool idle)
>  			 * idle_masks.smt handling is racy but that's fine as
>  			 * it's only for optimization and self-correcting.
>  			 */
> -			for_each_cpu(cpu, smt) {
> -				if (!cpumask_test_cpu(cpu, idle_masks.cpu))
> -					return;
> -			}
> +			if (!cpumask_subset(smt, idle_masks.cpu))
> +				return;
>  			cpumask_or(idle_masks.smt, idle_masks.smt, smt);
>  		} else {
>  			cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
> -- 
> 2.47.1
> 
> From yury