Re: [PATCH] sched/fair: Add SMT4 group_smt_balance handling

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 27, 2023 at 06:32:44AM -0700, Tim Chen wrote:

>  kernel/sched/fair.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a87988327f88..566686c5f2bd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9563,7 +9563,7 @@ static inline long sibling_imbalance(struct lb_env *env,
>  	imbalance /= ncores_local + ncores_busiest;
>  
>  	/* Take advantage of resource in an empty sched group */
> -	if (imbalance == 0 && local->sum_nr_running == 0 &&
> +	if (imbalance <= 1 && local->sum_nr_running == 0 &&
>  	    busiest->sum_nr_running > 1)
>  		imbalance = 2;
>  
> @@ -9751,6 +9751,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		break;
>  
>  	case group_smt_balance:
> +		/* no idle cpus on both groups handled by group_fully_busy below */
> +		if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
> +			if (sgs->idle_cpus > busiest->idle_cpus)
> +				return false;
> +			if (sgs->idle_cpus < busiest->idle_cpus)
> +				return true;
> +			if (sgs->sum_nr_running <= busiest->sum_nr_running)
> +				return false;
> +			else
> +				return true;
> +		}
> +		goto fully_busy;
> +		break;

This is really daft; why can't this simply be: fallthrough; ? At the
very least that break must go.

> +
>  	case group_fully_busy:
>  		/*
>  		 * Select the fully busy group with highest avg_load. In
> @@ -9763,7 +9777,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		 * select the 1st one, except if @sg is composed of SMT
>  		 * siblings.
>  		 */
> -
> +fully_busy:
>  		if (sgs->avg_load < busiest->avg_load)
>  			return false;
>  
> -- 
> 2.32.0
> 
>