Re: [PATCH 1/2] sched/fair: Improve consistency of allowed NUMA balance calculations

[Gautham R. Shenoy]

Hello Mel,

On Tue, Feb 08, 2022 at 09:43:33AM +0000, Mel Gorman wrote:
> There are inconsistencies when determining if a NUMA imbalance is allowed
> that should be corrected.
> 
> o allow_numa_imbalance changes types and is not always examining
>   the destination group so both the type should be corrected as
>   well as the naming.
> o find_idlest_group uses the sched_domain's weight instead of the
>   group weight which is different to find_busiest_group
> o find_busiest_group uses the source group instead of the destination
>   which is different to task_numa_find_cpu
> o Both find_idlest_group and find_busiest_group should account
>   for the number of running tasks if a move was allowed to be
>   consistent with task_numa_find_cpu
>

This patch looks good to me. Using the local_sgs.group_weight instead
of sd->span_weight is definitely helping improve the spread of the
stream threads on AMD EPYC processors.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>


> Fixes: 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA nodes")
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 095b0aa378df..4592ccf82c34 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9003,9 +9003,10 @@ static bool update_pick_idlest(struct sched_group *idlest,
>   * This is an approximation as the number of running tasks may not be
>   * related to the number of busy CPUs due to sched_setaffinity.
>   */
> -static inline bool allow_numa_imbalance(int dst_running, int dst_weight)
> +static inline bool
> +allow_numa_imbalance(unsigned int running, unsigned int weight)
>  {
> -	return (dst_running < (dst_weight >> 2));
> +	return (running < (weight >> 2));
>  }
>  
>  /*
> @@ -9139,12 +9140,13 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>  				return idlest;
>  #endif
>  			/*
> -			 * Otherwise, keep the task on this node to stay close
> -			 * its wakeup source and improve locality. If there is
> -			 * a real need of migration, periodic load balance will
> -			 * take care of it.
> +			 * Otherwise, keep the task close to the wakeup source
> +			 * and improve locality if the number of running tasks
> +			 * would remain below threshold where an imbalance is
> +			 * allowed. If there is a real need of migration,
> +			 * periodic load balance will take care of it.
>  			 */
> -			if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->span_weight))
> +			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
>  				return NULL;
>  		}
>  
> @@ -9350,7 +9352,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  		/* Consider allowing a small imbalance between NUMA groups */
>  		if (env->sd->flags & SD_NUMA) {
>  			env->imbalance = adjust_numa_imbalance(env->imbalance,
> -				busiest->sum_nr_running, busiest->group_weight);
> +				local->sum_nr_running + 1, local->group_weight);
>  		}
>  
>  		return;
> -- 
> 2.31.1
>