Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance calculation between asymmetric groups

[Shrikanth Hegde <sshegde@linux.vnet.ibm.com>]

On 7/8/23 4:27 AM, Tim Chen wrote:
> From: Tim C Chen <tim.c.chen@linux.intel.com>
> 
> In the current prefer sibling load balancing code, there is an implicit
> assumption that the busiest sched group and local sched group are
> equivalent, hence the tasks to be moved is simply the difference in
> number of tasks between the two groups (i.e. imbalance) divided by two.
> 
> However, we may have different number of cores between the cluster groups,
> say when we take CPU offline or we have hybrid groups.  In that case,
> we should balance between the two groups such that #tasks/#cores ratio
> is the same between the same between both groups.  Hence the imbalance

nit: type here. the same between is repeated.

> computed will need to reflect this.
> 
> Adjust the sibling imbalance computation to take into account of the
> above considerations.
> 
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++----
>  1 file changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f636d6c09dc6..f491b94908bf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9372,6 +9372,41 @@ static inline bool smt_balance(struct lb_env *env, struct sg_lb_stats *sgs,
>  	return false;
>  }
> 
> +static inline long sibling_imbalance(struct lb_env *env,
> +				    struct sd_lb_stats *sds,
> +				    struct sg_lb_stats *busiest,
> +				    struct sg_lb_stats *local)
> +{
> +	int ncores_busiest, ncores_local;
> +	long imbalance;

can imbalance be unsigned int or unsigned long? as sum_nr_running is unsigned int.

> +
> +	if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
> +		return 0;
> +
> +	ncores_busiest = sds->busiest->cores;
> +	ncores_local = sds->local->cores;
> +
> +	if (ncores_busiest == ncores_local) {
> +		imbalance = busiest->sum_nr_running;
> +		lsub_positive(&imbalance, local->sum_nr_running);
> +		return imbalance;
> +	}
> +
> +	/* Balance such that nr_running/ncores ratio are same on both groups */
> +	imbalance = ncores_local * busiest->sum_nr_running;
> +	lsub_positive(&imbalance, ncores_busiest * local->sum_nr_running);
> +	/* Normalize imbalance and do rounding on normalization */
> +	imbalance = 2 * imbalance + ncores_local + ncores_busiest;
> +	imbalance /= ncores_local + ncores_busiest;
> +

Could this work for case where number of CPU/cores would differ
between two sched groups in a sched domain? Such as problem pointed
by tobias on S390. It would be nice if this patch can work for that case 
as well. Ran numbers for a few cases. It looks to work.
https://lore.kernel.org/lkml/20230704134024.GV4253@hirez.programming.kicks-ass.net/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901



> +	/* Take advantage of resource in an empty sched group */
> +	if (imbalance == 0 && local->sum_nr_running == 0 &&
> +	    busiest->sum_nr_running > 1)
> +		imbalance = 2;
> +

I don't see how this case would be true. When there are unequal number of cores and local->sum_nr_ruuning 
is 0, and busiest->sum_nr_running is atleast 2, imbalance will be atleast 1. 


Reviewed-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>

> +	return imbalance;
> +}
> +
>  static inline bool
>  sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
>  {
> @@ -10230,14 +10265,12 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  		}
> 
>  		if (busiest->group_weight == 1 || sds->prefer_sibling) {
> -			unsigned int nr_diff = busiest->sum_nr_running;
>  			/*
>  			 * When prefer sibling, evenly spread running tasks on
>  			 * groups.
>  			 */
>  			env->migration_type = migrate_task;
> -			lsub_positive(&nr_diff, local->sum_nr_running);
> -			env->imbalance = nr_diff;
> +			env->imbalance = sibling_imbalance(env, sds, busiest, local);
>  		} else {
> 
>  			/*
> @@ -10424,7 +10457,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  	 * group's child domain.
>  	 */
>  	if (sds.prefer_sibling && local->group_type == group_has_spare &&
> -	    busiest->sum_nr_running > local->sum_nr_running + 1)
> +	    sibling_imbalance(env, &sds, busiest, local) > 1)
>  		goto force_balance;
> 
>  	if (busiest->group_type != group_overloaded) {