Re: [PATCH 2/6] sched/fair: Check whether active load balance is needed in busiest group

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 04, 2023 at 09:09:52AM -0700, Tim Chen wrote:
> From: Tim C Chen <tim.c.chen@linux.intel.com>
> 
> In the busiest group, we need to consider whether active load balance
> to a local group is needed even when it is not overloaded.  For example,
> when the busiest group is a SMT group that's fully busy and the destination group
> is a cluster group with idle CPU.  Such condition is considered by
> asym_active_balance() in load balancing but not when looking for busiest
> group and load imbalance.  Add this consideration in find_busiest_group()
> and calculate_imbalance().
> 
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 87317634fab2..bde962aa160a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9433,6 +9433,17 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  				sgs->group_capacity;
>  }
>  
> +/* One group is SMT while the other group is not */
> +static inline bool asymmetric_groups(struct sched_group *sg1,
> +				    struct sched_group *sg2)
> +{
> +	if (!sg1 || !sg2)
> +		return false;
> +
> +	return (sg1->flags & SD_SHARE_CPUCAPACITY) !=
> +		(sg2->flags & SD_SHARE_CPUCAPACITY);
> +}
> +
>  /**
>   * update_sd_pick_busiest - return 1 on busiest group
>   * @env: The load balancing environment.
> @@ -10079,6 +10090,31 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	update_idle_cpu_scan(env, sum_util);
>  }
>  
> +static inline bool asym_active_balance_busiest(struct lb_env *env, struct sd_lb_stats *sds)
> +{
> +	/*
> +	 * Don't balance to a group without spare capacity.
> +	 *
> +	 * Skip non asymmetric sched group balancing. That check
> +	 * is handled by code path handling imbalanced load between
> +	 * similar groups.
> +	 */
> +	if (env->idle == CPU_NOT_IDLE ||
> +	    sds->local_stat.group_type != group_has_spare ||
> +	    !asymmetric_groups(sds->local, sds->busiest))
> +		return false;
> +
> +	/*
> +	 * For SMT source group, pull when there are two or more
> +	 * tasks over-utilizing a core.
> +	 */
> +	if (sds->busiest->flags & SD_SHARE_CPUCAPACITY &&
> +	    sds->busiest_stat.sum_h_nr_running > 1)
> +		return true;
> +
> +	return false;
> +}

This all seems to be mixing two 'asymmetric' things in the 'asym'
namespace :/ One being the SD_ASYM_PACKING and then the above SMT/no-SMT
core thing.

> +
>  /**
>   * calculate_imbalance - Calculate the amount of imbalance present within the
>   *			 groups of a given sched_domain during load balance.
> @@ -10164,6 +10200,12 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  			return;
>  		}
>  
> +		if (asym_active_balance_busiest(env, sds)) {
> +			env->migration_type = migrate_task;
> +			env->imbalance = 1;
> +			return;
> +		}
> +
>  		if (busiest->group_weight == 1 || sds->prefer_sibling) {
>  			unsigned int nr_diff = busiest->sum_nr_running;
>  			/*
> @@ -10371,6 +10413,9 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  			 */
>  			goto out_balanced;
>  
> +		if (asym_active_balance_busiest(env, &sds))
> +			goto force_balance;
> +
>  		if (busiest->group_weight > 1 &&
>  		    local->idle_cpus <= (busiest->idle_cpus + 1))
>  			/*

All the cases here have a nice (CodingStyle busting) comment, perhaps
add the missing {} when hou add the comment?