Re: [PATCH v2 09/22] sched/fair: Use IPC class score to select a busiest runqueue

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi,

On Monday 28 Nov 2022 at 05:20:47 (-0800), Ricardo Neri wrote:
> For two runqueues of equal priority and equal number of running of tasks,
> select the one whose current task would have the highest IPC class score
> if placed on the destination CPU.
> 
[..]
> +static int ipcc_score_delta(struct task_struct *p, int alt_cpu)
> +{
> +	unsigned long ipcc = p->ipcc;
> +
> +	if (!sched_ipcc_enabled())
> +		return INT_MIN;
> +
> +	return arch_get_ipcc_score(ipcc, alt_cpu) -
> +	       arch_get_ipcc_score(ipcc, task_cpu(p));

Nit: arch_get_ipcc_score() return values are never checked for error.

> +}
> +
>  #else /* CONFIG_IPC_CLASSES */
>  static void update_sg_lb_ipcc_stats(struct sg_lb_ipcc_stats *sgcs,
>  				    struct rq *rq)
> @@ -9258,6 +9276,11 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
>  	return false;
>  }
>  
> +static int ipcc_score_delta(struct task_struct *p, int alt_cpu)
> +{
> +	return INT_MIN;
> +}
> +
>  #endif /* CONFIG_IPC_CLASSES */
>  
>  /**
> @@ -10419,8 +10442,8 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  {
>  	struct rq *busiest = NULL, *rq;
>  	unsigned long busiest_util = 0, busiest_load = 0, busiest_capacity = 1;
> +	int i, busiest_ipcc_delta = INT_MIN;
>  	unsigned int busiest_nr = 0;
> -	int i;
>  
>  	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
>  		unsigned long capacity, load, util;
> @@ -10526,8 +10549,37 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  
>  		case migrate_task:
>  			if (busiest_nr < nr_running) {
> +				struct task_struct *curr;
> +
>  				busiest_nr = nr_running;
>  				busiest = rq;
> +
> +				/*
> +				 * Remember the IPC score delta of busiest::curr.
> +				 * We may need it to break a tie with other queues
> +				 * with equal nr_running.
> +				 */
> +				curr = rcu_dereference(busiest->curr);
> +				busiest_ipcc_delta = ipcc_score_delta(curr,
> +								      env->dst_cpu);
> +			/*
> +			 * If rq and busiest have the same number of running
> +			 * tasks, pick rq if doing so would give rq::curr a
> +			 * bigger IPC boost on dst_cpu.
> +			 */
> +			} else if (sched_ipcc_enabled() &&
> +				   busiest_nr == nr_running) {
> +				struct task_struct *curr;
> +				int delta;
> +
> +				curr = rcu_dereference(rq->curr);
> +				delta = ipcc_score_delta(curr, env->dst_cpu);
> +
> +				if (busiest_ipcc_delta < delta) {
> +					busiest_ipcc_delta = delta;
> +					busiest_nr = nr_running;
> +					busiest = rq;
> +				}
>  			}
>  			break;
>  

While in the commit message you describe this as breaking a tie for
asym_packing, the code here does not only affect asym_packing. If
another architecture would have sched_ipcc_enabled() it would use this
as generic policy, and that might not be desired.

Hope it helps,
Ionela.

> -- 
> 2.25.1
> 
>