Re: [PATCH 6/7 v2] sched/fair: Add misfit case to push task callback for EAS

[Pierre Gondois <pierre.gondois@arm.com>]

On 12/17/24 17:07, Vincent Guittot wrote:
> Some task misfit cases can be handled directly by the push callback
> instead of triggering an idle load balance to pull the task on a better
> CPU.

Aren't misfit tasks migrated using active_load_balance_cpu_stop() rather than
the push mechanism ?

Also, I don't see cases where a misfit task would not be migrated by either
the push mechanism or the misfit handling present in this patch. Is it possible
to detail a case where the misfit load balancer would still be needed ?

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> 
> # Conflicts:
> #	kernel/sched/fair.c
> ---
>   kernel/sched/fair.c | 53 +++++++++++++++++++++++++++++----------------
>   1 file changed, 34 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2affc063da55..9bddb094ee21 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8541,6 +8541,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>   			target_stat.runnable = cpu_runnable(cpu_rq(cpu));
>   			target_stat.capa = capacity_of(cpu);
>   			target_stat.nr_running = cpu_rq(cpu)->cfs.h_nr_runnable;
> +			if ((p->on_rq) && (!p->se.sched_delayed) && (cpu == prev_cpu))
> +				target_stat.nr_running--;
>   
>   			/* If the target needs a lower OPP, then look up for
>   			 * the corresponding OPP and its associated cost.
> @@ -8623,48 +8625,58 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>   
>   static inline bool task_misfit_cpu(struct task_struct *p, int cpu)
>   {
> -	unsigned long max_capa = get_actual_cpu_capacity(cpu);
> -	unsigned long util = task_util_est(p);
> +	unsigned long max_capa, util;
> +
> +	if (p->nr_cpus_allowed == 1)
> +		return false;
>   
> -	max_capa = min(max_capa, uclamp_eff_value(p, UCLAMP_MAX));
> -	util = max(util, task_runnable(p));
> +	max_capa = min(get_actual_cpu_capacity(cpu),
> +		       uclamp_eff_value(p, UCLAMP_MAX));
> +	util = max(task_util_est(p), task_runnable(p));
>   
>   	/*
>   	 * Return true only if the task might not sleep/wakeup because of a low
>   	 * compute capacity. Tasks, which wake up regularly, will be handled by
>   	 * feec().
>   	 */
> -	return (util > max_capa);
> +	if (util > max_capa)
> +		return true;
> +
> +	/* Return true if the task doesn't fit anymore to run on the cpu */
> +	if ((arch_scale_cpu_capacity(cpu) < p->max_allowed_capacity) && !task_fits_cpu(p, cpu))
> +		return true;

This logic seems to already be present in update_misfit_status(). Maybe it would be
good to factorize it to have a common criteria for misfit tasks.

> +
> +	return false;
>   }
>   
>   static int active_load_balance_cpu_stop(void *data);
>   
> -static inline void migrate_misfit_task(struct task_struct *p, struct rq *rq)
> +static inline bool migrate_misfit_task(struct task_struct *p, struct rq *rq)
>   {
>   	int new_cpu, cpu = cpu_of(rq);
>   
>   	if (!sched_energy_enabled() || is_rd_overutilized(rq->rd))
> -		return;
> +		return false;
>   
>   	if (WARN_ON(!p))
> -		return;
> +		return false;
>   
> -	if (WARN_ON(p != rq->curr))
> -		return;
> +	if (WARN_ON(!task_current(rq, p)))
> +		return false;
>   
>   	if (is_migration_disabled(p))
> -		return;
> +		return false;
>   
> -	if ((rq->nr_running > 1) || (p->nr_cpus_allowed == 1))
> -		return;
> +	if (rq->nr_running > 1)
> +		return false;

NIT: Maybe the condition (p->nr_cpus_allowed == 1) could have already been
part of task_misfit_cpu() in the previous patch.

>   
>   	if (!task_misfit_cpu(p, cpu))
> -		return;
> +		return false;
>   
>   	new_cpu = find_energy_efficient_cpu(p, cpu);
>   
>   	if (new_cpu == cpu)
> -		return;
> +		return false;
>   
>   	/*
>   	 * ->active_balance synchronizes accesses to
> @@ -8675,13 +8687,15 @@ static inline void migrate_misfit_task(struct task_struct *p, struct rq *rq)
>   		rq->active_balance = 1;
>   		rq->push_cpu = new_cpu;
>   	} else
> -		return;
> +		return false;
>   
>   	raw_spin_rq_unlock(rq);
>   	stop_one_cpu_nowait(cpu,
>   		active_load_balance_cpu_stop, rq,
>   		&rq->active_balance_work);
>   	raw_spin_rq_lock(rq);
> +
> +	return true;
>   }
>   
>   static inline int has_pushable_tasks(struct rq *rq)
> @@ -13299,9 +13313,10 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
>   	if (static_branch_unlikely(&sched_numa_balancing))
>   		task_tick_numa(rq, curr);
>   
> -	migrate_misfit_task(curr, rq);
> -	update_misfit_status(curr, rq);
> -	check_update_overutilized_status(task_rq(curr));
> +	if (!migrate_misfit_task(curr, rq)) {
> +		update_misfit_status(curr, rq);

If the system is not-OU, the only case I see where migrate_misfit_task() would
not detect a misfit task and update_misfit_status() would is if there is another
task on the rq. I.e. through:
migrate_misfit_task()
\-if (rq->nr_running > 1) return false;

However in this case, the push callback should migrate the misfit task. So is it still
necessary to look for misfit task through sched_balance_find_src_group() ?

> +		check_update_overutilized_status(task_rq(curr));
> +	}
>   
>   	task_tick_core(rq, curr);
>   }