Re: [RFC PATCH 6/6 v7] sched/fair: Add EAS and idle cpu push trigger

[Christian Loehle <christian.loehle@arm.com>]

Some nits below for now

On 12/1/25 09:13, Vincent Guittot wrote:
> EAS is based on wakeup events to efficiently place tasks on the system, but
> there are cases where a task doesn't have wakeup events anymore or at a far
> too low pace. For such cases, we check if it's worht pushing hte task on

worth
the

> another CPUs instead of putting it back in the enqueued list.
> 
> Wake up events remain the main way to migrate tasks but we now detect
> situation where a task is stuck on a CPU by checking that its utilization
> is larger than the max available compute capacity (max cpu capacity or
> uclamp max setting)
> 
> When the system becomes overutilized and some CPUs are idle, we try to
> push tasks instead of waiting periodic load balance.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c     | 65 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/topology.c |  3 ++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9af8d0a61856..e9e1d0c05805 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6990,6 +6990,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  }
>  
>  static void fair_remove_pushable_task(struct rq *rq, struct task_struct *p);
> +

This doesn't belong here

>  /*
>   * Basically dequeue_task_fair(), except it can deal with dequeue_entity()
>   * failing half-way through and resume the dequeue later.
> @@ -8499,8 +8500,72 @@ static inline bool sched_push_task_enabled(void)
>  	return static_branch_unlikely(&sched_push_task);
>  }
>  
> +static inline bool task_stuck_on_cpu(struct task_struct *p, int cpu)
> +{
> +	unsigned long max_capa, util;
> +
> +	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);
> +}
> +
> +static inline bool sched_energy_push_task(struct task_struct *p, struct rq *rq)
> +{
> +	if (!sched_energy_enabled())
> +		return false;
> +
> +	if (is_rd_overutilized(rq->rd))
> +		return false;
> +
> +	if (task_stuck_on_cpu(p, cpu_of(rq)))
> +		return true;
> +
> +	if (!task_fits_cpu(p, cpu_of(rq)))
> +		return true;
> +
> +	return false;
> +}
> +
> +static inline bool sched_idle_push_task(struct task_struct *p, struct rq *rq)
> +{
> +	if (rq->nr_running == 1)
> +		return false;
> +
> +	if (!is_rd_overutilized(rq->rd))
> +		return false;
> +
> +	/* If there are idle cpus in the llc then try to push the task on it */
> +	if (test_idle_cores(cpu_of(rq)))
> +		return true;
> +
> +	return false;
> +}
> +
> +
>  static bool fair_push_task(struct rq *rq, struct task_struct *p)
>  {
> +	if (!task_on_rq_queued(p))
> +		return false;
> +
> +	if (p->se.sched_delayed)
> +		return false;
> +
> +	if (p->nr_cpus_allowed == 1)
> +		return false;
> +
> +	if (sched_energy_push_task(p, rq))
> +		return true;
> +
> +	if (sched_idle_push_task(p, rq))
> +		return true;
> +
>  	return false;
>  }
>  
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index cf643a5ddedd..5edf7b117ed9 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -391,10 +391,13 @@ static void sched_energy_set(bool has_eas)
>  		if (sched_debug())
>  			pr_info("%s: stopping EAS\n", __func__);
>  		static_branch_disable_cpuslocked(&sched_energy_present);
> +		static_branch_dec_cpuslocked(&sched_push_task);
> +	} else if (has_eas && !sched_energy_enabled()) {
>  	} else if (has_eas && !static_branch_unlikely(&sched_energy_present)) {

This could just be (has_eas && && sched_energy_enabled() && !static_branch_unlikely(&sched_energy_present))
to avoid the awkward else if above

>  		if (sched_debug())
>  			pr_info("%s: starting EAS\n", __func__);
>  		static_branch_enable_cpuslocked(&sched_energy_present);
> +		static_branch_inc_cpuslocked(&sched_push_task);
>  	}
>  }
>