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

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Message-ID: <20251202094454.9608-1-hdanton@sina.com>
Date: Tue,  2 Dec 2025 17:44:52 +0800
From: Hillf Danton <hdanton@...a.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: peterz@...radead.org,
	linux-kernel@...r.kernel.org,
	pierre.gondois@....com,
	kprateek.nayak@....com,
	qyousef@...alina.io,
	christian.loehle@....com,
	luis.machado@....com
Subject: Re: [RFC PATCH 6/6 v7] sched/fair: Add EAS and idle cpu push trigger

On Mon,  1 Dec 2025 10:13:08 +0100 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
> 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@...aro.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);
> +
>  /*
>   * 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;

Task is queued on rq.
> +
> +	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 task is stuck on CPU, it could not be on rq. Weird.
> +
> +	if (sched_idle_push_task(p, rq))
> +		return true;
> +
>  	return false;
>  }
>  
More, in the tick path,

task_tick_fair
  check_pushable_task
    fair_push_task
      task_on_rq_queued // this check makes no sense