linux-kernel - Re: [PATCH 5/7 v2] sched/fair: Add push task callback for EAS

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Message-ID: <0c3312f8-4e1c-49df-a084-8c236c9f7f53@arm.com>
Date: Thu, 16 Jan 2025 18:34:30 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, lukasz.luba@....com, rafael.j.wysocki@...el.com,
 linux-kernel@...r.kernel.org
Cc: qyousef@...alina.io, hongyan.xia2@....com, christian.loehle@....com,
 qperret@...gle.com
Subject: Re: [PATCH 5/7 v2] sched/fair: Add push task callback for EAS

Hello Vincent,

On 12/17/24 17:07, Vincent Guittot wrote:
> EAS is based on wakeup events to efficiently place tasks on the system, but
> there are cases where a task will not have wakeup events anymore or at a
> far too low pace. For such situation, we can take advantage of the task
> being put back in the enqueued list to check if it should be migrated on
> another CPU.
> 
> 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)

It seems there are 2 distinct cases:
a- The task is alone on a rq
b- The task shares the rq and is enqueued/dequeued

a. doesn't seem to need any of the push functions, and b. doesn't seem to
need any of the misfit functions. Maybe it's worth splitting the patch in 2.

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   kernel/sched/fair.c  | 206 +++++++++++++++++++++++++++++++++++++++++++
>   kernel/sched/sched.h |   2 +
>   2 files changed, 208 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cd046e8216a9..2affc063da55 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7088,6 +7088,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>   	hrtick_update(rq);
>   }
>   
> +static void dequeue_pushable_task(struct rq *rq, struct task_struct *p);
>   static void set_next_buddy(struct sched_entity *se);
>   
>   /*
> @@ -7118,6 +7119,9 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
>   		h_nr_idle = task_has_idle_policy(p);
>   		if (task_sleep || task_delayed || !se->sched_delayed)
>   			h_nr_runnable = 1;
> +
> +		if (task_sleep || task_on_rq_migrating(p))
> +			dequeue_pushable_task(rq, p);
>   	} else {
>   		cfs_rq = group_cfs_rq(se);
>   		slice = cfs_rq_min_slice(cfs_rq);
> @@ -8617,6 +8621,182 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>   	return target;
>   }
>   
> +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);
> +
> +	max_capa = min(max_capa, uclamp_eff_value(p, UCLAMP_MAX));
> +	util = max(util, 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().
> +	 */

NIT:
On a little CPU with min_OPP=256 and max_OPP=512,
a task with a util=100 and U_Max=10 will trigger this condition.
However:
- the task is already well placed from a power PoV
- the tasks has opportunities to sleep/wake-up
Shouldn't we ideally take:

unsigned long max_capa;
max_capa = max(min_capa(cpu), uclamp_eff_value(p, UCLAMP_MAX));
max_capa = min(get_actual_cpu_capacity(cpu), max_capa);

with min_capa(cpu) returning 256 in this case, i.e. the CPU capacity at the
lowest OPP ?

> +	return (util > max_capa);
> +}
> +

[...]