linux-kernel - Re: [PATCH 3/5] sched/fair: Rework feec() to use cost instead of spare capacity

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Message-ID: <4b5a6038-3b1e-4983-814b-280938173a2d@arm.com>
Date: Wed, 4 Sep 2024 17:07:15 +0200
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
Subject: Re: [PATCH 3/5] sched/fair: Rework feec() to use cost instead of
 spare capacity



On 8/30/24 15:03, Vincent Guittot wrote:
> feec() looks for the CPU with highest spare capacity in a PD assuming that
> it will be the best CPU from a energy efficiency PoV because it will
> require the smallest increase of OPP. Although this is true generally
> speaking, this policy also filters some others CPUs which will be as
> efficients because of using the same OPP.
> In fact, we really care about the cost of the new OPP that will be
> selected to handle the waking task. In many cases, several CPUs will end
> up selecting the same OPP and as a result using the same energy cost. In
> these cases, we can use other metrics to select the best CPU for the same
> energy cost.
> 
> Rework feec() to look 1st for the lowest cost in a PD and then the most
> performant CPU between CPUs.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   kernel/sched/fair.c | 466 +++++++++++++++++++++++---------------------
>   1 file changed, 244 insertions(+), 222 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e67d6029b269..2273eecf6086 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> -/*
> - * compute_energy(): Use the Energy Model to estimate the energy that @pd would
> - * consume for a given utilization landscape @eenv. When @dst_cpu < 0, the task
> - * contribution is ignored.
> - */
> -static inline unsigned long
> -compute_energy(struct energy_env *eenv, struct perf_domain *pd,
> -	       struct cpumask *pd_cpus, struct task_struct *p, int dst_cpu)
> +/*Check if the CPU can handle the waking task */
> +static int check_cpu_with_task(struct task_struct *p, int cpu)
>   {
> -	unsigned long max_util = eenv_pd_max_util(eenv, pd_cpus, p, dst_cpu);
> -	unsigned long busy_time = eenv->pd_busy_time;
> -	unsigned long energy;
> +	unsigned long p_util_min = uclamp_is_used() ? uclamp_eff_value(p, UCLAMP_MIN) : 0;
> +	unsigned long p_util_max = uclamp_is_used() ? uclamp_eff_value(p, UCLAMP_MAX) : 1024;
> +	unsigned long util_min = p_util_min;
> +	unsigned long util_max = p_util_max;
> +	unsigned long util = cpu_util(cpu, p, cpu, 0);
> +	struct rq *rq = cpu_rq(cpu);
>   
> -	if (dst_cpu >= 0)
> -		busy_time = min(eenv->pd_cap, busy_time + eenv->task_busy_time);
> +	/*
> +	 * Skip CPUs that cannot satisfy the capacity request.
> +	 * IOW, placing the task there would make the CPU
> +	 * overutilized. Take uclamp into account to see how
> +	 * much capacity we can get out of the CPU; this is
> +	 * aligned with sched_cpu_util().
> +	 */
> +	if (uclamp_is_used() && !uclamp_rq_is_idle(rq)) {
> +		unsigned long rq_util_min, rq_util_max;
> +		/*
> +		 * Open code uclamp_rq_util_with() except for
> +		 * the clamp() part. I.e.: apply max aggregation
> +		 * only. util_fits_cpu() logic requires to
> +		 * operate on non clamped util but must use the
> +		 * max-aggregated uclamp_{min, max}.
> +		 */
> +		rq_util_min = uclamp_rq_get(rq, UCLAMP_MIN);
> +		rq_util_max = uclamp_rq_get(rq, UCLAMP_MAX);
> +		util_min = max(rq_util_min, p_util_min);
> +		util_max = max(rq_util_max, p_util_max);
> +	}
> +	return util_fits_cpu(util, util_min, util_max, cpu);
> +}
>   
> -	energy = em_cpu_energy(pd->em_pd, max_util, busy_time, eenv->cpu_cap);

I think em_cpu_energy() would need to be removed with this patch,
if there are no more references to it.