linux-kernel - Re: [RFC PATCH] sched/feec: Simplify the traversal of pd'cpus

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Message-ID: <cf7847e4-78a6-4286-baba-60ace0c3d507@arm.com>
Date: Thu, 14 Aug 2025 09:46:08 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, vdonnefort@...gle.com, ke.wang@...soc.com,
 xuewen.yan94@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/feec: Simplify the traversal of pd'cpus



On 12.08.25 10:33, Xuewen Yan wrote:
> Now we use for_each_cpu() to traversal all pd's cpus,
> it is in order to compute the pd_cap. This approach may
> result in some unnecessary judgments.
> We can simply calculate pd_cap as follows:
> 
> pd_cap = cpu_actual_cap * cpumask_weight(pd_cpus);
> 
> Then we can AND pd'scpus, sd's cpus and task's cpus_ptr
> before traversing, which can save some unnecessary judgment.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/fair.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..e47fe94d6889 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8330,18 +8330,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)

Just a thought ...

for (; pd; pd = pd->next)

  cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);    <-- (1)
  cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);


  if (cpumask_empty(cpus))
    continue;                                               <-- (2)

Can you not mask cpus already early in the pd loop (1) and then profit
from (2) in these rare cases? IIRC, the sd only plays a role here in
exclusive cpusets scenarios which I don't thing anybody deploys with EAS?

>  		cpu_actual_cap = get_actual_cpu_capacity(cpu);
>  
>  		eenv.cpu_cap = cpu_actual_cap;
> -		eenv.pd_cap = 0;
> +		eenv.pd_cap = cpu_actual_cap * cpumask_weight(cpus);
>  
> -		for_each_cpu(cpu, cpus) {
> -			struct rq *rq = cpu_rq(cpu);
> -
> -			eenv.pd_cap += cpu_actual_cap;
> -
> -			if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> -				continue;
> +		cpumask_and(cpus, cpus, sched_domain_span(sd));
>  
> -			if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> -				continue;
> +		for_each_cpu_and(cpu, cpus, p->cpus_ptr) {
> +			struct rq *rq = cpu_rq(cpu);
>  
>  			util = cpu_util(cpu, p, cpu, 0);
>  			cpu_cap = capacity_of(cpu);