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

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Message-ID: <b16979c4-47cf-4a9f-8f67-30f784933c05@arm.com>
Date: Tue, 12 Aug 2025 22:29:13 +0100
From: Christian Loehle <christian.loehle@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, dietmar.eggemann@....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 8/12/25 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.

IMO this would be clearer if it's:
Calculate pd_cap as follows:
...

instead of traversing ...

Other than that LGTM.

> 
> 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)
>  		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);