linux-kernel - Re: [RFC PATCH] sched/fair: Refactor can_migrate

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Message-ID: <20250109094627.GP20870@noisy.programming.kicks-ass.net>
Date: Thu, 9 Jan 2025 10:46:27 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: I Hsin Cheng <richard120310@...il.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Refactor can_migrate_task() to elimate
 looping

On Thu, Jan 09, 2025 at 01:29:47AM +0800, I Hsin Cheng wrote:
>  kernel/sched/fair.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2d16c8545..ce46f61da 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9404,12 +9404,16 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  			return 0;
>  
>  		/* Prevent to re-select dst_cpu via env's CPUs: */
> -		for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
> -			if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
> -				env->flags |= LBF_DST_PINNED;
> -				env->new_dst_cpu = cpu;
> -				break;
> -			}
> +		struct cpumask dst_mask;

Except you cannot put cpumask on-stack...

> +
> +		cpumask_and(&dst_mask, env->dst_grpmask, env->cpus);
> +		cpumask_and(&dst_mask, &dst_mask, p->cpus_ptr);
> +
> +		cpu = cpumask_first(&dst_mask);
> +
> +		if (cpu < nr_cpu_ids) {
> +			env->flags |= LBF_DST_PINNED;
> +			env->new_dst_cpu = cpu;
>  		}
>  
>  		return 0;
> -- 
> 2.43.0
>