linux-kernel - Re: [PATCH 3/6 v8] sched/fair: Prepare select_task_rq

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Message-ID: <20260206180329.w4eduxwprf5etgs6@airbuntu>
Date: Fri, 6 Feb 2026 18:03:29 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: 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, linux-kernel@...r.kernel.org,
	pierre.gondois@....com, kprateek.nayak@....com,
	hongyan.xia2@....com, christian.loehle@....com,
	luis.machado@....com
Subject: Re: [PATCH 3/6 v8] sched/fair: Prepare select_task_rq_fair() to be
 called for new cases

On 12/02/25 19:12, Vincent Guittot wrote:
> Update select_task_rq_fair() to be called out of the 3 current cases which
> are :
> - wake up
> - exec
> - fork
> 
> We wants to select a rq in some new cases like pushing a runnable task on a
> better CPU than the local one. In such case, it's not a wakeup , nor an
> exec nor a fork. We make sure to not distrub these cases but still
> go through EAS and fast-path.

I'd add we have a fallback mechanism when moving between cpusets causes to pick
a random cpu. We have been carrying out of tree hack in Android for a while to
make this use the wake up path. Especially on HMP system, a random cpu could
mean bad placement decision as not all cores are equal. And it seems server
market is catching up with quirky caching systems.

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Reviewed-by: Qais Yousef <qyousef@...alina.io>

> ---
>  kernel/sched/fair.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f430ec890b72..80c4131fb35b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8518,6 +8518,7 @@ static int
>  select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  {
>  	int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> +	int want_sibling = !(wake_flags & (WF_EXEC | WF_FORK));
>  	struct sched_domain *tmp, *sd = NULL;
>  	int cpu = smp_processor_id();
>  	int new_cpu = prev_cpu;
> @@ -8535,16 +8536,21 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		if ((wake_flags & WF_CURRENT_CPU) &&
>  		    cpumask_test_cpu(cpu, p->cpus_ptr))
>  			return cpu;
> +	}
>  
> -		if (!is_rd_overutilized(this_rq()->rd)) {
> -			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> -			if (new_cpu >= 0)
> -				return new_cpu;
> -			new_cpu = prev_cpu;
> -		}
> +	/*
> +	 * We don't want EAS to be called for exec or fork but it should be
> +	 * called for any other case such as wake up or push callback.
> +	 */
> +	if (!is_rd_overutilized(this_rq()->rd) && want_sibling) {
> +		new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> +		if (new_cpu >= 0)
> +			return new_cpu;
> +		new_cpu = prev_cpu;
> +	}
>  
> +	if (wake_flags & WF_TTWU)
>  		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
> -	}
>  
>  	rcu_read_lock();
>  	for_each_domain(cpu, tmp) {
> @@ -8575,7 +8581,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	if (unlikely(sd)) {
>  		/* Slow path */
>  		new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> -	} else if (wake_flags & WF_TTWU) { /* XXX always ? */
> +	} else if (want_sibling) {
>  		/* Fast path */
>  		new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>  	}
> -- 
> 2.43.0
>