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

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Message-ID: <07c8435d-c396-4b53-bee5-765469fa8762@linux.ibm.com>
Date: Sun, 7 Dec 2025 18:53:38 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: qyousef@...alina.io, hongyan.xia2@....com, christian.loehle@....com,
        luis.machado@....com, 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
Subject: Re: [PATCH 3/6 v8] sched/fair: Prepare select_task_rq_fair() to be
 called for new cases



On 12/2/25 11:42 PM, 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

nit: s/distrub/disturb

> go through EAS and fast-path.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   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) {

It is going to find a idle core withing LLC first. then idle sibling. right?
So may need a better name than want_sibling.

>   		/* Fast path */
>   		new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>   	}