linux-kernel - [PATCH 3/6 v7] sched/fair: Prepare select_task_rq

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Message-ID: <20251201091308.761711-4-vincent.guittot@linaro.org>
Date: Mon,  1 Dec 2025 10:13:05 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: 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
Cc: qyousef@...alina.io,
	hongyan.xia2@....com,
	christian.loehle@....com,
	luis.machado@....com,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 3/6 v7] sched/fair: Prepare select_task_rq_fair() to be called for new cases

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.

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) {
 		/* Fast path */
 		new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
 	}
-- 
2.43.0