linux-kernel - [PATCH 4/4] sched/fair: Add more core cookie check in wake up fast path

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Message-ID: <a68bf0acdad9995fab15105cb26bd25f7d0edc8b.1758543008.git.sieberf@amazon.com>
Date: Mon, 22 Sep 2025 14:39:25 +0200
From: Fernand Sieber <sieberf@...zon.com>
To: <mingo@...hat.com>, <peterz@...radead.org>
CC: <linux-kernel@...r.kernel.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <dwmw@...zon.co.uk>,
	<jschoenh@...zon.de>, <liuyuxua@...zon.com>
Subject: [PATCH 4/4] sched/fair: Add more core cookie check in wake up fast path

The fast path in select_idle_sibling() can place tasks on CPUs without
considering core scheduling constraints, potentially causing immediate
force idle when the sibling runs an incompatible task.

Add cookie compatibility checks before selecting a CPU in the fast path.
This prevents placing waking tasks on CPUs where the sibling is running
an incompatible task, reducing force idle occurrences.

Signed-off-by: Fernand Sieber <sieberf@...zon.com>
---
 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 78b36225a039..a9cbb0e9bb43 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7578,7 +7578,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
 		 */
 		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
 			continue;
-		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
+		if (__select_idle_cpu(cpu, p) != -1)
 			return cpu;
 	}
 
@@ -7771,7 +7771,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	 */
 	lockdep_assert_irqs_disabled();
 
-	if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
+	if ((__select_idle_cpu(target, p) != -1) &&
 	    asym_fits_cpu(task_util, util_min, util_max, target))
 		return target;
 
@@ -7779,7 +7779,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	 * If the previous CPU is cache affine and idle, don't be stupid:
 	 */
 	if (prev != target && cpus_share_cache(prev, target) &&
-	    (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
+	    (__select_idle_cpu(prev, p) != -1) &&
 	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
 
 		if (!static_branch_unlikely(&sched_cluster_active) ||
@@ -7811,7 +7811,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if (recent_used_cpu != prev &&
 	    recent_used_cpu != target &&
 	    cpus_share_cache(recent_used_cpu, target) &&
-	    (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
+	    (__select_idle_cpu(recent_used_cpu, p) != -1) &&
 	    cpumask_test_cpu(recent_used_cpu, p->cpus_ptr) &&
 	    asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
 
-- 
2.43.0




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