linux-kernel - [PATCH 2/4] sched/fair: Still look for the idlest cpu with no matching cookie

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Message-ID: <beefbc1474a94868c22b0351dedc0d7398b79c33.1758543008.git.sieberf@amazon.com>
Date: Mon, 22 Sep 2025 14:39:23 +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 2/4] sched/fair: Still look for the idlest cpu with no matching cookie

The slow path for waking tasks currently discards all potential targets
when no cookie-matching CPU is found, leading to suboptimal task placement.

Fall back to selecting the idlest CPU when no cookie-matching target is
available, ensuring better CPU utilization while maintaining the preference
for cookie-compatible placements.

Signed-off-by: Fernand Sieber <sieberf@...zon.com>
---
 kernel/sched/fair.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 43ddfc25af99..67746899809e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7335,7 +7335,8 @@ sched_balance_find_dst_group(struct sched_domain *sd, struct task_struct *p, int
  * sched_balance_find_dst_group_cpu - find the idlest CPU among the CPUs in the group.
  */
 static int
-sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
+__sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *p,
+				   int this_cpu, bool cookie_match)
 {
 	unsigned long load, min_load = ULONG_MAX;
 	unsigned int min_exit_latency = UINT_MAX;
@@ -7352,7 +7353,8 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
 	for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
 		struct rq *rq = cpu_rq(i);
 
-		if (!sched_core_cookie_match(rq, p))
+		/* Only matching tasks if cookie_match, else only unmatching tasks */
+		if (cookie_match ^ sched_core_cookie_match(rq, p))
 			continue;
 
 		if (sched_idle_cpu(i))
@@ -7391,6 +7393,17 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
 	return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu;
 }
 
+/*
+ * sched_balance_find_dst_group_cpu - find the idlest CPU among the CPUs in the group.
+ */
+static inline int
+sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
+{
+	int cpu = __sched_balance_find_dst_group_cpu(group, p, this_cpu, true);
+
+	return cpu >= 0 ? cpu : __sched_balance_find_dst_group_cpu(group, p, this_cpu, false);
+}
+
 static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct task_struct *p,
 				  int cpu, int prev_cpu, int sd_flag)
 {
-- 
2.43.0




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