linux-kernel - [PATCH 1/4] sched/fair: Fix cookie check on __select_idle

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Message-ID: <64ca6d7f73625cc63fa0bc1800c9b199462a5ef3.1758543008.git.sieberf@amazon.com>
Date: Mon, 22 Sep 2025 14:39:22 +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 1/4] sched/fair: Fix cookie check on __select_idle_cpu()

The __select_idle_cpu() function uses sched_cpu_cookie_match() to determine
if a task can be placed on an idle CPU. This function incorrectly returns
false when the whole core is idle but the task has a cookie, preventing
proper task placement.

Replace sched_cpu_cookie_match() with sched_core_cookie_match() which
correctly handles the idle core case. Refactor select_idle_smt() to avoid
duplicate work by checking core cookie compatibility only once in the SMT
mask.

Fixes: 97886d9dcd868 ("sched: Migration changes for core scheduling")
Signed-off-by: Fernand Sieber <sieberf@...zon.com>
---
 kernel/sched/fair.c  |  5 ++++-
 kernel/sched/sched.h | 14 --------------
 2 files changed, 4 insertions(+), 15 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b173a059315c..43ddfc25af99 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7447,7 +7447,7 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
 static inline int __select_idle_cpu(int cpu, struct task_struct *p)
 {
 	if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
-	    sched_cpu_cookie_match(cpu_rq(cpu), p))
+	    sched_core_cookie_match(cpu_rq(cpu), p))
 		return cpu;
 
 	return -1;
@@ -7546,6 +7546,9 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
 {
 	int cpu;
 
+	if (!sched_core_cookie_match(cpu_rq(target), p))
+		return -1;
+
 	for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
 		if (cpu == target)
 			continue;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index be9745d104f7..4e7080123a4c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1386,15 +1386,6 @@ extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_f
  * A special case is that the task's cookie always matches with CPU's core
  * cookie if the CPU is in an idle core.
  */
-static inline bool sched_cpu_cookie_match(struct rq *rq, struct task_struct *p)
-{
-	/* Ignore cookie match if core scheduler is not enabled on the CPU. */
-	if (!sched_core_enabled(rq))
-		return true;
-
-	return rq->core->core_cookie == p->core_cookie;
-}
-
 static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
 {
 	bool idle_core = true;
@@ -1468,11 +1459,6 @@ static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
 	return &rq->__lock;
 }
 
-static inline bool sched_cpu_cookie_match(struct rq *rq, struct task_struct *p)
-{
-	return true;
-}
-
 static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
 {
 	return true;
-- 
2.43.0




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