[PATCH 1/4] sched/fair: Fix cookie check on __select_idle_cpu()

Fernand Sieber
2025-09-22T12:40:18Z

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@amazon.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