[tip: sched/core] sched/fair: Drop idle recency from slow-path CPU selection

[tip-bot2 for Christian Loehle]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     abe440b3770fce8eb8ee92a563dfb55a7b9a1c0e
Gitweb:        https://git.kernel.org/tip/abe440b3770fce8eb8ee92a563dfb55a7b9a1c0e
Author:        Christian Loehle <christian.loehle@xxxxxxx>
AuthorDate:    Thu, 17 Sep 2026 16:39:14 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 25 Sep 2026 12:45:58 +02:00

sched/fair: Drop idle recency from slow-path CPU selection

The slow-path CPU picker favours the most recently idle CPU as a proxy
for cache warmth.

A more recent idle stamp may make ongoing entry more likely. If entry
cannot be aborted, that CPU must finish entry and then exit, potentially
paying more than an already-resident CPU with the same advertised exit
latency. The advertised worst-case latency covers both cases. idle_stamp
does not timestamp the current CPUIdle entry, so this is only a heuristic.

A recent scheduler-idle transition may also mark a short gap in recurring
task activity, making the CPU likely to be busy again soon.

Drop the timestamp tie-break and retain the first candidate unless a
lower advertised exit latency is found.

Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://patch.msgid.link/20260917153915.1563875-2-christian.loehle@xxxxxxx
---
 kernel/sched/fair.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a03e7f4..c4d4b55 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8444,7 +8444,6 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
 {
 	unsigned long load, min_load = ULONG_MAX;
 	unsigned int min_exit_latency = UINT_MAX;
-	u64 latest_idle_timestamp = 0;
 	int least_loaded_cpu = this_cpu;
 	int shallowest_idle_cpu = -1;
 	int i;
@@ -8466,22 +8465,10 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
 		if (available_idle_cpu(i)) {
 			struct cpuidle_state *idle = idle_get_state(rq);
 			if (idle && idle->exit_latency < min_exit_latency) {
-				/*
-				 * We give priority to a CPU whose idle state
-				 * has the smallest exit latency irrespective
-				 * of any idle timestamp.
-				 */
 				min_exit_latency = idle->exit_latency;
-				latest_idle_timestamp = rq->idle_stamp;
 				shallowest_idle_cpu = i;
 			} else if ((!idle || idle->exit_latency == min_exit_latency) &&
-				   rq->idle_stamp > latest_idle_timestamp) {
-				/*
-				 * If equal or no active idle state, then
-				 * the most recently idled CPU might have
-				 * a warmer cache.
-				 */
-				latest_idle_timestamp = rq->idle_stamp;
+				   shallowest_idle_cpu == -1) {
 				shallowest_idle_cpu = i;
 			}
 		} else if (shallowest_idle_cpu == -1) {