[PATCH 1/2] sched/fair: Drop idle recency from slow-path CPU selection

[Christian Loehle <christian.loehle@arm.com>]

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. Among CPUs
with equal advertised exit latency, this may favour the one with the
highest wakeup cost: if entry cannot be aborted, it must finish entry and
then exit, while an already-resident CPU only needs to exit. The same
advertised worst-case latency covers both cases.

idle_stamp does not track the current CPUIdle entry, so an older
scheduler-idle CPU may also be re-entering.

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

Drop the timestamp tie-break, retaining the first idle candidate unless
a lower advertised exit latency is found.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
---
 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 7455a83a6a99..ff5793bddc35 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8459,7 +8459,6 @@ 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;
@@ -8480,23 +8479,11 @@ 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) {
-- 
2.34.1