sched: Update rq->avg_idle when a task is moved to an idle CPU

Currently, rq->idle_stamp is only used to calculate avg_idle during
wakeups. This means other paths that move a task to an idle CPU such as
fork/clone, execve, or migrations, do not end the CPU's idle status in
the scheduler's eyes, leading to an inaccurate avg_idle.

This patch introduces update_rq_avg_idle() to provide a more accurate
measurement of CPU idle duration. By invoking this helper in
put_prev_task_idle(), we ensure avg_idle is updated whenever a CPU
stops being idle, regardless of how the new task arrived.

Testing on an 80-core Ampere Altra (ARMv8) with 6.19-rc5 baseline:
 - Hackbench : +7.2% performance gain at 16 threads.
 - Schbench: Reduced p99.9 tail latencies at high concurrency.

Signed-off-by: Shubhang Kaushik <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Vincent Guittot <[email protected]>
Tested-by: Shubhang Kaushik <[email protected]>
Link: https://patch.msgid.link/20260121-v8-patch-series-v8-1-b7f1cbee5055@os.amperecomputing.com

kernel/sched/core.c

@@ -3613,6 +3613,18 @@ static inline void ttwu_do_wakeup(struct task_struct *p)
 	trace_sched_wakeup(p);
 }
 
+void update_rq_avg_idle(struct rq *rq)
+{
+	u64 delta = rq_clock(rq) - rq->idle_stamp;
+	u64 max = 2*rq->max_idle_balance_cost;
+
+	update_avg(&rq->avg_idle, delta);
+
+	if (rq->avg_idle > max)
+		rq->avg_idle = max;
+	rq->idle_stamp = 0;
+}
+
 static void
 ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 		 struct rq_flags *rf)
@@ -3648,18 +3660,6 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 		p->sched_class->task_woken(rq, p);
 		rq_repin_lock(rq, rf);
 	}
-
-	if (rq->idle_stamp) {
-		u64 delta = rq_clock(rq) - rq->idle_stamp;
-		u64 max = 2*rq->max_idle_balance_cost;
-
-		update_avg(&rq->avg_idle, delta);
-
-		if (rq->avg_idle > max)
-			rq->avg_idle = max;
-
-		rq->idle_stamp = 0;
-	}
 }
 
 /*

kernel/sched/idle.c

@@ -460,6 +460,7 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
 {
 	update_curr_idle(rq);
 	scx_update_idle(rq, false, true);
+	update_rq_avg_idle(rq);
 }
 
 static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)

kernel/sched/sched.h

@@ -1670,6 +1670,7 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 
 #endif /* !CONFIG_FAIR_GROUP_SCHED */
 
+extern void update_rq_avg_idle(struct rq *rq);
 extern void update_rq_clock(struct rq *rq);
 
 /*