sched/core: Skip rq->avg_idle update without a valid idle_stamp

Commit 4b603f1551 ("sched: Update rq->avg_idle when a task is moved
to an idle CPU") moved rq->avg_idle accounting out of the wakeup path and
into put_prev_task_idle(), so that the idle interval is consumed whenever
the idle task is switched out.

The wakeup-side accounting that it replaced only updated rq->avg_idle
when rq->idle_stamp was non-zero. The new helper lost that validity
check and unconditionally computes:

	rq_clock(rq) - rq->idle_stamp

If rq->idle_stamp is zero, this uses rq_clock(rq) as the sample. That is
not a valid idle duration and can immediately drive rq->avg_idle to its
clamp.

This can happen when sched_balance_newidle() returns before setting
rq->idle_stamp, for example when this_rq->ttwu_pending is set. In that
case the rq can switch to the idle task with idle_stamp still zero and
leave idle again when the pending wakeup is processed.

Other paths can also switch to the idle task without setting
rq->idle_stamp via newidle_balance(), for example find_proxy_task() or
force-idling.

Restore the idle_stamp validity check in update_rq_avg_idle() and skip
the rq->avg_idle update when there is no measured idle interval.

Fixes: 4b603f1551 ("sched: Update rq->avg_idle when a task is moved to an idle CPU")
Signed-off-by: Shubhang Kaushik (Ampere) <sh@gentwo.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260807-master-v3-1-c328354efed3@gentwo.org

kernel/sched/core.c

@@ -3742,11 +3742,17 @@ static inline void ttwu_do_wakeup(struct task_struct *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;
+	u64 idle_stamp = rq->idle_stamp;
+	u64 delta, max;
+
+	if (!idle_stamp)
+		return;
+
+	delta = rq_clock(rq) - idle_stamp;
 
 	update_avg(&rq->avg_idle, delta);
 
+	max = 2 * rq->max_idle_balance_cost;
 	if (rq->avg_idle > max)
 		rq->avg_idle = max;
 	rq->idle_stamp = 0;