sched/fair: Fix pelt lost idle time detection

JIRA: https://issues.redhat.com/browse/RHEL-128251

commit 17e3e88
Author: Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed Oct 8 15:12:14 2025 +0200

    sched/fair: Fix pelt lost idle time detection

    The check for some lost idle pelt time should be always done when
    pick_next_task_fair() fails to pick a task and not only when we call it
    from the fair fast-path.

    The case happens when the last running task on rq is a RT or DL task. When
    the latter goes to sleep and the /Sum of util_sum of the rq is at the max
    value, we don't account the lost of idle time whereas we should.

    Fixes: 6769243 ("sched: Rework pick_next_task() slow-path")
    Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/fair.c

@@ -8489,21 +8489,21 @@ done: __maybe_unused;
 	return p;
 
 idle:
-	if (!rf)
-		return NULL;
-
-	new_tasks = sched_balance_newidle(rq, rf);
+	if (rf) {
+		new_tasks = sched_balance_newidle(rq, rf);
 
-	/*
-	 * Because sched_balance_newidle() releases (and re-acquires) rq->lock, it is
-	 * possible for any higher priority task to appear. In that case we
-	 * must re-start the pick_next_entity() loop.
-	 */
-	if (new_tasks < 0)
-		return RETRY_TASK;
+		/*
+		 * Because sched_balance_newidle() releases (and re-acquires)
+		 * rq->lock, it is possible for any higher priority task to
+		 * appear. In that case we must re-start the pick_next_entity()
+		 * loop.
+		 */
+		if (new_tasks < 0)
+			return RETRY_TASK;
 
-	if (new_tasks > 0)
-		goto again;
+		if (new_tasks > 0)
+			goto again;
+	}
 
 	/*
 	 * rq is about to be idle, check if we need to update the