[PATCH v2 sched_ext/for-6.12] sched/fair: Make balance_fair() test sched_fair_runnable() instead of rq->nr_running

[Tejun Heo <tj@kernel.org>]

balance_fair() skips newidle balancing if rq->nr_running - there are already
tasks on the rq, so no need to try to pull tasks. This tests the total
number of queued tasks on the CPU instead of only the fair class, but is
still correct as the rq can currently only have fair class tasks while
balance_fair() is running.

However, with the addition of sched_ext below the fair class, this will not
hold anymore and make put_prev_task_balance() skip sched_ext's balance()
incorrectly as, when a CPU has only lower priority class tasks,
rq->nr_running would still be positive and balance_fair() would return 1
even when fair doesn't have any tasks to run.

Update balance_fair() to use sched_fair_runnable() which tests
rq->cfs.nr_running which is updated by bandwidth throttling. Note that
pick_next_task_fair() already uses sched_fair_runnable() in its optimized
path for the same purpose.

v2: K Prateek Nayak pointed out that the bw control issue described in v1
    was incorrect. Patch description updated. As this makes the patch only
    relevant for sched_ext, I'll carry this through the sched_ext tree
    unless there are objections.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
---
 kernel/sched/fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8323,7 +8323,7 @@ static void set_cpus_allowed_fair(struct
 static int
 balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
-	if (rq->nr_running)
+	if (sched_fair_runnable(rq))
 		return 1;
 
 	return sched_balance_newidle(rq, rf) != 0;