[PATCH 1/2] sched/core: Pass ENQUEUE_RQ_SELECTED on initial enqueue

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

select_task_rq() sets WF_RQ_SELECTED and ttwu_do_activate() maps it to
ENQUEUE_RQ_SELECTED so enqueue_task_scx() knows whether ops.select_cpu()
ran. wake_up_new_task() also calls select_task_rq() but activates the
new task without the flag, so on the fork path ops.enqueue() never sees
SCX_ENQ_CPU_SELECTED and SCX_EV_SELECT_CPU_FALLBACK never counts a
fork-path fallback: a misbehaving select_cpu() leaves no trace in the
counters, and schedulers testing SCX_ENQ_CPU_SELECTED run a redundant
select_cpu_dfl() on tasks select_cpu() already placed.

Set ENQUEUE_RQ_SELECTED on the initial enqueue when select_task_rq() was
called, matching the wakeup path. Only sched_ext consumes the flag.

Fixes: 9b671793c7d9 ("sched_ext, scx_qmap: Add and use SCX_ENQ_CPU_SELECTED")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 04b62c16fa8c..f7a452123ef5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5016,6 +5016,7 @@ void wake_up_new_task(struct task_struct *p)
 	struct rq_flags rf;
 	struct rq *rq;
 	int wake_flags = WF_FORK;
+	int en_flags = ENQUEUE_NOCLOCK | ENQUEUE_INITIAL;
 
 	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
 	WRITE_ONCE(p->__state, TASK_RUNNING);
@@ -5029,11 +5030,13 @@ void wake_up_new_task(struct task_struct *p)
 	 */
 	p->recent_used_cpu = task_cpu(p);
 	__set_task_cpu(p, select_task_rq(p, task_cpu(p), &wake_flags));
+	if (wake_flags & WF_RQ_SELECTED)
+		en_flags |= ENQUEUE_RQ_SELECTED;
 	rq = __task_rq_lock(p, &rf);
 	update_rq_clock(rq);
 	post_init_entity_util_avg(p);
 
-	activate_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_INITIAL);
+	activate_task(rq, p, en_flags);
 	trace_sched_wakeup_new(p);
 	wakeup_preempt(rq, p, wake_flags);
 	if (p->sched_class->task_woken) {
-- 
2.53.0

base-commit: 5b740a72b55c4267ba4d8010f17dde26b26f52d3