[tip: sched/core] sched/core: Fix the bug that task won't enqueue into core tree when update cookie

[tip-bot2 for Cruz Zhao]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     91caa5ae242465c3ab9fd473e50170faa7e944f4
Gitweb:        https://git.kernel.org/tip/91caa5ae242465c3ab9fd473e50170faa7e944f4
Author:        Cruz Zhao <CruzZhao@xxxxxxxxxxxxxxxxx>
AuthorDate:    Tue, 28 Jun 2022 15:57:23 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 21 Jul 2022 10:39:39 +02:00

sched/core: Fix the bug that task won't enqueue into core tree when update cookie

In function sched_core_update_cookie(), a task will enqueue into the
core tree only when it enqueued before, that is, if an uncookied task
is cookied, it will not enqueue into the core tree until it enqueue
again, which will result in unnecessary force idle.

Here follows the scenario:
  CPU x and CPU y are a pair of SMT siblings.
  1. Start task a running on CPU x without sleeping, and task b and
     task c running on CPU y without sleeping.
  2. We create a cookie and share it to task a and task b, and then
     we create another cookie and share it to task c.
  3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched

And we will find out that core_forceidle_sum of task a takes 30%
time of the sampling period, which shouldn't happen as task a and b
have the same cookie.

Then we migrate task a to CPU x', migrate task b and c to CPU y', where
CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we
will found out that core_forceidle_sum of task a and b are almost zero.

To solve this problem, we enqueue the task into the core tree if it's
on rq.

Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management")
Signed-off-by: Cruz Zhao <CruzZhao@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/1656403045-100840-2-git-send-email-CruzZhao@xxxxxxxxxxxxxxxxx
---
 kernel/sched/core_sched.c |  9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
index 5103502..93878cb 100644
--- a/kernel/sched/core_sched.c
+++ b/kernel/sched/core_sched.c
@@ -56,7 +56,6 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
 	unsigned long old_cookie;
 	struct rq_flags rf;
 	struct rq *rq;
-	bool enqueued;
 
 	rq = task_rq_lock(p, &rf);
 
@@ -68,14 +67,16 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
 	 */
 	SCHED_WARN_ON((p->core_cookie || cookie) && !sched_core_enabled(rq));
 
-	enqueued = sched_core_enqueued(p);
-	if (enqueued)
+	if (sched_core_enqueued(p))
 		sched_core_dequeue(rq, p, DEQUEUE_SAVE);
 
 	old_cookie = p->core_cookie;
 	p->core_cookie = cookie;
 
-	if (enqueued)
+	/*
+	 * Consider the cases: !prev_cookie and !cookie.
+	 */
+	if (cookie && task_on_rq_queued(p))
 		sched_core_enqueue(rq, p);
 
 	/*