[PATCH v2] sched_ext: Count cap-rejected local DSQ inserts in SCX_EV_SUB_REJECT

[luoliang@kylinos.cn]

From: Liang Luo <luoliang@kylinos.cn>

When a task is enqueued on a local DSQ without the caps its enqueue flags
require, __scx_resolve_local_dsq() has three outcomes: admit the task
anyway because the rq is draining offline, the task is migration-disabled
or a migration is pending (counted in SCX_EV_SUB_FORCED_ADMIT), divert it
to the rescue path when the enqueue requested rescue (counted in
SCX_EV_SUB_RESCUE), or divert it to the reject DSQ to be re-enqueued so
that the BPF scheduler can re-decide.

Unlike the other two, which require a draining rq, a task that cannot
migrate anywhere else, or an explicit SCX_ENQ_RESCUE opt-in, the reject
diversion has no preconditions: every other cap-missing insert lands
there. Its traffic is currently not visible in any counter -
SCX_EV_REENQ_REPEAT only counts it once the same task fails its
placement again, and nr_rejected counts tasks refused by ops.init_task()
through task->scx.disallow.

Add SCX_EV_SUB_REJECT and count the reject diversion so that the events
cover all three outcomes of a cap-missing local insert.

Signed-off-by: Liang Luo <luoliang@kylinos.cn>

---

v2: Rework the rationale in the commit message and drop the Fixes: tag
    per Tejun. No code changes.
---
 kernel/sched/ext/internal.h | 10 +++++++++-
 kernel/sched/ext/sub.c      |  2 ++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index 0bcceab612de..36da1a6560db 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -1308,6 +1308,13 @@ struct scx_event_stats {
 	 * caps for its cid and the task entered the rescue path.
 	 */
 	s64		SCX_EV_SUB_RESCUE;
+
+	/*
+	 * The number of times an insert lacked the caps for its cid and was
+	 * diverted to the reject DSQ to be re-enqueued so that the BPF
+	 * scheduler can re-decide.
+	 */
+	s64		SCX_EV_SUB_REJECT;
 };
 
 #define SCX_EVENTS_LIST(SCX_EVENT)					\
@@ -1331,7 +1338,8 @@ struct scx_event_stats {
 	SCX_EVENT(SCX_EV_SUB_KICK_DENIED);				\
 	SCX_EVENT(SCX_EV_SUB_REENQ_DENIED);				\
 	SCX_EVENT(SCX_EV_SUB_CIDPERF_DENIED);				\
-	SCX_EVENT(SCX_EV_SUB_RESCUE)
+	SCX_EVENT(SCX_EV_SUB_RESCUE);					\
+	SCX_EVENT(SCX_EV_SUB_REJECT)
 
 struct scx_sched;
 
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 10567196be96..3429b84f319a 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -748,6 +748,8 @@ struct scx_dispatch_q *__scx_resolve_local_dsq(struct scx_sched *sch, struct rq
 		return &rq->scx.rescue.dsq;
 	}
 
+	__scx_add_event(sch, SCX_EV_SUB_REJECT, 1);
+
 	p->scx.reenq_reason_caps = missing;
 	p->scx.reenq_reason_cid = cid;
 
-- 
2.43.0