[PATCH 2/5] sched_ext: Gate local DSQ reenq on baseline cid access

scx_bpf_dsq_reenq() with an SCX_DSQ_LOCAL_ON target schedules deferred reenq
work on the cid's cpu, raising an IPI when the target rq isn't the locked
one. Nothing checks caps along the way, so a sub-sched holding no cap at all
on a cid can force its cpu to take IPIs and rq lock cycles at will. The
analogous scx_bpf_kick_cid() path gates delivery on SCX_CAP_BASE in
kick_one_cpu() to prevent exactly this.

Apply the same rule at the reenq scheduling point: if the calling sched
lacks SCX_CAP_BASE on the target cid, drop the reenq and count it in the new
SCX_EV_SUB_REENQ_DENIED event. The check is lockless, which is fine: a reenq
slipping through right after a revoke is harmless, and a wrong denial can't
happen - if the caller has seen its ownership of the cpu, the check sees it
too.

Signed-off-by: Tejun Heo <[email protected]>
---
 kernel/sched/ext/ext.c      | 12 ++++++++++++
 kernel/sched/ext/internal.h |  9 ++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 81c2d8eeae41..fd88b4d4f12a 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1071,6 +1071,18 @@ void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
 	if (dsq->id == SCX_DSQ_LOCAL) {
 		rq = container_of(dsq, struct rq, scx.local_dsq);
 
+		/*
+		 * A sub-sched lacking baseline access on the target cid has no
+		 * business triggering IPIs. The lockless test is fine: slipping
+		 * through right after a revoke is harmless and a wrong denial
+		 * can't happen - if the caller has seen its ownership, so does
+		 * this test.
+		 */
+		if (unlikely(scx_missing_caps(sch, cpu_of(rq), SCX_CAP_BASE))) {
+			__scx_add_event(sch, SCX_EV_SUB_REENQ_DENIED, 1);
+			return;
+		}
+
 		struct scx_sched_pcpu *sch_pcpu = per_cpu_ptr(sch->pcpu, cpu_of(rq));
 		struct scx_deferred_reenq_local *drl = &sch_pcpu->deferred_reenq_local;
 
diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index a9a853a71061..1bdf34f3fc3c 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -1193,6 +1193,12 @@ struct scx_event_stats {
 	 * kick degrades to a plain reschedule.
 	 */
 	s64		SCX_EV_SUB_PREEMPT_DENIED;
+
+	/*
+	 * The number of times a local DSQ reenq was dropped because the
+	 * sub-sched lacked baseline access on the target cid.
+	 */
+	s64		SCX_EV_SUB_REENQ_DENIED;
 };
 
 #define SCX_EVENTS_LIST(SCX_EVENT)					\
@@ -1212,7 +1218,8 @@ struct scx_event_stats {
 	SCX_EVENT(SCX_EV_INSERT_NOT_OWNED);				\
 	SCX_EVENT(SCX_EV_SUB_BYPASS_DISPATCH);				\
 	SCX_EVENT(SCX_EV_SUB_FORCED_ADMIT);				\
-	SCX_EVENT(SCX_EV_SUB_PREEMPT_DENIED)
+	SCX_EVENT(SCX_EV_SUB_PREEMPT_DENIED);				\
+	SCX_EVENT(SCX_EV_SUB_REENQ_DENIED)
 
 struct scx_sched;
 
-- 
2.55.0