[PATCH 09/15] sched_ext: Generalize the reject DSQ reenqueue path

The reject DSQ is currently specific to sub-scheduler cap failures. Its
storage and initialization depend on CONFIG_EXT_SUB_SCHED, and the drain
path assumes every rejected task was rejected for SCX_TASK_REENQ_CAP.

Other transient placement failures need the same ability to park a task
on its source rq and return it to the owning BPF scheduler. Make the
reject DSQ unconditional, move its drain path into the core sched_ext
implementation, and record the reenqueue reason on each parked task.

This is a preparatory change to support proxy execution with sched_ext.

Signed-off-by: Andrea Righi <[email protected]>
---
 include/linux/sched/ext.h |  1 +
 kernel/sched/ext/ext.c    | 59 ++++++++++++++++++++++++++++++++++++---
 kernel/sched/ext/sub.c    | 47 ++-----------------------------
 kernel/sched/ext/sub.h    |  2 --
 kernel/sched/sched.h      |  4 +--
 5 files changed, 59 insertions(+), 54 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 4785a02905ecc..0f25d99f5fdd0 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -197,6 +197,7 @@ struct sched_ext_entity {
 	struct rb_node		dsq_priq;	/* p->scx.dsq_vtime order */
 	u32			dsq_seq;
 	u32			dsq_flags;	/* protected by DSQ lock */
+	u32			reject_reason;	/* protected by rq lock */
 	u32			flags;		/* protected by rq lock */
 	u32			weight;
 	u32			reenq_cnt;	/* reenqueues since last run */
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 240d84a0aaa37..fd4860b7fce4e 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1521,11 +1521,10 @@ static void scx_dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
 				 struct scx_dispatch_q *dsq, struct task_struct *p,
 				 u64 enq_flags)
 {
-	bool is_rq_owned = false;
+	bool is_rq_owned = dsq_is_rq_owned(dsq);
 
 	if (dsq->id == SCX_DSQ_LOCAL) {
 		dsq = scx_local_or_reject_dsq(sch, rq, p, &enq_flags);
-		is_rq_owned = true;
 	}
 
 	WARN_ON_ONCE(p->scx.dsq || !list_empty(&p->scx.dsq_list.node));
@@ -1725,6 +1724,9 @@ void scx_dispatch_dequeue(struct rq *rq, struct task_struct *p)
 	}
 	p->scx.dsq = NULL;
 
+	if (dsq->id == SCX_DSQ_REJECT)
+		p->scx.reject_reason = SCX_TASK_REENQ_NONE;
+
 	if (!is_rq_owned)
 		raw_spin_unlock(&dsq->lock);
 }
@@ -4396,6 +4398,57 @@ static void process_deferred_reenq_users(struct rq *rq)
 	}
 }
 
+/*
+ * Drain @rq->scx.reject_dsq and reenqueue each task so that its owning BPF
+ * scheduler chooses placement again.
+ *
+ * A task can be re-rejected repeatedly. Reenqueues are bounded per task by
+ * SCX_REENQ_MAX_REPEAT in scx_do_enqueue_task(), which ejects the owning
+ * scheduler. The private list below prevents a task from being revisited in
+ * the same round.
+ */
+static void scx_reenq_reject(struct rq *rq)
+{
+	LIST_HEAD(tasks);
+	struct task_struct *p, *n;
+
+	lockdep_assert_rq_held(rq);
+
+	if (list_empty(&rq->scx.reject_dsq.list))
+		return;
+
+	/*
+	 * Move tasks to a private list so a task re-rejected by
+	 * scx_do_enqueue_task() below isn't revisited this round.
+	 */
+	list_for_each_entry_safe(p, n, &rq->scx.reject_dsq.list, scx.dsq_list.node) {
+		u32 reason = p->scx.reject_reason;
+
+		/* migration_pending tasks should have bypassed to local DSQ */
+		if (WARN_ON_ONCE(p->migration_pending))
+			continue;
+		if (WARN_ON_ONCE(!reason))
+			continue;
+
+		scx_dispatch_dequeue(rq, p);
+		p->scx.reject_reason = SCX_TASK_REENQ_NONE;
+
+		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
+			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
+		p->scx.flags |= reason;
+
+		list_add_tail(&p->scx.dsq_list.node, &tasks);
+	}
+
+	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
+		list_del_init(&p->scx.dsq_list.node);
+
+		scx_do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);
+
+		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
+	}
+}
+
 static void run_deferred(struct rq *rq)
 {
 	process_ddsp_deferred_locals(rq);
@@ -8421,9 +8474,7 @@ void __init init_sched_ext_class(void)
 
 		/* local_dsq's sch will be set during scx_root_enable() */
 		BUG_ON(init_dsq(&rq->scx.local_dsq, SCX_DSQ_LOCAL, NULL));
-#ifdef CONFIG_EXT_SUB_SCHED
 		BUG_ON(init_dsq(&rq->scx.reject_dsq, SCX_DSQ_REJECT, NULL));
-#endif
 
 		INIT_LIST_HEAD(&rq->scx.runnable_list);
 		INIT_LIST_HEAD(&rq->scx.ddsp_deferred_locals);
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 8758317035754..640e4a4f3c07f 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -284,6 +284,8 @@ struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq
 
 	p->scx.reenq_reason_caps = missing;
 	p->scx.reenq_reason_cid = cid;
+	WARN_ON_ONCE(p->scx.reject_reason);
+	p->scx.reject_reason = SCX_TASK_REENQ_CAP;
 
 	/*
 	 * Only local DSQ can honor IMMED and dsq_inc_nr() WARNs on IMMED into
@@ -315,51 +317,6 @@ bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p)
 	return true;
 }
 
-/*
- * Drain @rq->scx.reject_dsq, reenqueueing each task so the BPF re-decides
- * from p->scx.reenq_reason_*.
- *
- * A task can be re-rejected repeatedly. The reenqueue is bounded per task in
- * scx_do_enqueue_task(), which ejects the owning sub past SCX_REENQ_MAX_REPEAT.
- * Rejection can't happen for root.
- */
-void scx_reenq_reject(struct rq *rq)
-{
-	LIST_HEAD(tasks);
-	struct task_struct *p, *n;
-
-	lockdep_assert_rq_held(rq);
-
-	if (!scx_has_subs() || list_empty(&rq->scx.reject_dsq.list))
-		return;
-
-	/*
-	 * Move to a private list so a task re-rejected by the
-	 * scx_do_enqueue_task() below isn't revisited this round.
-	 */
-	list_for_each_entry_safe(p, n, &rq->scx.reject_dsq.list, scx.dsq_list.node) {
-		/* migration_pending tasks should have bypassed to local DSQ */
-		if (WARN_ON_ONCE(p->migration_pending))
-			continue;
-
-		scx_dispatch_dequeue(rq, p);
-
-		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
-			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
-		p->scx.flags |= SCX_TASK_REENQ_CAP;
-
-		list_add_tail(&p->scx.dsq_list.node, &tasks);
-	}
-
-	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
-		list_del_init(&p->scx.dsq_list.node);
-
-		scx_do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);
-
-		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
-	}
-}
-
 /* record a caps change, see struct scx_caps_updated */
 static void caps_updated_record(struct scx_pshard *ps, const struct scx_cmask *cids, u64 caps,
 				struct list_head *to_deliver)
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index 0019b75a2560e..b5870aeb9c625 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -37,7 +37,6 @@ void scx_discard_stale_ecaps_syncs(void);
 struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq *rq,
 					       struct task_struct *p, u64 *enq_flags);
 bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p);
-void scx_reenq_reject(struct rq *rq);
 
 /*
  * cgrp->scx_sched is written by root/sub enable/disable under all of
@@ -86,7 +85,6 @@ static inline void scx_discard_ecaps_to_sync(s32 cpu, struct scx_sched_pcpu *pcp
 static inline void scx_discard_stale_ecaps_syncs(void) {}
 static inline struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq *rq, struct task_struct *p, u64 *enq_flags) { return &rq->scx.local_dsq; }
 static inline bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p) { return false; }
-static inline void scx_reenq_reject(struct rq *rq) {}
 static inline void scx_dec_has_subs(struct scx_sched *sch) {}
 
 #endif	/* CONFIG_EXT_SUB_SCHED */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 79daacc2389e1..ade8bb393786a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -796,9 +796,7 @@ enum scx_rq_flags {
 
 struct scx_rq {
 	struct scx_dispatch_q	local_dsq;
-#ifdef CONFIG_EXT_SUB_SCHED
-	struct scx_dispatch_q	reject_dsq;		/* staging for cap-rejected tasks */
-#endif
+	struct scx_dispatch_q	reject_dsq;		/* staging for rejected tasks */
 	struct list_head	runnable_list;		/* runnable tasks on this rq */
 	struct list_head	ddsp_deferred_locals;	/* deferred ddsps from enq */
 	unsigned long		ops_qseq;
-- 
2.55.0