[RFC][PATCH 3/3] sched/eevdf: Fix sched idle task group preemption

[Peter Zijlstra <peterz@infradead.org>]

From: Vincent Guittot <vincent.guittot@linaro.org>

Even with flat hierarchy which puts all runnable tasks in the same cfs rq,
we need to parse the hierarchy to check for sched_idle task group.

Fixes: 85570f10a4c6 ("sched/eevdf: Move to a single runqueue")
Reported-by: Jakub Okoński <jakub@okonski.org>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/58dc8e8a-7260-43ce-aebb-8d9c29a6afc9@okonski.org/
Link: https://patch.msgid.link/20261019182622.1584454-1-vincent.guittot@linaro.org
---

Changes since v1:
- fix prototype and cleanup unsued variable

 kernel/sched/fair.c |   87 ++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 74 insertions(+), 13 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -433,6 +433,38 @@ static inline struct sched_entity *paren
 	return se->parent;
 }
 
+static void
+find_matching_se(struct sched_entity **se, struct sched_entity **pse)
+{
+	int se_depth, pse_depth;
+
+	/*
+	 * preemption test can be made between sibling entities who are in the
+	 * same cfs_rq i.e who have a common parent. Walk up the hierarchy of
+	 * both tasks until we find their ancestors who are siblings of common
+	 * parent.
+	 */
+
+	/* First walk up until both entities are at same depth */
+	se_depth = (*se)->depth;
+	pse_depth = (*pse)->depth;
+
+	while (se_depth > pse_depth) {
+		se_depth--;
+		*se = parent_entity(*se);
+	}
+
+	while (pse_depth > se_depth) {
+		pse_depth--;
+		*pse = parent_entity(*pse);
+	}
+
+	while (!is_same_group(*se, *pse)) {
+		*se = parent_entity(*se);
+		*pse = parent_entity(*pse);
+	}
+}
+
 static int tg_is_idle(struct task_group *tg)
 {
 	return tg->idle > 0;
@@ -476,6 +508,11 @@ static inline struct sched_entity *paren
 	return NULL;
 }
 
+static inline void
+find_matching_se(struct sched_entity **se, struct sched_entity **pse)
+{
+}
+
 static inline int tg_is_idle(struct task_group *tg)
 {
 	return 0;
@@ -10130,7 +10167,40 @@ preempt_sync(struct rq *rq, int wake_fla
 }
 
 /*
- * Preempt the current task with a newly woken task if needed:
+ * Preempt an idle entity in favor of a non-idle entity (and don't preempt
+ * in the inverse case).
+ */
+static inline enum preempt_wakeup_action
+wakeup_preempt_sched_idle(struct sched_entity *cse, struct sched_entity *pse)
+{
+	int cse_is_idle, pse_is_idle;
+
+	cse_is_idle = se_is_idle(cse);
+	pse_is_idle = se_is_idle(pse);
+
+	if (cse_is_idle || pse_is_idle) {
+		find_matching_se(&cse, &pse);
+		WARN_ON_ONCE(!pse);
+
+		cse_is_idle = se_is_idle(cse);
+		pse_is_idle = se_is_idle(pse);
+
+		/*
+		 * Preempt an idle entity in favor of a non-idle entity (and
+		 * don't preempt in the inverse case).
+		 */
+		if (cse_is_idle && !pse_is_idle)
+			return PREEMPT_WAKEUP_RESCHED;
+
+		if (cse_is_idle != pse_is_idle)
+			return PREEMPT_WAKEUP_NONE;
+	}
+
+	return PREEMPT_WAKEUP_PICK;
+}
+
+/*
+ * Preempt the current task with a newly woken task if needed.
  */
 static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_flags)
 {
@@ -10138,7 +10208,6 @@ static void wakeup_preempt_fair(struct r
 	struct task_struct *donor = rq->donor;
 	struct sched_entity *nse, *se = &donor->se, *pse = &p->se;
 	struct cfs_rq *cfs_rq = &rq->cfs;
-	int cse_is_idle, pse_is_idle;
 
 	/*
 	 * XXX Getting preempted by higher class, try and find idle CPU?
@@ -10175,22 +10244,14 @@ static void wakeup_preempt_fair(struct r
 	if (!sched_feat(WAKEUP_PREEMPTION))
 		return;
 
-	WARN_ON_ONCE(!pse);
-
-	cse_is_idle = se_is_idle(se);
-	pse_is_idle = se_is_idle(pse);
+	preempt_action = wakeup_preempt_sched_idle(se, pse);
 
-	nse = se;
-	/*
-	 * Preempt an idle entity in favor of a non-idle entity (and don't preempt
-	 * in the inverse case).
-	 */
-	if (cse_is_idle && !pse_is_idle)
+	if (preempt_action == PREEMPT_WAKEUP_RESCHED)
 		goto preempt;
 
 	update_curr_fair(rq);
 
-	if (cse_is_idle != pse_is_idle)
+	if (preempt_action == PREEMPT_WAKEUP_NONE)
 		goto update;
 
 	/*