sched/deadline: Fix schedstats vs deadline servers

In dl_server_start(), when schedstats is enabled, the following
happens:

  dl_server_start()
    dl_se->dl_server = 1;
    enqueue_dl_entity()
      update_stats_enqueue_dl()
        __schedstats_from_dl_se()
          dl_task_of()
            BUG_ON(dl_server(dl_se));

Since only tasks have schedstats and internal entries do not, avoid
trying to update stats in this case.

Fixes: 63ba8422 ("sched/deadline: Introduce deadline servers")
Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20240829031111.12142-1-shijie@os.amperecomputing.com

kernel/sched/deadline.c

@@ -1896,46 +1896,40 @@ static inline bool __dl_less(struct rb_node *a, const struct rb_node *b)
 	return dl_time_before(__node_2_dle(a)->deadline, __node_2_dle(b)->deadline);
 }
 
-static inline struct sched_statistics *
+static __always_inline struct sched_statistics *
 __schedstats_from_dl_se(struct sched_dl_entity *dl_se)
 {
+	if (!schedstat_enabled())
+		return NULL;
+
+	if (dl_server(dl_se))
+		return NULL;
+
 	return &dl_task_of(dl_se)->stats;
 }
 
 static inline void
 update_stats_wait_start_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se)
 {
-	struct sched_statistics *stats;
-
-	if (!schedstat_enabled())
-		return;
-
-	stats = __schedstats_from_dl_se(dl_se);
-	__update_stats_wait_start(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
+	struct sched_statistics *stats = __schedstats_from_dl_se(dl_se);
+	if (stats)
+		__update_stats_wait_start(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
 }
 
 static inline void
 update_stats_wait_end_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se)
 {
-	struct sched_statistics *stats;
-
-	if (!schedstat_enabled())
-		return;
-
-	stats = __schedstats_from_dl_se(dl_se);
-	__update_stats_wait_end(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
+	struct sched_statistics *stats = __schedstats_from_dl_se(dl_se);
+	if (stats)
+		__update_stats_wait_end(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
 }
 
 static inline void
 update_stats_enqueue_sleeper_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se)
 {
-	struct sched_statistics *stats;
-
-	if (!schedstat_enabled())
-		return;
-
-	stats = __schedstats_from_dl_se(dl_se);
-	__update_stats_enqueue_sleeper(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
+	struct sched_statistics *stats = __schedstats_from_dl_se(dl_se);
+	if (stats)
+		__update_stats_enqueue_sleeper(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
 }
 
 static inline void