sched_ext: Always call put_prev_task() with scx enabled

With the consolidation of put_prev_task/set_next_task(), we are now
skipping the sched_ext ops.stopping/running() transitions when the
previous and next tasks are the same, see commit 436f3ee ("sched:
Combine the last put_prev_task() and the first set_next_task()").

While this optimization makes sense in general, it can negatively impact
performance in some user-space schedulers, that expect to handle such
transitions when tasks exhaust their timeslice (see SCX_OPS_ENQ_LAST).

For example, scx_rustland suffers a significant performance regression
(e.g., gaming benchmarks drop from ~60fps to ~10fps).

To fix this, ensure that put_prev_task()/set_next_task() are never
skipped when the scx scheduling class is enabled, allowing the scx class
to handle such transitions.

This change restores the previous behavior, fixing the performance
regression in scx_rustland.

Link: sched-ext/scx#788
Fixes: 7c65ae8 ("sched_ext: Don't call put_prev_task_scx() before picking the next task")
Signed-off-by: Andrea Righi <[email protected]>

kernel/sched/sched.h

@@ -2470,21 +2470,6 @@ __put_prev_set_next_dl_server(struct rq *rq,
 	rq->dl_server = NULL;
 }
 
-static inline void put_prev_set_next_task(struct rq *rq,
-					  struct task_struct *prev,
-					  struct task_struct *next)
-{
-	WARN_ON_ONCE(rq->curr != prev);
-
-	__put_prev_set_next_dl_server(rq, prev, next);
-
-	if (next == prev)
-		return;
-
-	prev->sched_class->put_prev_task(rq, prev, next);
-	next->sched_class->set_next_task(rq, next, true);
-}
-
 /*
  * Helper to define a sched_class instance; each one is placed in a separate
  * section which is ordered by the linker script:
@@ -2523,6 +2508,21 @@ DECLARE_STATIC_KEY_FALSE(__scx_switched_all);	/* all fair class tasks on SCX */
 #define scx_switched_all()	false
 #endif /* !CONFIG_SCHED_CLASS_EXT */
 
+static inline void put_prev_set_next_task(struct rq *rq,
+					  struct task_struct *prev,
+					  struct task_struct *next)
+{
+	WARN_ON_ONCE(rq->curr != prev);
+
+	__put_prev_set_next_dl_server(rq, prev, next);
+
+	if (next == prev && !scx_enabled())
+		return;
+
+	prev->sched_class->put_prev_task(rq, prev, next);
+	next->sched_class->set_next_task(rq, next, true);
+}
+
 /*
  * Iterate only active classes. SCX can take over all fair tasks or be
  * completely disabled. If the former, skip fair. If the latter, skip SCX.