sched: Use set_next_task(.first) where required

Turns out the core_sched bits forgot to use the
set_next_task(.first=true) variant. Notably:

  pick_next_task() := pick_task() + set_next_task(.first = true)
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240813224015.614146342@infradead.org

kernel/sched/core.c

@@ -6010,7 +6010,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 		next = rq->core_pick;
 		if (next != prev) {
 			put_prev_task(rq, prev);
-			set_next_task(rq, next);
+			set_next_task_first(rq, next);
 		}
 
 		rq->core_pick = NULL;
@@ -6184,7 +6184,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	}
 
 out_set_next:
-	set_next_task(rq, next);
+	set_next_task_first(rq, next);
 out:
 	if (rq->core->core_forceidle_count && next == rq->idle)
 		queue_core_balance(rq);

kernel/sched/sched.h

@@ -2363,6 +2363,10 @@ static inline void set_next_task(struct rq *rq, struct task_struct *next)
 	next->sched_class->set_next_task(rq, next, false);
 }
 
+static inline void set_next_task_first(struct rq *rq, struct task_struct *next)
+{
+	next->sched_class->set_next_task(rq, next, true);
+}
 
 /*
  * Helper to define a sched_class instance; each one is placed in a separate