sched: track highest prio task queued

This patch adds accounting to each runqueue to keep track of the
highest prio task queued on the run queue. We only care about
RT tasks, so if the run queue does not contain any active RT tasks
its priority will be considered MAX_RT_PRIO.

This information will be used for later patches.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -343,6 +343,8 @@ struct rt_rq {
 	int rt_load_balance_idx;
 	struct list_head *rt_load_balance_head, *rt_load_balance_curr;
 	unsigned long rt_nr_running;
+	/* highest queued rt task prio */
+	int highest_prio;
 };
 
 /*
@@ -6864,6 +6866,7 @@ void __init sched_init(void)
 		rq->cpu = i;
 		rq->migration_thread = NULL;
 		INIT_LIST_HEAD(&rq->migration_queue);
+		rq->rt.highest_prio = MAX_RT_PRIO;
 #endif
 		atomic_set(&rq->nr_iowait, 0);
 

kernel/sched_rt.c

@@ -30,6 +30,10 @@ static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
 {
 	WARN_ON(!rt_task(p));
 	rq->rt.rt_nr_running++;
+#ifdef CONFIG_SMP
+	if (p->prio < rq->rt.highest_prio)
+		rq->rt.highest_prio = p->prio;
+#endif /* CONFIG_SMP */
 }
 
 static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
@@ -37,6 +41,20 @@ static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
 	WARN_ON(!rt_task(p));
 	WARN_ON(!rq->rt.rt_nr_running);
 	rq->rt.rt_nr_running--;
+#ifdef CONFIG_SMP
+	if (rq->rt.rt_nr_running) {
+		struct rt_prio_array *array;
+
+		WARN_ON(p->prio < rq->rt.highest_prio);
+		if (p->prio == rq->rt.highest_prio) {
+			/* recalculate */
+			array = &rq->rt.active;
+			rq->rt.highest_prio =
+				sched_find_first_bit(array->bitmap);
+		} /* otherwise leave rq->highest prio alone */
+	} else
+		rq->rt.highest_prio = MAX_RT_PRIO;
+#endif /* CONFIG_SMP */
 }
 
 static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)