[RFC PATCH 1/3] sched: track the next-highest priority on eachrunqueue

[Gregory Haskins]

We will use this later in the series to reduce the amount of rq-lock
contention during a pull operation

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
---

 kernel/sched.c    |    8 +++-
 kernel/sched_rt.c |  115 +++++++++++++++++++++++++++++++++++------------------
 2 files changed, 81 insertions(+), 42 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 6625c3c..a29193b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -448,7 +448,10 @@ struct rt_rq {
 	struct rt_prio_array active;
 	unsigned long rt_nr_running;
 #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
-	int highest_prio; /* highest queued rt task prio */
+	struct {
+		int curr; /* highest queued rt task prio */
+		int next; /* next highest */
+	} highest_prio;
 #endif
 #ifdef CONFIG_SMP
 	unsigned long rt_nr_migratory;
@@ -8040,7 +8043,8 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
 	__set_bit(MAX_RT_PRIO, array->bitmap);
 
 #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
-	rt_rq->highest_prio = MAX_RT_PRIO;
+	rt_rq->highest_prio.curr = MAX_RT_PRIO;
+	rt_rq->highest_prio.next = MAX_RT_PRIO;
 #endif
 #ifdef CONFIG_SMP
 	rt_rq->rt_nr_migratory = 0;
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index b446dc8..d3b54ba 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -108,7 +108,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 	if (rt_rq->rt_nr_running) {
 		if (rt_se && !on_rt_rq(rt_se))
 			enqueue_rt_entity(rt_se);
-		if (rt_rq->highest_prio < curr->prio)
+		if (rt_rq->highest_prio.curr < curr->prio)
 			resched_task(curr);
 	}
 }
@@ -473,7 +473,7 @@ static inline int rt_se_prio(struct sched_rt_entity *rt_se)
 	struct rt_rq *rt_rq = group_rt_rq(rt_se);
 
 	if (rt_rq)
-		return rt_rq->highest_prio;
+		return rt_rq->highest_prio.curr;
 #endif
 
 	return rt_task_of(rt_se)->prio;
@@ -547,33 +547,66 @@ static void update_curr_rt(struct rq *rq)
 	}
 }
 
+#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
+
+static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu);
+
+static inline int next_prio(struct rq *rq)
+{
+	struct task_struct *next = pick_next_highest_task_rt(rq, rq->cpu);
+
+	if (next && rt_prio(next->prio))
+		return next->prio;
+	else
+		return MAX_RT_PRIO;
+}
+#endif
+
 static inline
 void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 {
-	WARN_ON(!rt_prio(rt_se_prio(rt_se)));
-	rt_rq->rt_nr_running++;
 #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
-	if (rt_se_prio(rt_se) < rt_rq->highest_prio) {
+	int prio = rt_se_prio(rt_se);
+#endif
 #ifdef CONFIG_SMP
-		struct rq *rq = rq_of_rt_rq(rt_rq);
+	struct rq *rq = rq_of_rt_rq(rt_rq);
 #endif
 
-		rt_rq->highest_prio = rt_se_prio(rt_se);
+	WARN_ON(!rt_prio(prio));
+	rt_rq->rt_nr_running++;
+#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
+	if (prio < rt_rq->highest_prio.curr) {
+
+		/*
+		 * If the new task is higher in priority than anything on the
+		 * run-queue, we have a new high that must be published to
+		 * the world.  We also know that the previous high becomes
+		 * our next-highest.
+		 */
+		rt_rq->highest_prio.next = rt_rq->highest_prio.curr;
+		rt_rq->highest_prio.curr = prio;
 #ifdef CONFIG_SMP
 		if (rq->online)
-			cpupri_set(&rq->rd->cpupri, rq->cpu,
-				   rt_se_prio(rt_se));
+			cpupri_set(&rq->rd->cpupri, rq->cpu, prio);
 #endif
-	}
+	} else if (prio == rt_rq->highest_prio.curr)
+		/*
+		 * If the next task is equal in priority to the highest on
+		 * the run-queue, then we implicitly know that the next highest
+		 * task cannot be any lower than current
+		 */
+		rt_rq->highest_prio.next = prio;
+	else if (prio < rt_rq->highest_prio.next)
+		/*
+		 * Otherwise, we need to recompute next-highest
+		 */
+		rt_rq->highest_prio.next = next_prio(rq);
 #endif
 #ifdef CONFIG_SMP
-	if (rt_se->nr_cpus_allowed > 1) {
-		struct rq *rq = rq_of_rt_rq(rt_rq);
-
+	if (rt_se->nr_cpus_allowed > 1)
 		rq->rt.rt_nr_migratory++;
-	}
 
-	update_rt_migration(rq_of_rt_rq(rt_rq));
+	update_rt_migration(rq);
 #endif
 #ifdef CONFIG_RT_GROUP_SCHED
 	if (rt_se_boosted(rt_se))
@@ -589,8 +622,9 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 static inline
 void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 {
-#ifdef CONFIG_SMP
-	int highest_prio = rt_rq->highest_prio;
+#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
+	struct rq *rq = rq_of_rt_rq(rt_rq);
+	int highest_prio = rt_rq->highest_prio.curr;
 #endif
 
 	WARN_ON(!rt_prio(rt_se_prio(rt_se)));
@@ -598,33 +632,34 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 	rt_rq->rt_nr_running--;
 #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
 	if (rt_rq->rt_nr_running) {
-		struct rt_prio_array *array;
+		int prio = rt_se_prio(rt_se);
+
+		WARN_ON(prio < rt_rq->highest_prio.curr);
 
-		WARN_ON(rt_se_prio(rt_se) < rt_rq->highest_prio);
-		if (rt_se_prio(rt_se) == rt_rq->highest_prio) {
-			/* recalculate */
-			array = &rt_rq->active;
-			rt_rq->highest_prio =
+		/*
+		 * This may have been our highest or next-highest priority
+		 * task and therefore we may have some recomputation to do
+		 */
+		if (prio == rt_rq->highest_prio.curr) {
+			struct rt_prio_array *array = &rt_rq->active;
+
+			rt_rq->highest_prio.curr =
 				sched_find_first_bit(array->bitmap);
-		} /* otherwise leave rq->highest prio alone */
+		}
+
+		if (prio == rt_rq->highest_prio.next)
+			rt_rq->highest_prio.next = next_prio(rq);
 	} else
-		rt_rq->highest_prio = MAX_RT_PRIO;
+		rt_rq->highest_prio.curr = MAX_RT_PRIO;
 #endif
 #ifdef CONFIG_SMP
-	if (rt_se->nr_cpus_allowed > 1) {
-		struct rq *rq = rq_of_rt_rq(rt_rq);
+	if (rt_se->nr_cpus_allowed > 1)
 		rq->rt.rt_nr_migratory--;
-	}
 
-	if (rt_rq->highest_prio != highest_prio) {
-		struct rq *rq = rq_of_rt_rq(rt_rq);
-
-		if (rq->online)
-			cpupri_set(&rq->rd->cpupri, rq->cpu,
-				   rt_rq->highest_prio);
-	}
+	if (rq->online && rt_rq->highest_prio.curr != highest_prio)
+		cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio.curr);
 
-	update_rt_migration(rq_of_rt_rq(rt_rq));
+	update_rt_migration(rq);
 #endif /* CONFIG_SMP */
 #ifdef CONFIG_RT_GROUP_SCHED
 	if (rt_se_boosted(rt_se))
@@ -1069,7 +1104,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 		}
 
 		/* If this rq is still suitable use it. */
-		if (lowest_rq->rt.highest_prio > task->prio)
+		if (lowest_rq->rt.highest_prio.curr > task->prio)
 			break;
 
 		/* try again */
@@ -1257,7 +1292,7 @@ static int pull_rt_task(struct rq *this_rq)
 static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
 {
 	/* Try to pull RT tasks here if we lower this rq's prio */
-	if (unlikely(rt_task(prev)) && rq->rt.highest_prio > prev->prio)
+	if (unlikely(rt_task(prev)) && rq->rt.highest_prio.curr > prev->prio)
 		pull_rt_task(rq);
 }
 
@@ -1343,7 +1378,7 @@ static void rq_online_rt(struct rq *rq)
 
 	__enable_runtime(rq);
 
-	cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
+	cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio.curr);
 }
 
 /* Assumes rq->lock is held */
@@ -1426,7 +1461,7 @@ static void prio_changed_rt(struct rq *rq, struct task_struct *p,
 		 * can release the rq lock and p could migrate.
 		 * Only reschedule if p is still on the same runqueue.
 		 */
-		if (p->prio > rq->rt.highest_prio && rq->curr == p)
+		if (p->prio > rq->rt.highest_prio.curr && rq->curr == p)
 			resched_task(p);
 #else
 		/* For UP simply resched on drop of prio */

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