[PATCH] sched/rt: Introduce prio_lower() helper for comparing RT task prority

[PATCH] sched/rt: Introduce prio_lower() helper for comparing RT task prority

[Muchun Song]

We use a value to represent the priority of the RT task. But a smaller
value corresponds to a higher priority. If there are two RT task A and B,
their priorities are prio_a and prio_b, respectively. If prio_a is larger
than prio_b, which means that the priority of RT task A is lower than RT
task B. It may seem a bit strange.

In rt.c, there are many if condition of priority comparison. We need to
think carefully about which priority is higher because of this opposite
logic when read those code. So we introduce prio_lower() helper for
comparing RT task prority, which can make code more readable.

Signed-off-by: Muchun Song <smuchun@gmail.com>
---
 kernel/sched/rt.c | 54 ++++++++++++++++++++++++++++-------------------
 1 file changed, 32 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 9aa3287ce301..4808684607b9 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -101,6 +101,15 @@ void init_rt_rq(struct rt_rq *rt_rq)
 	raw_spin_lock_init(&rt_rq->rt_runtime_lock);
 }
 
+/**
+ * prio_lower(a, b) returns true if the priority a is
+ * lower than the priority b, otherwise return false.
+ */
+static inline bool prio_lower(int a, int b)
+{
+	return a > b;
+}
+
 #ifdef CONFIG_RT_GROUP_SCHED
 static void destroy_rt_bandwidth(struct rt_bandwidth *rt_b)
 {
@@ -262,7 +271,7 @@ static void pull_rt_task(struct rq *this_rq);
 static inline bool need_pull_rt_task(struct rq *rq, struct task_struct *prev)
 {
 	/* Try to pull RT tasks here if we lower this rq's prio */
-	return rq->rt.highest_prio.curr > prev->prio;
+	return prio_lower(rq->rt.highest_prio.curr, prev->prio);
 }
 
 static inline int rt_overloaded(struct rq *rq)
@@ -377,7 +386,7 @@ static void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
 	plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks);
 
 	/* Update the highest prio pushable task */
-	if (p->prio < rq->rt.highest_prio.next)
+	if (prio_lower(rq->rt.highest_prio.next, p->prio))
 		rq->rt.highest_prio.next = p->prio;
 }
 
@@ -498,7 +507,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 		else if (!on_rt_rq(rt_se))
 			enqueue_rt_entity(rt_se, 0);
 
-		if (rt_rq->highest_prio.curr < curr->prio)
+		if (prio_lower(curr->prio, rt_rq->highest_prio.curr))
 			resched_curr(rq);
 	}
 }
@@ -1044,7 +1053,7 @@ inc_rt_prio_smp(struct rt_rq *rt_rq, int prio, int prev_prio)
 	if (&rq->rt != rt_rq)
 		return;
 #endif
-	if (rq->online && prio < prev_prio)
+	if (rq->online && prio_lower(prev_prio, prio))
 		cpupri_set(&rq->rd->cpupri, rq->cpu, prio);
 }
 
@@ -1079,7 +1088,7 @@ inc_rt_prio(struct rt_rq *rt_rq, int prio)
 {
 	int prev_prio = rt_rq->highest_prio.curr;
 
-	if (prio < prev_prio)
+	if (prio_lower(prev_prio, prio))
 		rt_rq->highest_prio.curr = prio;
 
 	inc_rt_prio_smp(rt_rq, prio, prev_prio);
@@ -1092,7 +1101,7 @@ dec_rt_prio(struct rt_rq *rt_rq, int prio)
 
 	if (rt_rq->rt_nr_running) {
 
-		WARN_ON(prio < prev_prio);
+		WARN_ON(prio_lower(prev_prio, prio));
 
 		/*
 		 * This may have been our highest task, and therefore
@@ -1424,7 +1433,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	 */
 	if (curr && unlikely(rt_task(curr)) &&
 	    (curr->nr_cpus_allowed < 2 ||
-	     curr->prio <= p->prio)) {
+	     !prio_lower(curr->prio, p->prio))) {
 		int target = find_lowest_rq(p);
 
 		/*
@@ -1432,7 +1441,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 		 * not running a lower priority task.
 		 */
 		if (target != -1 &&
-		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
+		    prio_lower(cpu_rq(target)->rt.highest_prio.curr, p->prio))
 			cpu = target;
 	}
 	rcu_read_unlock();
@@ -1475,7 +1484,7 @@ static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
  */
 static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p, int flags)
 {
-	if (p->prio < rq->curr->prio) {
+	if (prio_lower(rq->curr->prio, p->prio)) {
 		resched_curr(rq);
 		return;
 	}
@@ -1732,7 +1741,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 
 		lowest_rq = cpu_rq(cpu);
 
-		if (lowest_rq->rt.highest_prio.curr <= task->prio) {
+		if (!prio_lower(lowest_rq->rt.highest_prio.curr, task->prio)) {
 			/*
 			 * Target rq has tasks of equal or higher priority,
 			 * retrying does not release any lock and is unlikely
@@ -1763,7 +1772,7 @@ 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.curr > task->prio)
+		if (prio_lower(lowest_rq->rt.highest_prio.curr, task->prio))
 			break;
 
 		/* try again */
@@ -1823,7 +1832,7 @@ static int push_rt_task(struct rq *rq)
 	 * higher priority than current. If that's the case
 	 * just reschedule current.
 	 */
-	if (unlikely(next_task->prio < rq->curr->prio)) {
+	if (unlikely(prio_lower(rq->curr->prio, next_task->prio))) {
 		resched_curr(rq);
 		return 0;
 	}
@@ -2100,8 +2109,8 @@ static void pull_rt_task(struct rq *this_rq)
 		 * logically higher, the src_rq will push this task away.
 		 * And if its going logically lower, we do not care
 		 */
-		if (src_rq->rt.highest_prio.next >=
-		    this_rq->rt.highest_prio.curr)
+		if (!prio_lower(this_rq->rt.highest_prio.curr,
+				src_rq->rt.highest_prio.next))
 			continue;
 
 		/*
@@ -2121,7 +2130,7 @@ static void pull_rt_task(struct rq *this_rq)
 		 * Do we have an RT task that preempts
 		 * the to-be-scheduled task?
 		 */
-		if (p && (p->prio < this_rq->rt.highest_prio.curr)) {
+		if (p && prio_lower(this_rq->rt.highest_prio.curr, p->prio)) {
 			WARN_ON(p == src_rq->curr);
 			WARN_ON(!task_on_rq_queued(p));
 
@@ -2133,7 +2142,7 @@ static void pull_rt_task(struct rq *this_rq)
 			 * p if it is lower in priority than the
 			 * current task on the run queue
 			 */
-			if (p->prio < src_rq->curr->prio)
+			if (prio_lower(src_rq->curr->prio, p->prio))
 				goto skip;
 
 			resched = true;
@@ -2167,7 +2176,7 @@ static void task_woken_rt(struct rq *rq, struct task_struct *p)
 	    p->nr_cpus_allowed > 1 &&
 	    (dl_task(rq->curr) || rt_task(rq->curr)) &&
 	    (rq->curr->nr_cpus_allowed < 2 ||
-	     rq->curr->prio <= p->prio))
+	     !prio_lower(rq->curr->prio, p->prio)))
 		push_rt_tasks(rq);
 }
 
@@ -2242,7 +2251,8 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
 		if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
 			rt_queue_push_tasks(rq);
 #endif /* CONFIG_SMP */
-		if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
+		if (prio_lower(rq->curr->prio, p->prio) &&
+		    cpu_online(cpu_of(rq)))
 			resched_curr(rq);
 	}
 }
@@ -2263,18 +2273,18 @@ prio_changed_rt(struct rq *rq, struct task_struct *p, int oldprio)
 		 * If our priority decreases while running, we
 		 * may need to pull tasks to this runqueue.
 		 */
-		if (oldprio < p->prio)
+		if (prio_lower(p->prio, oldprio))
 			rt_queue_pull_task(rq);
 
 		/*
 		 * If there's a higher priority task waiting to run
 		 * then reschedule.
 		 */
-		if (p->prio > rq->rt.highest_prio.curr)
+		if (prio_lower(p->prio, rq->rt.highest_prio.curr))
 			resched_curr(rq);
 #else
 		/* For UP simply resched on drop of prio */
-		if (oldprio < p->prio)
+		if (prio_lower(p->prio, oldprio))
 			resched_curr(rq);
 #endif /* CONFIG_SMP */
 	} else {
@@ -2283,7 +2293,7 @@ prio_changed_rt(struct rq *rq, struct task_struct *p, int oldprio)
 		 * greater than the current running task
 		 * then reschedule.
 		 */
-		if (p->prio < rq->curr->prio)
+		if (prio_lower(rq->curr->prio, p->prio))
 			resched_curr(rq);
 	}
 }
-- 
2.17.1

Re: [PATCH] sched/rt: Introduce prio_lower() helper for comparing RT task prority

[Muchun Song]

Hi Peter,

This is v2 patch. I'm sorry that I forgot to add the
word "v2" to the subject.

Yours,
Muchun Song