[PATCH v2] sched/rt : return accurate release rq lock info

[Peng Hao]

find_lock_lowest_rq may or not releease rq lock, but it is fuzzy.
If not releasing rq lock, it is unnecessary to re-call
pick_next_pushable_task.
When CONFIG_PREEMPT=n, not releasing rq lock frequently happens
in a simple test case:
Four different rt priority tasks run on limited two cpus.

Signed-off-by: Peng Hao <peng.hao2@xxxxxxxxxx>
---
 kernel/sched/rt.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2e2955a..7c5382a 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1718,6 +1718,7 @@ static int find_lowest_rq(struct task_struct *task)
 static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 {
 	struct rq *lowest_rq = NULL;
+	bool release_lock = false;
 	int tries;
 	int cpu;
 
@@ -1741,6 +1742,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 
 		/* if the prio of this runqueue changed, try again */
 		if (double_lock_balance(rq, lowest_rq)) {
+			release_lock = true;
 			/*
 			 * We had to unlock the run queue. In
 			 * the mean time, task could have
@@ -1768,6 +1770,8 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 		lowest_rq = NULL;
 	}
 
+	if (!lowest_rq && release_lock)
+		lowest_rq = RETRY_TASK;
 	return lowest_rq;
 }
 
@@ -1830,7 +1834,7 @@ static int push_rt_task(struct rq *rq)
 
 	/* find_lock_lowest_rq locks the rq if found */
 	lowest_rq = find_lock_lowest_rq(next_task, rq);
-	if (!lowest_rq) {
+	if (lowest_rq == RETRY_TASK) {
 		struct task_struct *task;
 		/*
 		 * find_lock_lowest_rq releases rq->lock
@@ -1863,6 +1867,8 @@ static int push_rt_task(struct rq *rq)
 		goto retry;
 	}
 
+	if (!lowest_rq)
+		goto out;
 	deactivate_task(rq, next_task, 0);
 	set_task_cpu(next_task, lowest_rq->cpu);
 	activate_task(lowest_rq, next_task, 0);
-- 
1.8.3.1