[PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU

[Qais Yousef <qais.yousef@arm.com>]

If a task was running on unfit CPU we could ignore migrating if the
priority level of the new fitting CPU is the *same* as the unfit one.

Add an extra check to select_task_rq_rt() to allow the push in case:

	* old_cpu.highest_priority == new_cpu.highest_priority
	* task_fits(p, new_cpu)

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---

I was seeing some delays in migrating a task to a big CPU sometimes although it
was free, and I think this fixes it.

TBH, I fail to see how the check of

	p->prio < cpu_rq(target)->rt.highest_prio.curr

is necessary as find_lowest_rq() surely implies the above condition by
definition?

Unless we're fighting a race condition here where the rt_rq priority has
changed between the time we selected the lowest_rq and taking the decision to
migrate, then this makes sense.


 kernel/sched/rt.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 0c8bac134d3a..5ea235f2cfe8 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1430,7 +1430,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 {
 	struct task_struct *curr;
 	struct rq *rq;
-	bool test;
+	bool test, fit;
 
 	/* For anything but wake ups, just return the task_cpu */
 	if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK)
@@ -1471,16 +1471,32 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	       unlikely(rt_task(curr)) &&
 	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
 
-	if (test || !rt_task_fits_capacity(p, cpu)) {
+	fit = rt_task_fits_capacity(p, cpu);
+
+	if (test || !fit) {
 		int target = find_lowest_rq(p);
 
-		/*
-		 * Don't bother moving it if the destination CPU is
-		 * not running a lower priority task.
-		 */
-		if (target != -1 &&
-		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
-			cpu = target;
+		if (target != -1) {
+			/*
+			 * Don't bother moving it if the destination CPU is
+			 * not running a lower priority task.
+			 */
+			if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
+
+				cpu = target;
+
+			} else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
+
+				/*
+				 * If the priority is the same and the new CPU
+				 * is a better fit, then move, otherwise don't
+				 * bother here either.
+				 */
+				fit = rt_task_fits_capacity(p, target);
+				if (fit)
+					cpu = target;
+			}
+		}
 	}
 	rcu_read_unlock();
 
-- 
2.17.1