[PATCH v2 2/6] sched/rt: Re-instate old behavior in select_task_rq_rt

[Qais Yousef]

When RT Capacity Aware support was added, the logic in select_task_rq_rt
was modified to force a search for a fitting CPU if the task currently
doesn't run on one.

But if the search failed, and the search was only triggered to fulfill
the fitness request; we could end up selecting a new CPU unnecessarily.

Fix this and re-instate the original behavior by ensuring we bail out
in that case.

This behavior change only affected asymmetric systems that are using
util_clamp to implement capacity aware. None asymmetric systems weren't
affected.

Reported-by: Pavan Kondeti <pkondeti@xxxxxxxxxxxxxx>
Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
LINK: https://lore.kernel.org/lkml/20200218041620.GD28029@xxxxxxxxxxxxxx/
Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
---
 kernel/sched/rt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe45459..2c3fae637cef 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1474,6 +1474,13 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	if (test || !rt_task_fits_capacity(p, cpu)) {
 		int target = find_lowest_rq(p);
 
+		/*
+		 * Bail out if we were forcing a migration to find a better
+		 * fitting CPU but our search failed.
+		 */
+		if (!test && !rt_task_fits_capacity(p, target))
+			goto out_unlock;
+
 		/*
 		 * Don't bother moving it if the destination CPU is
 		 * not running a lower priority task.
@@ -1482,6 +1489,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
 			cpu = target;
 	}
+
+out_unlock:
 	rcu_read_unlock();
 
 out:
-- 
2.17.1