[PATCH] sched/rt: Fix rq's cpupri leak while enqueue/dequeue child RT entities

[Kirill Tkhai]

This patch touches RT group scheduling case.

Functions inc_rt_prio_smp() and dec_rt_prio_smp() change (global) rq's priority,
while rt_rq passed to them may be not the top-level rt_rq. This is wrong, because
changing of priority on a child level does not guarantee that the priority is
the highest all over the rq. So, this leak makes RT balancing unusable.

The short example: the task having the highest priority among all rq's RT tasks
(no one other task has the same priority) are waking on a throttle rt_rq.
The rq's cpupri is set to the task's priority equivalent, but real
rq->rt.highest_prio.curr is less.

The patch below fixes the problem.

It looks like all version have this bug, so I CC'ed stable mailing list.

Signed-off-by: Kirill Tkhai <tkhai@xxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx
---
 kernel/sched/rt.c |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7d57275..1c40655 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -901,6 +901,13 @@ inc_rt_prio_smp(struct rt_rq *rt_rq, int prio, int prev_prio)
 {
 	struct rq *rq = rq_of_rt_rq(rt_rq);
 
+#ifdef CONFIG_RT_GROUP_SCHED
+	/*
+	 * Change rq's cpupri only if rt_rq is the top queue.
+	 */
+	if (&rq->rt != rt_rq)
+		return;
+#endif
 	if (rq->online && prio < prev_prio)
 		cpupri_set(&rq->rd->cpupri, rq->cpu, prio);
 }
@@ -910,6 +917,13 @@ dec_rt_prio_smp(struct rt_rq *rt_rq, int prio, int prev_prio)
 {
 	struct rq *rq = rq_of_rt_rq(rt_rq);
 
+#ifdef CONFIG_RT_GROUP_SCHED
+	/*
+	 * Change rq's cpupri only if rt_rq is the top queue.
+	 */
+	if (&rq->rt != rt_rq)
+		return;
+#endif
 	if (rq->online && rt_rq->highest_prio.curr != prev_prio)
 		cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio.curr);
 }
--
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