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

[tip-bot for Kirill Tkhai]

Commit-ID:  757dfcaa41844595964f1220f1d33182dae49976
Gitweb:     http://git.kernel.org/tip/757dfcaa41844595964f1220f1d33182dae49976
Author:     Kirill Tkhai <tkhai@xxxxxxxxx>
AuthorDate: Wed, 27 Nov 2013 19:59:13 +0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 17 Dec 2013 15:08:44 +0100

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

This patch touches the 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.

Signed-off-by: Kirill Tkhai <tkhai@xxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/49231385567953@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/rt.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

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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