linux-kernel - [PATCH 3.2 46/79] sched/rt: Fix rq's cpupri leak while enqueue/dequeue child RT entities

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Message-ID: <lsq.1392192612.216027001@decadent.org.uk>
Date:	Wed, 12 Feb 2014 08:10:12 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Kirill Tkhai" <tkhai@...dex.ru>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Steven Rostedt" <rostedt@...dmis.org>
Subject: [PATCH 3.2 46/79] sched/rt: Fix rq's cpupri leak while
 enqueue/dequeue child RT entities

3.2.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Kirill Tkhai <tkhai@...dex.ru>

commit 757dfcaa41844595964f1220f1d33182dae49976 upstream.

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@...dex.ru>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
CC: Steven Rostedt <rostedt@...dmis.org>
Link: http://lkml.kernel.org/r/49231385567953@web4m.yandex.ru
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/sched_rt.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -719,6 +719,13 @@ inc_rt_prio_smp(struct rt_rq *rt_rq, int
 {
 	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);
 }
@@ -728,6 +735,13 @@ dec_rt_prio_smp(struct rt_rq *rt_rq, int
 {
 	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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