[PATCH 3.12 24/83] sched: Fix RLIMIT_RTTIME when PI-boosting to RT

[Jiri Slaby]

From: Brian Silverman <brian@xxxxxxxxxxxxxxxx>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 746db9443ea57fd9c059f62c4bfbf41cf224fe13 upstream.

When non-realtime tasks get priority-inheritance boosted to a realtime
scheduling class, RLIMIT_RTTIME starts to apply to them. However, the
counter used for checking this (the same one used for SCHED_RR
timeslices) was not getting reset. This meant that tasks running with a
non-realtime scheduling class which are repeatedly boosted to a realtime
one, but never block while they are running realtime, eventually hit the
timeout without ever running for a time over the limit. This patch
resets the realtime timeslice counter when un-PI-boosting from an RT to
a non-RT scheduling class.

I have some test code with two threads and a shared PTHREAD_PRIO_INHERIT
mutex which induces priority boosting and spins while boosted that gets
killed by a SIGXCPU on non-fixed kernels but doesn't with this patch
applied. It happens much faster with a CONFIG_PREEMPT_RT kernel, and
does happen eventually with PREEMPT_VOLUNTARY kernels.

Signed-off-by: Brian Silverman <brian@xxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: austin@xxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1424305436-6716-1-git-send-email-brian@xxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 kernel/sched/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f09e22163be3..0030db473c99 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3060,8 +3060,11 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 
 	if (rt_prio(prio))
 		p->sched_class = &rt_sched_class;
-	else
+	else {
+		if (rt_prio(oldprio))
+			p->rt.timeout = 0;
 		p->sched_class = &fair_sched_class;
+	}
 
 	p->prio = prio;
 
-- 
2.3.5

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