sched: Fix RLIMIT_RTTIME when PI-boosting to RT

commit 746db944 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@peloton-tech.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: austin@peloton-tech.com
Link: http://lkml.kernel.org/r/1424305436-6716-1-git-send-email-brian@peloton-tech.comSigned-off-by: Ingo Molnar <mingo@kernel.org>
[lizf: Backported to 3.4: adjust contest]
Signed-off-by: Zefan Li <lizefan@huawei.com>

kernel/sched/core.c

@@ -4031,10 +4031,13 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 	if (running)
 		p->sched_class->put_prev_task(rq, p);
 
-	if (rt_prio(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;