sched: Fix RLIMIT_RTTIME when PI-boosting to RT

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@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.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f09e22163be33f1448e4fc0d0e5506819ef687d2..0030db473c994f0c3b7ef8b4a5d04fa98bb8eeb0 100644 (file)
--- 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;