sched/rt: Do not throttle when PI boosting

When a runqueue has rt_runtime_us = 0 then the only way it can
accumulate rt_time is via PI boosting. That causes the runqueue
to be throttled and replenishing does not change anything due to
rt_runtime_us = 0. So avoid that situation by clearing rt_time and
skip the throttling alltogether.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
[ Changelog ]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/n/tip-7x70cypsotjb4jvcor3edctk@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 18 insertions, 2 deletions

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 6d1eb0be187..7f7e7cdcb47 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -857,8 +857,24 @@ static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
                 return 0;
 
         if (rt_rq->rt_time > runtime) {
-                rt_rq->rt_throttled = 1;
-                printk_once(KERN_WARNING "sched: RT throttling activated\n");
+                struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);
+
+                /*
+                 * Don't actually throttle groups that have no runtime assigned
+                 * but accrue some time due to boosting.
+                 */
+                if (likely(rt_b->rt_runtime)) {
+                        rt_rq->rt_throttled = 1;
+                        printk_once(KERN_WARNING "sched: RT throttling activated\n");
+                } else {
+                        /*
+                         * In case we did anyway, make it go away,
+                         * replenishment is a joke, since it will replenish us
+                         * with exactly 0 ns.
+                         */
+                        rt_rq->rt_time = 0;
+                }
+
                 if (rt_rq_throttled(rt_rq)) {
                         sched_rt_rq_dequeue(rt_rq);
                         return 1;