[RFC 8/8] Do not reclaim the whole CPU bandwidth

[Luca Abeni]

Original GRUB tends to reclaim 100% of the CPU time... And this allows a
"CPU hog" (i.e., a busy loop) to starve non-deadline tasks.
To address this issue, allow the scheduler to reclaim only a specified
fraction of CPU time.
NOTE: the fraction of CPU time that cannot be reclaimed is currently
hardcoded as (1 << 20) / 10 -> 90%, but it must be made configurable!
---
 kernel/sched/deadline.c | 3 ++-
 kernel/sched/sched.h    | 4 ++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 712cc6d..57b693b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -185,6 +185,7 @@ void init_dl_rq(struct dl_rq *dl_rq)
 #else
 	init_dl_bw(&dl_rq->dl_bw);
 #endif
+	dl_rq->unusable_bw = (1 << 20) / 10;		// FIXME: allow to set this!
 }
 
 #ifdef CONFIG_SMP
@@ -825,7 +826,7 @@ extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq);
 
 u64 grub_reclaim(u64 delta, struct rq *rq, u64 u)
 {
-	return (delta * rq->dl.running_bw) >> 20;
+	return (delta * (rq->dl.unusable_bw + rq->dl.running_bw)) >> 20;
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d06005b..76df0ff 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -524,6 +524,10 @@ struct dl_rq {
 	 * and decreased when a task blocks
 	 */
 	s64 running_bw;
+	/* This is the amount of utilization that GRUB can not
+         * reclaim (per runqueue)
+         */
+	s64 unusable_bw;
 
 	s64 this_bw;
 };
-- 
1.9.1