[RFC v5 5/9] sched/deadline: do not reclaim the whole CPU bandwidth

[luca abeni <luca.abeni@santannapisa.it>]

From: Luca Abeni <luca.abeni@santannapisa.it>

Original GRUB tends to reclaim 100% of the CPU time... And this
allows a CPU hog to starve non-deadline tasks.
To address this issue, allow the scheduler to reclaim only a
specified fraction of CPU time.

Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>
---
 kernel/sched/core.c     | 6 ++++++
 kernel/sched/deadline.c | 7 ++++++-
 kernel/sched/sched.h    | 6 ++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20c62e7..efa88eb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6716,6 +6716,12 @@ static void sched_dl_do_global(void)
 		raw_spin_unlock_irqrestore(&dl_b->lock, flags);
 
 		rcu_read_unlock_sched();
+		if (dl_b->bw == -1)
+			cpu_rq(cpu)->dl.deadline_bw_inv = 1 << 8;
+		else
+			cpu_rq(cpu)->dl.deadline_bw_inv =
+				to_ratio(global_rt_runtime(),
+					 global_rt_period()) >> 12;
 	}
 }
 
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 6035311..e964051 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -212,6 +212,11 @@ void init_dl_rq(struct dl_rq *dl_rq)
 #else
 	init_dl_bw(&dl_rq->dl_bw);
 #endif
+	if (global_rt_runtime() == RUNTIME_INF)
+		dl_rq->deadline_bw_inv = 1 << 8;
+	else
+		dl_rq->deadline_bw_inv =
+			to_ratio(global_rt_runtime(), global_rt_period()) >> 12;
 }
 
 #ifdef CONFIG_SMP
@@ -871,7 +876,7 @@ extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq);
  */
 u64 grub_reclaim(u64 delta, struct rq *rq)
 {
-	return (delta * rq->dl.running_bw) >> 20;
+	return (delta * rq->dl.running_bw * rq->dl.deadline_bw_inv) >> 20 >> 8;
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 57bb79b..141549b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -565,6 +565,12 @@ struct dl_rq {
 	 * task blocks
 	 */
 	u64 running_bw;
+
+	/*
+	 * Inverse of the fraction of CPU utilization that can be reclaimed
+	 * by the GRUB algorithm.
+	 */
+	u64 deadline_bw_inv;
 };
 
 #ifdef CONFIG_SMP
-- 
2.7.4