linux-kernel - [PATCH v2 2/2] sched/deadline: Change the way to replenish runtime for sleep tasks

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Date:   Thu, 23 Feb 2017 12:10:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: [PATCH v2 2/2] sched/deadline: Change the way to replenish runtime for sleep tasks

Let's consider the following example.

timeline : o...................o.........o.......o..o
           ^                   ^         ^       ^  ^
           |                   |         |       |  |
       start                   |         |       |  |
                original runtime         |       |  |
                     sleep with (-)runtime       |  |
                                 original deadline  |
                                              wake up

When this task is woken up, a negative runtime should be considered,
which means that the task should get penalized when assigning runtime,
becasue it already spent more than expected.

Current code handles this by replenishing a runtime in hrtimer callback
for deadline. But this approach has room for improvement in two ways:

   1. No need to keep the hrtimer for a sleep task because it can be
      handled when waking it up.

   2. It will be replenished twice unnecessarily if the task sleeps for
      long time so that the deadline, assigned in the hrtimer callback,
      also passed. In other words, one happens in the callback and the
      other happens in update_dl_entiry() when waking it up.

So try to cancel the hrtimer for a sleep task and make all these things
handled when waking it up.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/sched/deadline.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 69e3fbb..b59ee7b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -507,8 +507,10 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
 	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
 	struct rq *rq = rq_of_dl_rq(dl_rq);
 
-	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
-	    dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
+	if (dl_time_before(dl_se->deadline, rq_clock(rq)))
+		__replenish_dl_entity(dl_se, pi_se);
+
+	if (dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
 		dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
 		dl_se->runtime = pi_se->dl_runtime;
 	}
@@ -981,6 +983,9 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 {
 	update_curr_dl(rq);
 	__dequeue_task_dl(rq, p, flags);
+
+	if (flags & DEQUEUE_SLEEP)
+		hrtimer_try_to_cancel(&p->dl.dl_timer);
 }
 
 /*
-- 
1.9.1