[RFC v4 2/6] sched/deadline: improve the tracking of active utilization

[Luca Abeni]

From: Luca Abeni <luca.abeni@xxxxxxxx>

This patch implements a more theoretically sound algorithm for
tracking active utilization: instead of decreasing it when a
task blocks, use a timer (the "inactive timer", named after the
"Inactive" task state of the GRUB algorithm) to decrease the
active utilization at the so called "0-lag time".

Signed-off-by: Luca Abeni <luca.abeni@xxxxxxxx>
---
 include/linux/sched.h   |  18 +++++-
 kernel/sched/core.c     |   2 +
 kernel/sched/deadline.c | 150 ++++++++++++++++++++++++++++++++++++++++++++----
 kernel/sched/sched.h    |   1 +
 4 files changed, 158 insertions(+), 13 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4d19052..f34633c2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1451,14 +1451,30 @@ struct sched_dl_entity {
 	 *
 	 * @dl_yielded tells if task gave up the cpu before consuming
 	 * all its available runtime during the last job.
+	 *
+	 * @dl_non_contending tells if task is inactive while still
+	 * contributing to the active utilization. In other words, it
+	 * indicates if the inactive timer has been armed and its handler
+	 * has not been executed yet. This flag is useful to avoid race
+	 * conditions between the inactive timer handler and the wakeup
+	 * code.
 	 */
-	int dl_throttled, dl_boosted, dl_yielded;
+	int dl_throttled, dl_boosted, dl_yielded, dl_non_contending;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its
 	 * own bandwidth to be enforced, thus we need one timer per task.
 	 */
 	struct hrtimer dl_timer;
+
+	/*
+	 * Inactive timer, responsible for decreasing the active utilization
+	 * at the "0-lag time". When a -deadline task blocks, it contributes
+	 * to GRUB's active utilization until the "0-lag time", hence a
+	 * timer is needed to decrease the active utilization at the correct
+	 * time.
+	 */
+	struct hrtimer inactive_timer;
 };
 
 union rcu_special {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c56fb57..98f9944 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2187,6 +2187,7 @@ void __dl_clear_params(struct task_struct *p)
 
 	dl_se->dl_throttled = 0;
 	dl_se->dl_yielded = 0;
+	dl_se->dl_non_contending = 0;
 }
 
 /*
@@ -2218,6 +2219,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 	RB_CLEAR_NODE(&p->dl.rb_node);
 	init_dl_task_timer(&p->dl);
+	init_inactive_task_timer(&p->dl);
 	__dl_clear_params(p);
 
 	INIT_LIST_HEAD(&p->rt.run_list);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 23c840e..cdb7274 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -65,6 +65,46 @@ void sub_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 		dl_rq->running_bw = 0;
 }
 
+static void task_go_inactive(struct task_struct *p)
+{
+	struct sched_dl_entity *dl_se = &p->dl;
+	struct hrtimer *timer = &dl_se->inactive_timer;
+	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
+	struct rq *rq = rq_of_dl_rq(dl_rq);
+	s64 zerolag_time;
+
+	WARN_ON(dl_se->dl_runtime == 0);
+
+	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
+	WARN_ON(dl_se->dl_non_contending);
+
+	zerolag_time = dl_se->deadline -
+		 div64_long((dl_se->runtime * dl_se->dl_period),
+			dl_se->dl_runtime);
+
+	/*
+	 * Using relative times instead of the absolute "0-lag time"
+	 * allows to simplify the code
+	 */
+	zerolag_time -= rq_clock(rq);
+
+	/*
+	 * If the "0-lag time" already passed, decrease the active
+	 * utilization now, instead of starting a timer
+	 */
+	if (zerolag_time < 0) {
+		sub_running_bw(dl_se, dl_rq);
+		if (!dl_task(p))
+			__dl_clear_params(p);
+
+		return;
+	}
+
+	dl_se->dl_non_contending = 1;
+	get_task_struct(p);
+	hrtimer_start(timer, ns_to_ktime(zerolag_time), HRTIMER_MODE_REL);
+}
+
 static inline int is_leftmost(struct task_struct *p, struct dl_rq *dl_rq)
 {
 	struct sched_dl_entity *dl_se = &p->dl;
@@ -610,10 +650,8 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	 * The task might have changed its scheduling policy to something
 	 * different than SCHED_DEADLINE (through switched_from_dl()).
 	 */
-	if (!dl_task(p)) {
-		__dl_clear_params(p);
+	if (!dl_task(p))
 		goto unlock;
-	}
 
 	/*
 	 * The task might have been boosted by someone else and might be in the
@@ -800,6 +838,48 @@ static void update_curr_dl(struct rq *rq)
 	}
 }
 
+static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
+{
+	struct sched_dl_entity *dl_se = container_of(timer,
+						     struct sched_dl_entity,
+						     inactive_timer);
+	struct task_struct *p = dl_task_of(dl_se);
+	struct rq_flags rf;
+	struct rq *rq;
+
+	rq = task_rq_lock(p, &rf);
+
+	if (!dl_task(p) || p->state == TASK_DEAD) {
+		if (p->state == TASK_DEAD && dl_se->dl_non_contending)
+			sub_running_bw(&p->dl, dl_rq_of_se(&p->dl));
+
+		__dl_clear_params(p);
+
+		goto unlock;
+	}
+	if (dl_se->dl_non_contending == 0)
+		goto unlock;
+
+	sched_clock_tick();
+	update_rq_clock(rq);
+
+	sub_running_bw(dl_se, &rq->dl);
+	dl_se->dl_non_contending = 0;
+unlock:
+	task_rq_unlock(rq, p, &rf);
+	put_task_struct(p);
+
+	return HRTIMER_NORESTART;
+}
+
+void init_inactive_task_timer(struct sched_dl_entity *dl_se)
+{
+	struct hrtimer *timer = &dl_se->inactive_timer;
+
+	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	timer->function = inactive_task_timer;
+}
+
 #ifdef CONFIG_SMP
 
 static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
@@ -934,7 +1014,28 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se,
 	if (flags & ENQUEUE_WAKEUP) {
 		struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
 
-		add_running_bw(dl_se, dl_rq);
+		if (dl_se->dl_non_contending) {
+			/*
+			 * If the timer handler is currently running and the
+			 * timer cannot be cancelled, inactive_task_timer()
+			 * will see that dl_not_contending is not set, and
+			 * will do nothing, so we are still safe.
+			 */
+			if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == 1)
+				put_task_struct(dl_task_of(dl_se));
+			WARN_ON(dl_task_of(dl_se)->nr_cpus_allowed > 1);
+			dl_se->dl_non_contending = 0;
+		} else {
+			/*
+			 * Since "dl_non_contending" is not set, the
+			 * task's utilization has already been removed from
+			 * active utilization (either when the task blocked,
+			 * when the "inactive timer" fired, or when it has
+			 * been cancelled in select_task_rq_dl()).
+			 * So, add it back.
+			 */
+			add_running_bw(dl_se, dl_rq);
+		}
 		update_dl_entity(dl_se, pi_se);
 	}
 	else if (flags & ENQUEUE_REPLENISH)
@@ -1023,7 +1124,7 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 	 * or "inactive")
 	 */
 	if (flags & DEQUEUE_SLEEP)
-		sub_running_bw(&p->dl, &rq->dl);
+		task_go_inactive(p);
 }
 
 /*
@@ -1097,6 +1198,22 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
 	}
 	rcu_read_unlock();
 
+	rq = task_rq(p);
+	raw_spin_lock(&rq->lock);
+	if (p->dl.dl_non_contending) {
+		sub_running_bw(&p->dl, &rq->dl);
+		p->dl.dl_non_contending = 0;
+		/*
+		 * If the timer handler is currently running and the
+		 * timer cannot be cancelled, inactive_task_timer()
+		 * will see that dl_not_contending is not set, and
+		 * will do nothing, so we are still safe.
+		 */
+		if (hrtimer_try_to_cancel(&p->dl.inactive_timer) == 1)
+			put_task_struct(p);
+	}
+	raw_spin_unlock(&rq->lock);
+
 out:
 	return cpu;
 }
@@ -1743,16 +1860,25 @@ void __init init_sched_dl_class(void)
 static void switched_from_dl(struct rq *rq, struct task_struct *p)
 {
 	/*
-	 * Start the deadline timer; if we switch back to dl before this we'll
-	 * continue consuming our current CBS slice. If we stay outside of
-	 * SCHED_DEADLINE until the deadline passes, the timer will reset the
-	 * task.
+	 * task_go_inactive() can start the "inactive timer" (if the 0-lag
+	 * time is in the future). If the task switches back to dl before
+	 * the "inactive timer" fires, it can continue to consume its current
+	 * runtime using its current deadline. If it stays outside of
+	 * SCHED_DEADLINE until the 0-lag time passes, inactive_task_timer()
+	 * will reset the task parameters.
 	 */
-	if (!start_dl_timer(p))
-		__dl_clear_params(p);
+	if (task_on_rq_queued(p) && p->dl.dl_runtime)
+		task_go_inactive(p);
 
-	if (task_on_rq_queued(p))
+	/*
+	 * We cannot use inactive_task_timer() to invoke sub_running_bw()
+	 * at the 0-lag time, because the task could have been migrated
+	 * while SCHED_OTHER in the meanwhile.
+	 */
+	if (p->dl.dl_non_contending) {
 		sub_running_bw(&p->dl, &rq->dl);
+		p->dl.dl_non_contending = 0;
+	}
 
 	/*
 	 * Since this might be the only -deadline task on the rq,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 0659772..e422803 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1367,6 +1367,7 @@ extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime
 extern struct dl_bandwidth def_dl_bandwidth;
 extern void init_dl_bandwidth(struct dl_bandwidth *dl_b, u64 period, u64 runtime);
 extern void init_dl_task_timer(struct sched_dl_entity *dl_se);
+extern void init_inactive_task_timer(struct sched_dl_entity *dl_se);
 
 unsigned long to_ratio(u64 period, u64 runtime);
 
-- 
2.7.4