[NETEM]: avoid excessive requeues

The netem code would call getnstimeofday() and dequeue/requeue after
every packet, even if it was waiting. Avoid this overhead by using
the throttled flag.
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -298,6 +298,7 @@ static enum hrtimer_restart qdisc_watchdog(struct hrtimer *timer)
 						 timer);
 
 	wd->qdisc->flags &= ~TCQ_F_THROTTLED;
+	smp_wmb();
 	netif_schedule(wd->qdisc->dev);
 	return HRTIMER_NORESTART;
 }
@@ -315,6 +316,7 @@ void qdisc_watchdog_schedule(struct qdisc_watchdog *wd, psched_time_t expires)
 	ktime_t time;
 
 	wd->qdisc->flags |= TCQ_F_THROTTLED;
+	smp_wmb();
 	time = ktime_set(0, 0);
 	time = ktime_add_ns(time, PSCHED_US2NS(expires));
 	hrtimer_start(&wd->timer, time, HRTIMER_MODE_ABS);
@@ -325,6 +327,7 @@ void qdisc_watchdog_cancel(struct qdisc_watchdog *wd)
 {
 	hrtimer_cancel(&wd->timer);
 	wd->qdisc->flags &= ~TCQ_F_THROTTLED;
+	smp_wmb();
 }
 EXPORT_SYMBOL(qdisc_watchdog_cancel);
 

net/sched/sch_netem.c

@@ -273,6 +273,10 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 	struct netem_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *skb;
 
+	smp_mb();
+	if (sch->flags & TCQ_F_THROTTLED)
+		return NULL;
+
 	skb = q->qdisc->dequeue(q->qdisc);
 	if (skb) {
 		const struct netem_skb_cb *cb
@@ -285,18 +289,17 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 		if (PSCHED_TLESS(cb->time_to_send, now)) {
 			pr_debug("netem_dequeue: return skb=%p\n", skb);
 			sch->q.qlen--;
-			sch->flags &= ~TCQ_F_THROTTLED;
 			return skb;
-		} else {
-			qdisc_watchdog_schedule(&q->watchdog, cb->time_to_send);
-
-			if (q->qdisc->ops->requeue(skb, q->qdisc) != NET_XMIT_SUCCESS) {
-				qdisc_tree_decrease_qlen(q->qdisc, 1);
-				sch->qstats.drops++;
-				printk(KERN_ERR "netem: queue discpline %s could not requeue\n",
-				       q->qdisc->ops->id);
-			}
 		}
+
+		if (unlikely(q->qdisc->ops->requeue(skb, q->qdisc) != NET_XMIT_SUCCESS)) {
+			qdisc_tree_decrease_qlen(q->qdisc, 1);
+			sch->qstats.drops++;
+			printk(KERN_ERR "netem: %s could not requeue\n",
+			       q->qdisc->ops->id);
+		}
+
+		qdisc_watchdog_schedule(&q->watchdog, cb->time_to_send);
 	}
 
 	return NULL;