net_sched: TCQ_F_CAN_BYPASS generalization

Now qdisc stab is handled before TCQ_F_CAN_BYPASS test in
__dev_xmit_skb(), we can generalize TCQ_F_CAN_BYPASS to other qdiscs
than pfifo_fast : pfifo, bfifo, pfifo_head_drop and sfq

SFQ is special because it can have external classifiers, and in these
cases, we cannot bypass queue discipline (packet could be dropped by
classifier) without admin asking it, or further changes.

Its worth doing this, especially for SFQ, avoiding dirtying memory in
case no packets are already waiting in queue.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fifo.c

@@ -64,11 +64,13 @@ static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 static int fifo_init(struct Qdisc *sch, struct nlattr *opt)
 {
 	struct fifo_sched_data *q = qdisc_priv(sch);
+	bool bypass;
+	bool is_bfifo = sch->ops == &bfifo_qdisc_ops;
 
 	if (opt == NULL) {
 		u32 limit = qdisc_dev(sch)->tx_queue_len ? : 1;
 
-		if (sch->ops == &bfifo_qdisc_ops)
+		if (is_bfifo)
 			limit *= psched_mtu(qdisc_dev(sch));
 
 		q->limit = limit;
@@ -81,6 +83,15 @@ static int fifo_init(struct Qdisc *sch, struct nlattr *opt)
 		q->limit = ctl->limit;
 	}
 
+	if (is_bfifo)
+		bypass = q->limit >= psched_mtu(qdisc_dev(sch));
+	else
+		bypass = q->limit >= 1;
+
+	if (bypass)
+		sch->flags |= TCQ_F_CAN_BYPASS;
+	else
+		sch->flags &= ~TCQ_F_CAN_BYPASS;
 	return 0;
 }
 

net/sched/sch_generic.c

@@ -527,6 +527,8 @@ static int pfifo_fast_init(struct Qdisc *qdisc, struct nlattr *opt)
 	for (prio = 0; prio < PFIFO_FAST_BANDS; prio++)
 		skb_queue_head_init(band2list(priv, prio));
 
+	/* Can by-pass the queue discipline */
+	qdisc->flags |= TCQ_F_CAN_BYPASS;
 	return 0;
 }
 
@@ -691,9 +693,6 @@ static void attach_one_default_qdisc(struct net_device *dev,
 			netdev_info(dev, "activation failed\n");
 			return;
 		}
-
-		/* Can by-pass the queue discipline for default qdisc */
-		qdisc->flags |= TCQ_F_CAN_BYPASS;
 	}
 	dev_queue->qdisc_sleeping = qdisc;
 }

net/sched/sch_mq.c

@@ -61,7 +61,6 @@ static int mq_init(struct Qdisc *sch, struct nlattr *opt)
 						    TC_H_MIN(ntx + 1)));
 		if (qdisc == NULL)
 			goto err;
-		qdisc->flags |= TCQ_F_CAN_BYPASS;
 		priv->qdiscs[ntx] = qdisc;
 	}
 

net/sched/sch_mqprio.c

@@ -130,7 +130,6 @@ static int mqprio_init(struct Qdisc *sch, struct nlattr *opt)
 			err = -ENOMEM;
 			goto err;
 		}
-		qdisc->flags |= TCQ_F_CAN_BYPASS;
 		priv->qdiscs[i] = qdisc;
 	}
 

net/sched/sch_sfq.c

@@ -560,6 +560,10 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt)
 		slot_queue_init(&q->slots[i]);
 		sfq_link(q, i);
 	}
+	if (q->limit >= 1)
+		sch->flags |= TCQ_F_CAN_BYPASS;
+	else
+		sch->flags &= ~TCQ_F_CAN_BYPASS;
 	return 0;
 }
 
@@ -611,6 +615,8 @@ static unsigned long sfq_get(struct Qdisc *sch, u32 classid)
 static unsigned long sfq_bind(struct Qdisc *sch, unsigned long parent,
 			      u32 classid)
 {
+	/* we cannot bypass queue discipline anymore */
+	sch->flags &= ~TCQ_F_CAN_BYPASS;
 	return 0;
 }