net/sched: taprio: don't segment unnecessarily

Improve commit 497cc002 ("taprio: Handle short intervals and large
packets") to only perform segmentation when skb->len exceeds what
taprio_dequeue() expects.

In practice, this will make the biggest difference when a traffic class
gate is always open in the schedule. This is because the max_frm_len
will be U32_MAX, and such large skb->len values as Kurt reported will be
sent just fine unsegmented.

What I don't seem to know how to handle is how to make sure that the
segmented skbs themselves are smaller than the maximum frame size given
by the current queueMaxSDU[tc]. Nonetheless, we still need to drop
those, otherwise the Qdisc will hang.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_taprio.c

@@ -566,9 +566,6 @@ static int taprio_enqueue_one(struct sk_buff *skb, struct Qdisc *sch,
 			return qdisc_drop(skb, sch, to_free);
 	}
 
-	if (taprio_skb_exceeds_queue_max_sdu(sch, skb))
-		return qdisc_drop(skb, sch, to_free);
-
 	qdisc_qstats_backlog_inc(sch, skb);
 	sch->q.qlen++;
 
@@ -593,7 +590,14 @@ static int taprio_enqueue_segmented(struct sk_buff *skb, struct Qdisc *sch,
 		qdisc_skb_cb(segs)->pkt_len = segs->len;
 		slen += segs->len;
 
+		/* FIXME: we should be segmenting to a smaller size
+		 * rather than dropping these
+		 */
+		if (taprio_skb_exceeds_queue_max_sdu(sch, segs))
+			ret = qdisc_drop(segs, sch, to_free);
+		else
 			ret = taprio_enqueue_one(segs, sch, child, to_free);
+
 		if (ret != NET_XMIT_SUCCESS) {
 			if (net_xmit_drop_count(ret))
 				qdisc_qstats_drop(sch);
@@ -625,13 +629,18 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	if (unlikely(!child))
 		return qdisc_drop(skb, sch, to_free);
 
-	/* Large packets might not be transmitted when the transmission duration
-	 * exceeds any configured interval. Therefore, segment the skb into
-	 * smaller chunks. Drivers with full offload are expected to handle
-	 * this in hardware.
+	if (taprio_skb_exceeds_queue_max_sdu(sch, skb)) {
+		/* Large packets might not be transmitted when the transmission
+		 * duration exceeds any configured interval. Therefore, segment
+		 * the skb into smaller chunks. Drivers with full offload are
+		 * expected to handle this in hardware.
 		 */
 		if (skb_is_gso(skb))
-		return taprio_enqueue_segmented(skb, sch, child, to_free);
+			return taprio_enqueue_segmented(skb, sch, child,
+							to_free);
+
+		return qdisc_drop(skb, sch, to_free);
+	}
 
 	return taprio_enqueue_one(skb, sch, child, to_free);
 }