Subject: [RFC net-next PATCH V2 2/3] qdisc: bulk dequeue support for qdiscs with TCQ_F_ONETXQUEUE

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 04 Sep 2014 14:55:49 +0200

Based on DaveM's recent API work on dev_hard_start_xmit(), that allows
sending/processing an entire skb list.

This patch implements qdisc bulk dequeue, by allowing multiple packets
to be dequeued in dequeue_skb().

The optimization principle for this is two fold, (1) to amortize
locking cost and (2) avoid expensive tailptr update for notifying HW.
 (1) Several packets are dequeued while holding the qdisc root_lock,
amortizing locking cost over several packet.  The dequeued SKB list is
processed under the TXQ lock in dev_hard_start_xmit(), thus also
amortizing the cost of the TXQ lock.
 (2) Further more, dev_hard_start_xmit() will utilize the skb->xmit_more
API to delay HW tailptr update, which also reduces the cost per
packet.

One restriction of the new API is that every SKB must belong to the
same TXQ.  This patch takes the easy way out, by restricting bulk
dequeue to qdisc's with the TCQ_F_ONETXQUEUE flag, that specifies the
qdisc only have attached a single TXQ.

Some detail about the flow; dev_hard_start_xmit() will process the skb
list, and transmit packets individually towards the driver (see
xmit_one()).  In case the driver stops midway in the list, the
remaining skb list is returned by dev_hard_start_xmit().  In
sch_direct_xmit() this returned list is requeued by dev_requeue_skb().

The patch also tries to limit the amount of bytes dequeued, based on
the drivers BQL limits.  It also tries to avoid and stop dequeuing
when seeing a GSO packet (both real GSO and segmented GSO skb lists).

Signed-off-by: Jesper Dangaard Brouer <[email-masked]>

---
V2:
 - Restruct functions, split out functionality
 - Use BQL bytelimit to avoid overshooting driver limits, causing
   too large skb lists to be sitting on the requeue gso_skb.

 net/sched/sch_generic.c |   67 +++++++++++++++++++++++++++++++++++++++++++++--
 1 files changed, 64 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 19696eb..a0c8070 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -56,6 +56,67 @@ static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
 	return 0;
 }
 
+static inline bool qdisc_may_bulk(const struct Qdisc *qdisc,
+				  const struct sk_buff *skb)
+{
+	return (qdisc->flags & TCQ_F_ONETXQUEUE);
+}
+
+static inline struct sk_buff *qdisc_dequeue_validate(struct Qdisc *qdisc)
+{
+	struct sk_buff *skb = qdisc->dequeue(qdisc);
+
+	if (skb != NULL)
+		skb = validate_xmit_skb(skb, qdisc_dev(qdisc));
+
+	return skb;
+}
+
+static inline struct sk_buff *qdisc_bulk_dequeue_skb(struct Qdisc *q,
+						     struct sk_buff *head)
+{
+	struct sk_buff *new, *skb = head;
+	struct netdev_queue *txq = q->dev_queue;
+	int bytelimit = netdev_tx_avail_queue(txq);
+	int limit = 5;
+
+	if (bytelimit <= 0)
+		return head;
+
+	do {
+		if (skb->next || skb_is_gso(skb)) {
+			/* Stop processing if the skb is already a skb
+			 * list (e.g a segmented GSO packet) or a real
+			 * GSO packet */
+			break;
+		}
+		new = qdisc_dequeue_validate(q);
+		if (new) {
+			skb->next = new;
+			skb = new;
+			bytelimit -= skb->len;
+			cnt++;
+			/* One problem here is it is difficult to
+			 * requeue the "new" dequeued skb, e.g. in
+			 * case of GSO, thus a "normal" packet can
+			 * have a GSO packet on its ->next ptr.
+			 *
+			 * Requeue is difficult because if requeuing
+			 * on q->gso_skb, then a second requeue can
+			 * happen from sch_direct_xmit e.g. if driver
+			 * returns NETDEV_TX_BUSY, which would
+			 * overwrite this requeue.
+			 */
+		}
+	} while (new && --limit && (bytelimit > 0));
+	skb = head;
+
+	return skb;
+}
+
+/* Note that dequeue_skb can possibly return a SKB list (via skb->next).
+ * A requeued skb (via q->gso_skb) can also be a SKB list.
+ */
 static inline struct sk_buff *dequeue_skb(struct Qdisc *q)
 {
 	struct sk_buff *skb = q->gso_skb;
@@ -71,9 +132,9 @@ static inline struct sk_buff *dequeue_skb(struct Qdisc *q)
 			skb = NULL;
 	} else {
 		if (!(q->flags & TCQ_F_ONETXQUEUE) || !netif_xmit_frozen_or_stopped(txq)) {
-			skb = q->dequeue(q);
-			if (skb)
-				skb = validate_xmit_skb(skb, qdisc_dev(q));
+			skb = qdisc_dequeue_validate(q);
+			if (skb && qdisc_may_bulk(q, skb))
+				skb = qdisc_bulk_dequeue_skb(q, skb);
 		}
 	}