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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 02 Sep 2014 16:35:48 +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().

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.

Testing if this have the desired effect is the challenging part.
Generating enough packets for a backlog queue to form at the qdisc is
a challenge (because overhead else-where is a limiting factor
e.g. I've measured the pure skb_alloc/free cycle to cost 80ns).

After trying many qdisc setups, I figured out that, the easiest way to
make a backlog form is to fully load the system, all CPUs.  And I can
even demonstrate this with the default MQ disc.

This is a 12 core CPU (without HT) running trafgen on all 12 cores,
via qdisc-path using sendto():
 * trafgen --cpp --dev $DEV --conf udp_example02_const.trafgen --qdisc-path -t0 --cpus 12

Measuring TX pps:
 * Baseline  : 12,815,925 pps
 * This patch: 14,892,001 pps

This is crazy fast. This measurement is actually "too-high" as
10Gbit/s wirespeed is 14,880,952 (11049 pps too fast).

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

 net/sched/sch_generic.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 5b261e9..30814ef 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -56,6 +56,9 @@ static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
 	return 0;
 }
 
+/* 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;
@@ -70,10 +73,28 @@ static inline struct sk_buff *dequeue_skb(struct Qdisc *q)
 		} else
 			skb = NULL;
 	} else {
-		if (!(q->flags & TCQ_F_ONETXQUEUE) || !netif_xmit_frozen_or_stopped(txq)) {
+		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));
+			/* bulk dequeue */
+			if (skb && !skb->next && (q->flags & TCQ_F_ONETXQUEUE)) {
+				struct sk_buff *new, *head = skb;
+				int limit = 7;
+
+				do {
+					new = q->dequeue(q);
+					if (new)
+						new = validate_xmit_skb(
+							new, qdisc_dev(q));
+					if (new) {
+						skb->next = new;
+						skb = new;
+					}
+				} while (new && --limit);
+				skb = head;
+			}
 		}
 	}