net_sched: drop packets after root qdisc lock is released

Qdisc performance suffers when packets are dropped at enqueue()
time because drops (kfree_skb()) are done while qdisc lock is held,
delaying a dequeue() draining the queue.

Nominal throughput can be reduced by 50 % when this happens,
at a time we would like the dequeue() to proceed as fast as possible.

Even FQ is vulnerable to this problem, while one of FQ goals was
to provide some flow isolation.

This patch adds a 'struct sk_buff **to_free' parameter to all
qdisc->enqueue(), and in qdisc_drop() helper.

I measured a performance increase of up to 12 %, but this patch
is a prereq so that future batches in enqueue() can fly.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 7 deletions

diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c
index c12df84d1078..303355c449ab 100644
--- a/net/sched/sch_tbf.c
+++ b/net/sched/sch_tbf.c
@@ -155,7 +155,8 @@ static unsigned int skb_gso_mac_seglen(const struct sk_buff *skb)
 /* GSO packet is too big, segment it so that tbf can transmit
  * each segment in time
  */
-static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch)
+static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch,
+		       struct sk_buff **to_free)
 {
 	struct tbf_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *segs, *nskb;
@@ -166,7 +167,7 @@ static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch)
 	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
 
 	if (IS_ERR_OR_NULL(segs))
-		return qdisc_drop(skb, sch);
+		return qdisc_drop(skb, sch, to_free);
 
 	nb = 0;
 	while (segs) {
@@ -174,7 +175,7 @@ static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch)
 		segs->next = NULL;
 		qdisc_skb_cb(segs)->pkt_len = segs->len;
 		len += segs->len;
-		ret = qdisc_enqueue(segs, q->qdisc);
+		ret = qdisc_enqueue(segs, q->qdisc, to_free);
 		if (ret != NET_XMIT_SUCCESS) {
 			if (net_xmit_drop_count(ret))
 				qdisc_qstats_drop(sch);
@@ -190,17 +191,18 @@ static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch)
 	return nb > 0 ? NET_XMIT_SUCCESS : NET_XMIT_DROP;
 }
 
-static int tbf_enqueue(struct sk_buff *skb, struct Qdisc *sch)
+static int tbf_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+		       struct sk_buff **to_free)
 {
 	struct tbf_sched_data *q = qdisc_priv(sch);
 	int ret;
 
 	if (qdisc_pkt_len(skb) > q->max_size) {
 		if (skb_is_gso(skb) && skb_gso_mac_seglen(skb) <= q->max_size)
-			return tbf_segment(skb, sch);
-		return qdisc_drop(skb, sch);
+			return tbf_segment(skb, sch, to_free);
+		return qdisc_drop(skb, sch, to_free);
 	}
-	ret = qdisc_enqueue(skb, q->qdisc);
+	ret = qdisc_enqueue(skb, q->qdisc, to_free);
 	if (ret != NET_XMIT_SUCCESS) {
 		if (net_xmit_drop_count(ret))
 			qdisc_qstats_drop(sch);