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, 15 insertions, 10 deletions

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index e271967439bf..ccca8ca4c722 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -397,7 +397,8 @@ static void tfifo_enqueue(struct sk_buff *nskb, struct Qdisc *sch)
  * when we statistically choose to corrupt one, we instead segment it, returning
  * the first packet to be corrupted, and re-enqueue the remaining frames
  */
-static struct sk_buff *netem_segment(struct sk_buff *skb, struct Qdisc *sch)
+static struct sk_buff *netem_segment(struct sk_buff *skb, struct Qdisc *sch,
+				     struct sk_buff **to_free)
 {
 	struct sk_buff *segs;
 	netdev_features_t features = netif_skb_features(skb);
@@ -405,7 +406,7 @@ static struct sk_buff *netem_segment(struct sk_buff *skb, struct Qdisc *sch)
 	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
 
 	if (IS_ERR_OR_NULL(segs)) {
-		qdisc_drop(skb, sch);
+		qdisc_drop(skb, sch, to_free);
 		return NULL;
 	}
 	consume_skb(skb);
@@ -418,7 +419,8 @@ static struct sk_buff *netem_segment(struct sk_buff *skb, struct Qdisc *sch)
  * 	NET_XMIT_DROP: queue length didn't change.
  *      NET_XMIT_SUCCESS: one skb was queued.
  */
-static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
+static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+			 struct sk_buff **to_free)
 {
 	struct netem_sched_data *q = qdisc_priv(sch);
 	/* We don't fill cb now as skb_unshare() may invalidate it */
@@ -443,7 +445,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	}
 	if (count == 0) {
 		qdisc_qstats_drop(sch);
-		kfree_skb(skb);
+		__qdisc_drop(skb, to_free);
 		return NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
 	}
 
@@ -463,7 +465,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		u32 dupsave = q->duplicate; /* prevent duplicating a dup... */
 
 		q->duplicate = 0;
-		rootq->enqueue(skb2, rootq);
+		rootq->enqueue(skb2, rootq, to_free);
 		q->duplicate = dupsave;
 	}
 
@@ -475,7 +477,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	 */
 	if (q->corrupt && q->corrupt >= get_crandom(&q->corrupt_cor)) {
 		if (skb_is_gso(skb)) {
-			segs = netem_segment(skb, sch);
+			segs = netem_segment(skb, sch, to_free);
 			if (!segs)
 				return NET_XMIT_DROP;
 		} else {
@@ -488,7 +490,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		if (!(skb = skb_unshare(skb, GFP_ATOMIC)) ||
 		    (skb->ip_summed == CHECKSUM_PARTIAL &&
 		     skb_checksum_help(skb))) {
-			rc = qdisc_drop(skb, sch);
+			rc = qdisc_drop(skb, sch, to_free);
 			goto finish_segs;
 		}
 
@@ -497,7 +499,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	}
 
 	if (unlikely(skb_queue_len(&sch->q) >= sch->limit))
-		return qdisc_drop(skb, sch);
+		return qdisc_drop(skb, sch, to_free);
 
 	qdisc_qstats_backlog_inc(sch, skb);
 
@@ -557,7 +559,7 @@ finish_segs:
 			segs->next = NULL;
 			qdisc_skb_cb(segs)->pkt_len = segs->len;
 			last_len = segs->len;
-			rc = qdisc_enqueue(segs, sch);
+			rc = qdisc_enqueue(segs, sch, to_free);
 			if (rc != NET_XMIT_SUCCESS) {
 				if (net_xmit_drop_count(rc))
 					qdisc_qstats_drop(sch);
@@ -615,8 +617,11 @@ deliver:
 #endif
 
 			if (q->qdisc) {
-				int err = qdisc_enqueue(skb, q->qdisc);
+				struct sk_buff *to_free = NULL;
+				int err;
 
+				err = qdisc_enqueue(skb, q->qdisc, &to_free);
+				kfree_skb_list(to_free);
 				if (unlikely(err != NET_XMIT_SUCCESS)) {
 					if (net_xmit_drop_count(err)) {
 						qdisc_qstats_drop(sch);