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_choke.c b/net/sched/sch_choke.c
index 789b69ee9e51..3b6d5bd69101 100644
--- a/net/sched/sch_choke.c
+++ b/net/sched/sch_choke.c
@@ -115,7 +115,8 @@ static void choke_zap_tail_holes(struct choke_sched_data *q)
 }
 
 /* Drop packet from queue array by creating a "hole" */
-static void choke_drop_by_idx(struct Qdisc *sch, unsigned int idx)
+static void choke_drop_by_idx(struct Qdisc *sch, unsigned int idx,
+			      struct sk_buff **to_free)
 {
 	struct choke_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *skb = q->tab[idx];
@@ -129,7 +130,7 @@ static void choke_drop_by_idx(struct Qdisc *sch, unsigned int idx)
 
 	qdisc_qstats_backlog_dec(sch, skb);
 	qdisc_tree_reduce_backlog(sch, 1, qdisc_pkt_len(skb));
-	qdisc_drop(skb, sch);
+	qdisc_drop(skb, sch, to_free);
 	--sch->q.qlen;
 }
 
@@ -261,7 +262,8 @@ static bool choke_match_random(const struct choke_sched_data *q,
 	return choke_match_flow(oskb, nskb);
 }
 
-static int choke_enqueue(struct sk_buff *skb, struct Qdisc *sch)
+static int choke_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+			 struct sk_buff **to_free)
 {
 	int ret = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
 	struct choke_sched_data *q = qdisc_priv(sch);
@@ -288,7 +290,7 @@ static int choke_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		/* Draw a packet at random from queue and compare flow */
 		if (choke_match_random(q, skb, &idx)) {
 			q->stats.matched++;
-			choke_drop_by_idx(sch, idx);
+			choke_drop_by_idx(sch, idx, to_free);
 			goto congestion_drop;
 		}
 
@@ -331,16 +333,16 @@ static int choke_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	}
 
 	q->stats.pdrop++;
-	return qdisc_drop(skb, sch);
+	return qdisc_drop(skb, sch, to_free);
 
 congestion_drop:
-	qdisc_drop(skb, sch);
+	qdisc_drop(skb, sch, to_free);
 	return NET_XMIT_CN;
 
 other_drop:
 	if (ret & __NET_XMIT_BYPASS)
 		qdisc_qstats_drop(sch);
-	kfree_skb(skb);
+	__qdisc_drop(skb, to_free);
 	return ret;
 }