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, 4 insertions, 3 deletions

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 6eb0667..e5458b9 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -368,18 +368,19 @@ static void flow_queue_add(struct fq_flow *flow, struct sk_buff *skb)
 	}
 }
 
-static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
+static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+		      struct sk_buff **to_free)
 {
 	struct fq_sched_data *q = qdisc_priv(sch);
 	struct fq_flow *f;
 
 	if (unlikely(sch->q.qlen >= sch->limit))
-		return qdisc_drop(skb, sch);
+		return qdisc_drop(skb, sch, to_free);
 
 	f = fq_classify(skb, q);
 	if (unlikely(f->qlen >= q->flow_plimit && f != &q->internal)) {
 		q->stat_flows_plimit++;
-		return qdisc_drop(skb, sch);
+		return qdisc_drop(skb, sch, to_free);
 	}
 
 	f->qlen++;