[PATCH v2 net-next 09/14] net_sched: sch_fq: prefetch one skb ahead in dequeue()

[Eric Dumazet <edumazet@google.com>]

prefetch the skb that we are likely to dequeue at the next dequeue().

Also call fq_dequeue_skb() a bit sooner in fq_dequeue().

This reduces the window between read of q.qlen and
changes of fields in the cache line that could be dirtied
by another cpu trying to queue a packet.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/sched/sch_fq.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 0b0ca1aa9251f959e87dd5dc504fbe0f4cbc75eb..6e5f2f4f241546605f8ba37f96275446c8836eee 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -480,7 +480,10 @@ static void fq_erase_head(struct Qdisc *sch, struct fq_flow *flow,
 			  struct sk_buff *skb)
 {
 	if (skb == flow->head) {
-		flow->head = skb->next;
+		struct sk_buff *next = skb->next;
+
+		prefetch(next);
+		flow->head = next;
 	} else {
 		rb_erase(&skb->rbnode, &flow->t_root);
 		skb->dev = qdisc_dev(sch);
@@ -712,6 +715,7 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 			goto begin;
 		}
 		prefetch(&skb->end);
+		fq_dequeue_skb(sch, f, skb);
 		if ((s64)(now - time_next_packet - q->ce_threshold) > 0) {
 			INET_ECN_set_ce(skb);
 			q->stat_ce_mark++;
@@ -719,7 +723,6 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 		if (--f->qlen == 0)
 			q->inactive_flows++;
 		q->band_pkt_count[fq_skb_cb(skb)->band]--;
-		fq_dequeue_skb(sch, f, skb);
 	} else {
 		head->first = f->next;
 		/* force a pass through old_flows to prevent starvation */
-- 
2.52.0.rc1.455.g30608eb744-goog