net_sched: sch_fq: prefetch one skb ahead in dequeue()

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 <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/sched/sch_fq.c

···
 			  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);
···
 			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++;
···
 		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 */