sched: add head drop fifo queue

This adds an additional queuing strategy, called pfifo_head_drop,
to remove the oldest skb in the case of an overflow within the queue -
the head element - instead of the last skb (tail). To remove the oldest
skb in congested situations is useful for sensor network environments
where newer packets reflect the superior information.

Reviewed-by: Florian Westphal <[e-mail]>
Acked-by: Patrick McHardy <[e-mail]>
Signed-off-by: Hagen Paul Pfeifer <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/pkt_sched.h

···
 
 extern struct Qdisc_ops pfifo_qdisc_ops;
 extern struct Qdisc_ops bfifo_qdisc_ops;
+extern struct Qdisc_ops pfifo_head_drop_qdisc_ops;
 
 extern int fifo_set_limit(struct Qdisc *q, unsigned int limit);
 extern struct Qdisc *fifo_create_dflt(struct Qdisc *sch, struct Qdisc_ops *ops,

include/net/sch_generic.h

···
 	return __qdisc_dequeue_head(sch, &sch->q);
 }
 
+static inline unsigned int __qdisc_queue_drop_head(struct Qdisc *sch,
+					      struct sk_buff_head *list)
+{
+	struct sk_buff *skb = __qdisc_dequeue_head(sch, list);
+
+	if (likely(skb != NULL)) {
+		unsigned int len = qdisc_pkt_len(skb);
+		kfree_skb(skb);
+		return len;
+	}
+
+	return 0;
+}
+
+static inline unsigned int qdisc_queue_drop_head(struct Qdisc *sch)
+{
+	return __qdisc_queue_drop_head(sch, &sch->q);
+}
+
 static inline struct sk_buff *__qdisc_dequeue_tail(struct Qdisc *sch,
 						   struct sk_buff_head *list)
 {

net/sched/sch_api.c

···
 {
 	register_qdisc(&pfifo_qdisc_ops);
 	register_qdisc(&bfifo_qdisc_ops);
+	register_qdisc(&pfifo_head_drop_qdisc_ops);
 	register_qdisc(&mq_qdisc_ops);
 	proc_net_fops_create(&init_net, "psched", 0, &psched_fops);
 

net/sched/sch_fifo.c

···
 	return qdisc_reshape_fail(skb, sch);
 }
 
+static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc* sch)
+{
+	struct sk_buff *skb_head;
+	struct fifo_sched_data *q = qdisc_priv(sch);
+
+	if (likely(skb_queue_len(&sch->q) < q->limit))
+		return qdisc_enqueue_tail(skb, sch);
+
+	/* queue full, remove one skb to fulfill the limit */
+	skb_head = qdisc_dequeue_head(sch);
+	sch->bstats.bytes -= qdisc_pkt_len(skb_head);
+	sch->bstats.packets--;
+	sch->qstats.drops++;
+	kfree_skb(skb_head);
+
+	qdisc_enqueue_tail(skb, sch);
+
+	return NET_XMIT_CN;
+}
+
 static int fifo_init(struct Qdisc *sch, struct nlattr *opt)
 {
 	struct fifo_sched_data *q = qdisc_priv(sch);
···
 	.owner		=	THIS_MODULE,
 };
 EXPORT_SYMBOL(bfifo_qdisc_ops);
+
+struct Qdisc_ops pfifo_head_drop_qdisc_ops __read_mostly = {
+	.id		=	"pfifo_head_drop",
+	.priv_size	=	sizeof(struct fifo_sched_data),
+	.enqueue	=	pfifo_tail_enqueue,
+	.dequeue	=	qdisc_dequeue_head,
+	.peek		=	qdisc_peek_head,
+	.drop		=	qdisc_queue_drop_head,
+	.init		=	fifo_init,
+	.reset		=	qdisc_reset_queue,
+	.change		=	fifo_init,
+	.dump		=	fifo_dump,
+	.owner		=	THIS_MODULE,
+};
 
 /* Pass size change message down to embedded FIFO */
 int fifo_set_limit(struct Qdisc *q, unsigned int limit)