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 <fw@strlen.de>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 35 insertions, 0 deletions

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 75fd1c672c61..6cd491013b50 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1707,6 +1707,7 @@ static int __init pktsched_init(void)
 {
         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);
 
diff --git a/net/sched/sch_fifo.c b/net/sched/sch_fifo.c
index 69188e8358b4..4b0a6cc44c77 100644
--- a/net/sched/sch_fifo.c
+++ b/net/sched/sch_fifo.c
@@ -43,6 +43,26 @@ static int pfifo_enqueue(struct sk_buff *skb, struct Qdisc* sch)
         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);
@@ -108,6 +128,20 @@ struct Qdisc_ops bfifo_qdisc_ops __read_mostly = {
 };
 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)
 {