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, 20 insertions, 0 deletions

diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index 2d567265363e..b6cdc33b39c1 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -71,6 +71,7 @@ extern void qdisc_watchdog_cancel(struct qdisc_watchdog *wd);
 
 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,
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index dad558bc06fa..67dc08eaaa45 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -427,6 +427,25 @@ static inline struct sk_buff *qdisc_dequeue_head(struct Qdisc *sch)
         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)
 {