dccp: Policy-based packet dequeueing infrastructure

This patch adds a generic infrastructure for policy-based dequeueing of
TX packets and provides two policies:
 * a simple FIFO policy (which is the default) and
 * a priority based policy (set via socket options).
Both policies honour the tx_qlen sysctl for the maximum size of the write
queue (can be overridden via socket options).

The priority policy uses skb->priority internally to assign an u32 priority
identifier, using the same ranking as SO_PRIORITY. The skb->priority field
is set to 0 when the packet leaves DCCP. The priority is supplied as ancillary
data using cmsg(3), the patch also provides the requisite parsing routines.

Signed-off-by: Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 126 insertions, 0 deletions

diff --git a/net/dccp/qpolicy.c b/net/dccp/qpolicy.c
new file mode 100644
index 000000000000..4b0fd6b11f6d
--- /dev/null
+++ b/net/dccp/qpolicy.c
@@ -0,0 +1,126 @@
+/*
+ *  net/dccp/qpolicy.c
+ *
+ *  Policy-based packet dequeueing interface for DCCP.
+ *
+ *  Copyright (c) 2008 Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
+ *
+ *  This program is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU General Public License v2
+ *  as published by the Free Software Foundation.
+ */
+#include "dccp.h"
+
+/*
+ *      Simple Dequeueing Policy:
+ *      If tx_qlen is different from 0, enqueue up to tx_qlen elements.
+ */
+static void qpolicy_simple_push(struct sock *sk, struct sk_buff *skb)
+{
+        skb_queue_tail(&sk->sk_write_queue, skb);
+}
+
+static bool qpolicy_simple_full(struct sock *sk)
+{
+        return dccp_sk(sk)->dccps_tx_qlen &&
+               sk->sk_write_queue.qlen >= dccp_sk(sk)->dccps_tx_qlen;
+}
+
+static struct sk_buff *qpolicy_simple_top(struct sock *sk)
+{
+        return skb_peek(&sk->sk_write_queue);
+}
+
+/*
+ *      Priority-based Dequeueing Policy:
+ *      If tx_qlen is different from 0 and the queue has reached its upper bound
+ *      of tx_qlen elements, replace older packets lowest-priority-first.
+ */
+static struct sk_buff *qpolicy_prio_best_skb(struct sock *sk)
+{
+        struct sk_buff *skb, *best = NULL;
+
+        skb_queue_walk(&sk->sk_write_queue, skb)
+                if (best == NULL || skb->priority > best->priority)
+                        best = skb;
+        return best;
+}
+
+static struct sk_buff *qpolicy_prio_worst_skb(struct sock *sk)
+{
+        struct sk_buff *skb, *worst = NULL;
+
+        skb_queue_walk(&sk->sk_write_queue, skb)
+                if (worst == NULL || skb->priority < worst->priority)
+                        worst = skb;
+        return worst;
+}
+
+static bool qpolicy_prio_full(struct sock *sk)
+{
+        if (qpolicy_simple_full(sk))
+                dccp_qpolicy_drop(sk, qpolicy_prio_worst_skb(sk));
+        return false;
+}
+
+/**
+ * struct dccp_qpolicy_operations  -  TX Packet Dequeueing Interface
+ * @push: add a new @skb to the write queue
+ * @full: indicates that no more packets will be admitted
+ * @top:  peeks at whatever the queueing policy defines as its `top'
+ */
+static struct dccp_qpolicy_operations {
+        void            (*push) (struct sock *sk, struct sk_buff *skb);
+        bool            (*full) (struct sock *sk);
+        struct sk_buff* (*top)  (struct sock *sk);
+
+} qpol_table[DCCPQ_POLICY_MAX] = {
+        [DCCPQ_POLICY_SIMPLE] = {
+                .push = qpolicy_simple_push,
+                .full = qpolicy_simple_full,
+                .top  = qpolicy_simple_top,
+        },
+        [DCCPQ_POLICY_PRIO] = {
+                .push = qpolicy_simple_push,
+                .full = qpolicy_prio_full,
+                .top  = qpolicy_prio_best_skb,
+        },
+};
+
+/*
+ *      Externally visible interface
+ */
+void dccp_qpolicy_push(struct sock *sk, struct sk_buff *skb)
+{
+        qpol_table[dccp_sk(sk)->dccps_qpolicy].push(sk, skb);
+}
+
+bool dccp_qpolicy_full(struct sock *sk)
+{
+        return qpol_table[dccp_sk(sk)->dccps_qpolicy].full(sk);
+}
+
+void dccp_qpolicy_drop(struct sock *sk, struct sk_buff *skb)
+{
+        if (skb != NULL) {
+                skb_unlink(skb, &sk->sk_write_queue);
+                kfree_skb(skb);
+        }
+}
+
+struct sk_buff *dccp_qpolicy_top(struct sock *sk)
+{
+        return qpol_table[dccp_sk(sk)->dccps_qpolicy].top(sk);
+}
+
+struct sk_buff *dccp_qpolicy_pop(struct sock *sk)
+{
+        struct sk_buff *skb = dccp_qpolicy_top(sk);
+
+        if (skb != NULL) {
+                /* Clear any skb fields that we used internally */
+                skb->priority = 0;
+                skb_unlink(skb, &sk->sk_write_queue);
+        }
+        return skb;
+}