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>

5 files changed, 206 insertions, 8 deletions

diff --git a/net/dccp/Makefile b/net/dccp/Makefile
index b68440bd7fa2..0c1c9af2bf7e 100644
--- a/net/dccp/Makefile
+++ b/net/dccp/Makefile
@@ -1,7 +1,7 @@
 obj-$(CONFIG_IP_DCCP) += dccp.o dccp_ipv4.o
 
 dccp-y := ccid.o feat.o input.o minisocks.o options.o \
-          output.o proto.o timer.o ackvec.o
+          qpolicy.o output.o proto.o timer.o ackvec.o
 
 dccp_ipv4-y := ipv4.o
 
diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index 74c90cd27677..ce2dd6f6f34d 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -234,6 +234,18 @@ extern void dccp_reqsk_send_ack(struct sock *sk, struct sk_buff *skb,
 extern void dccp_send_sync(struct sock *sk, const u64 seq,
                            const enum dccp_pkt_type pkt_type);
 
+/*
+ * TX Packet Dequeueing Interface
+ */
+extern void             dccp_qpolicy_push(struct sock *sk, struct sk_buff *skb);
+extern bool             dccp_qpolicy_full(struct sock *sk);
+extern void             dccp_qpolicy_drop(struct sock *sk, struct sk_buff *skb);
+extern struct sk_buff   *dccp_qpolicy_top(struct sock *sk);
+extern struct sk_buff   *dccp_qpolicy_pop(struct sock *sk);
+
+/*
+ * TX Packet Output and TX Timers
+ */
 extern void dccp_write_xmit(struct sock *sk);
 extern void dccp_write_space(struct sock *sk);
 extern void dccp_flush_write_queue(struct sock *sk, long *time_budget);
diff --git a/net/dccp/output.c b/net/dccp/output.c
index b1eaf7bcfb11..2532797a8009 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -241,7 +241,7 @@ static void dccp_xmit_packet(struct sock *sk)
 {
         int err, len;
         struct dccp_sock *dp = dccp_sk(sk);
-        struct sk_buff *skb = skb_dequeue(&sk->sk_write_queue);
+        struct sk_buff *skb = dccp_qpolicy_pop(sk);
 
         if (unlikely(skb == NULL))
                 return;
@@ -344,7 +344,7 @@ void dccp_write_xmit(struct sock *sk)
         struct dccp_sock *dp = dccp_sk(sk);
         struct sk_buff *skb;
 
-        while ((skb = skb_peek(&sk->sk_write_queue))) {
+        while ((skb = dccp_qpolicy_top(sk))) {
                 int rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);
 
                 switch (ccid_packet_dequeue_eval(rc)) {
@@ -358,8 +358,7 @@ void dccp_write_xmit(struct sock *sk)
                         dccp_xmit_packet(sk);
                         break;
                 case CCID_PACKET_ERR:
-                        skb_dequeue(&sk->sk_write_queue);
-                        kfree_skb(skb);
+                        dccp_qpolicy_drop(sk, skb);
                         dccp_pr_debug("packet discarded due to err=%d\n", rc);
                 }
         }
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 8c125ffab1c5..b56efdd2a421 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -189,6 +189,7 @@ int dccp_init_sock(struct sock *sk, const __u8 ctl_sock_initialized)
         dp->dccps_rate_last     = jiffies;
         dp->dccps_role          = DCCP_ROLE_UNDEFINED;
         dp->dccps_service       = DCCP_SERVICE_CODE_IS_ABSENT;
+        dp->dccps_tx_qlen       = sysctl_dccp_tx_qlen;
 
         dccp_init_xmit_timers(sk);
 
@@ -541,6 +542,20 @@ static int do_dccp_setsockopt(struct sock *sk, int level, int optname,
         case DCCP_SOCKOPT_RECV_CSCOV:
                 err = dccp_setsockopt_cscov(sk, val, true);
                 break;
+        case DCCP_SOCKOPT_QPOLICY_ID:
+                if (sk->sk_state != DCCP_CLOSED)
+                        err = -EISCONN;
+                else if (val < 0 || val >= DCCPQ_POLICY_MAX)
+                        err = -EINVAL;
+                else
+                        dp->dccps_qpolicy = val;
+                break;
+        case DCCP_SOCKOPT_QPOLICY_TXQLEN:
+                if (val < 0)
+                        err = -EINVAL;
+                else
+                        dp->dccps_tx_qlen = val;
+                break;
         default:
                 err = -ENOPROTOOPT;
                 break;
@@ -648,6 +663,12 @@ static int do_dccp_getsockopt(struct sock *sk, int level, int optname,
         case DCCP_SOCKOPT_RECV_CSCOV:
                 val = dp->dccps_pcrlen;
                 break;
+        case DCCP_SOCKOPT_QPOLICY_ID:
+                val = dp->dccps_qpolicy;
+                break;
+        case DCCP_SOCKOPT_QPOLICY_TXQLEN:
+                val = dp->dccps_tx_qlen;
+                break;
         case 128 ... 191:
                 return ccid_hc_rx_getsockopt(dp->dccps_hc_rx_ccid, sk, optname,
                                              len, (u32 __user *)optval, optlen);
@@ -690,6 +711,43 @@ int compat_dccp_getsockopt(struct sock *sk, int level, int optname,
 EXPORT_SYMBOL_GPL(compat_dccp_getsockopt);
 #endif
 
+static int dccp_msghdr_parse(struct msghdr *msg, struct sk_buff *skb)
+{
+        struct cmsghdr *cmsg = CMSG_FIRSTHDR(msg);
+
+        /*
+         * Assign an (opaque) qpolicy priority value to skb->priority.
+         *
+         * We are overloading this skb field for use with the qpolicy subystem.
+         * The skb->priority is normally used for the SO_PRIORITY option, which
+         * is initialised from sk_priority. Since the assignment of sk_priority
+         * to skb->priority happens later (on layer 3), we overload this field
+         * for use with queueing priorities as long as the skb is on layer 4.
+         * The default priority value (if nothing is set) is 0.
+         */
+        skb->priority = 0;
+
+        for (; cmsg != NULL; cmsg = CMSG_NXTHDR(msg, cmsg)) {
+
+                if (!CMSG_OK(msg, cmsg))
+                        return -EINVAL;
+
+                if (cmsg->cmsg_level != SOL_DCCP)
+                        continue;
+
+                switch (cmsg->cmsg_type) {
+                case DCCP_SCM_PRIORITY:
+                        if (cmsg->cmsg_len != CMSG_LEN(sizeof(__u32)))
+                                return -EINVAL;
+                        skb->priority = *(__u32 *)CMSG_DATA(cmsg);
+                        break;
+                default:
+                        return -EINVAL;
+                }
+        }
+        return 0;
+}
+
 int dccp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
                  size_t len)
 {
@@ -705,8 +763,7 @@ int dccp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 
         lock_sock(sk);
 
-        if (sysctl_dccp_tx_qlen &&
-            (sk->sk_write_queue.qlen >= sysctl_dccp_tx_qlen)) {
+        if (dccp_qpolicy_full(sk)) {
                 rc = -EAGAIN;
                 goto out_release;
         }
@@ -734,7 +791,11 @@ int dccp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
         if (rc != 0)
                 goto out_discard;
 
-        skb_queue_tail(&sk->sk_write_queue, skb);
+        rc = dccp_msghdr_parse(msg, skb);
+        if (rc != 0)
+                goto out_discard;
+
+        dccp_qpolicy_push(sk, skb);
         dccp_write_xmit(sk);
 out_release:
         release_sock(sk);
diff --git a/net/dccp/qpolicy.c b/net/dccp/qpolicy.c
new file mode 100644
index 000000000000..414696b0d830
--- /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);
+
+        /* Clear any skb fields that we used internally */
+        skb->priority = 0;
+
+        if (skb)
+                skb_unlink(skb, &sk->sk_write_queue);
+        return skb;
+}