[DCCP] CCID2: Halve cwnd once upon multiple losses in a single RTT

When multiple losses occur in one RTT, the window should be halved
only once [a single "congestion event"].  This is now implemented,
although not perfectly.  Slightly changed the interface for changing
the cwnd: pass hctx instead of dp.  This is required in order to allow
for change_cwnd to be called from _init().

Signed-off-by: Andrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 32 insertions, 18 deletions

diff --git a/net/dccp/ccids/ccid2.c b/net/dccp/ccids/ccid2.c
index 93a30ae8d07a..b88da035865f 100644
--- a/net/dccp/ccids/ccid2.c
+++ b/net/dccp/ccids/ccid2.c
@@ -187,10 +187,8 @@ static void ccid2_change_l_ack_ratio(struct sock *sk, int val)
         dp->dccps_l_ack_ratio = val;
 }
 
-static void ccid2_change_cwnd(struct sock *sk, int val)
+static void ccid2_change_cwnd(struct ccid2_hc_tx_sock *hctx, int val)
 {
-        struct ccid2_hc_tx_sock *hctx = ccid2_hc_tx_sk(sk);
-
         if (val == 0)
                 val = 1;
 
@@ -234,7 +232,7 @@ static void ccid2_hc_tx_rto_expire(unsigned long data)
         hctx->ccid2hctx_ssthresh = hctx->ccid2hctx_cwnd >> 1;
         if (hctx->ccid2hctx_ssthresh < 2)
                 hctx->ccid2hctx_ssthresh = 2;
-        ccid2_change_cwnd(sk, 1);
+        ccid2_change_cwnd(hctx, 1);
 
         /* clear state about stuff we sent */
         hctx->ccid2hctx_seqt    = hctx->ccid2hctx_seqh;
@@ -444,7 +442,7 @@ static inline void ccid2_new_ack(struct sock *sk,
                         /* increase every 2 acks */
                         hctx->ccid2hctx_ssacks++;
                         if (hctx->ccid2hctx_ssacks == 2) {
-                                ccid2_change_cwnd(sk, hctx->ccid2hctx_cwnd + 1);
+                                ccid2_change_cwnd(hctx, hctx->ccid2hctx_cwnd+1);
                                 hctx->ccid2hctx_ssacks = 0;
                                 *maxincr = *maxincr - 1;
                         }
@@ -457,7 +455,7 @@ static inline void ccid2_new_ack(struct sock *sk,
                 hctx->ccid2hctx_acks++;
 
                 if (hctx->ccid2hctx_acks >= hctx->ccid2hctx_cwnd) {
-                        ccid2_change_cwnd(sk, hctx->ccid2hctx_cwnd + 1);
+                        ccid2_change_cwnd(hctx, hctx->ccid2hctx_cwnd + 1);
                         hctx->ccid2hctx_acks = 0;
                 }
         }
@@ -532,6 +530,22 @@ static void ccid2_hc_tx_dec_pipe(struct sock *sk)
                 ccid2_hc_tx_kill_rto_timer(sk);
 }
 
+static void ccid2_congestion_event(struct ccid2_hc_tx_sock *hctx,
+                                   struct ccid2_seq *seqp)
+{
+        if (time_before(seqp->ccid2s_sent, hctx->ccid2hctx_last_cong)) {
+                ccid2_pr_debug("Multiple losses in an RTT---treating as one\n");
+                return;
+        }
+
+        hctx->ccid2hctx_last_cong = jiffies;
+
+        ccid2_change_cwnd(hctx, hctx->ccid2hctx_cwnd >> 1);
+        hctx->ccid2hctx_ssthresh = hctx->ccid2hctx_cwnd;
+        if (hctx->ccid2hctx_ssthresh < 2)
+                hctx->ccid2hctx_ssthresh = 2;
+}
+
 static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 {
         struct dccp_sock *dp = dccp_sk(sk);
@@ -542,7 +556,6 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
         unsigned char veclen;
         int offset = 0;
         int done = 0;
-        int loss = 0;
         unsigned int maxincr = 0;
 
         ccid2_hc_tx_check_sanity(hctx);
@@ -636,7 +649,8 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                                     !seqp->ccid2s_acked) {
                                         if (state ==
                                             DCCP_ACKVEC_STATE_ECN_MARKED) {
-                                                loss = 1;
+                                                ccid2_congestion_event(hctx,
+                                                                       seqp);
                                         } else
                                                 ccid2_new_ack(sk, seqp,
                                                               &maxincr);
@@ -688,7 +702,13 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                 /* check for lost packets */
                 while (1) {
                         if (!seqp->ccid2s_acked) {
-                                loss = 1;
+                                ccid2_pr_debug("Packet lost: %llu\n",
+                                               seqp->ccid2s_seq);
+                                /* XXX need to traverse from tail -> head in
+                                 * order to detect multiple congestion events in
+                                 * one ack vector.
+                                 */
+                                ccid2_congestion_event(hctx, seqp);
                                 ccid2_hc_tx_dec_pipe(sk);
                         }
                         if (seqp == hctx->ccid2hctx_seqt)
@@ -707,14 +727,6 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                 hctx->ccid2hctx_seqt = hctx->ccid2hctx_seqt->ccid2s_next;
         }
 
-        if (loss) {
-                /* XXX do bit shifts guarantee a 0 as the new bit? */
-                ccid2_change_cwnd(sk, hctx->ccid2hctx_cwnd >> 1);
-                hctx->ccid2hctx_ssthresh = hctx->ccid2hctx_cwnd;
-                if (hctx->ccid2hctx_ssthresh < 2)
-                        hctx->ccid2hctx_ssthresh = 2;
-        }
-
         ccid2_hc_tx_check_sanity(hctx);
 }
 
@@ -722,7 +734,7 @@ static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk)
 {
         struct ccid2_hc_tx_sock *hctx = ccid_priv(ccid);
 
-        hctx->ccid2hctx_cwnd      = 1;
+        ccid2_change_cwnd(hctx, 1);
         /* Initialize ssthresh to infinity.  This means that we will exit the
          * initial slow-start after the first packet loss.  This is what we
          * want.
@@ -741,6 +753,7 @@ static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk)
         hctx->ccid2hctx_rttvar   = -1;
         hctx->ccid2hctx_lastrtt  = 0;
         hctx->ccid2hctx_rpdupack = -1;
+        hctx->ccid2hctx_last_cong = jiffies;
 
         hctx->ccid2hctx_rtotimer.function = &ccid2_hc_tx_rto_expire;
         hctx->ccid2hctx_rtotimer.data     = (unsigned long)sk;
diff --git a/net/dccp/ccids/ccid2.h b/net/dccp/ccids/ccid2.h
index 2a02ce04ba85..5b2ef4acb300 100644
--- a/net/dccp/ccids/ccid2.h
+++ b/net/dccp/ccids/ccid2.h
@@ -71,6 +71,7 @@ struct ccid2_hc_tx_sock {
         u64                     ccid2hctx_rpseq;
         int                     ccid2hctx_rpdupack;
         int                     ccid2hctx_sendwait;
+        unsigned long           ccid2hctx_last_cong;
 };
 
 struct ccid2_hc_rx_sock {