tcp: early retransmit: delayed fast retransmit

Implementing the advanced early retransmit (sysctl_tcp_early_retrans==2).
Delays the fast retransmit by an interval of RTT/4. We borrow the
RTO timer to implement the delay. If we receive another ACK or send
a new packet, the timer is cancelled and restored to original RTO
value offset by time elapsed.  When the delayed-ER timer fires,
we enter fast recovery and perform fast retransmit.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 7859b416d46e19a9e9e3b1f7f190dfee083e9667..d9b42c5be088655f3ede5a1bfe396018cb3aac6c 100644 (file)
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -372,7 +372,8 @@ struct tcp_sock {
                repair      : 1,
                unused      : 1;
        u8      repair_queue;
-       u8      do_early_retrans:1;/* Enable RFC5827 early-retransmit  */
+       u8      do_early_retrans:1,/* Enable RFC5827 early-retransmit  */
+               early_retrans_delayed:1; /* Delayed ER timer installed */
 
 /* RTT measurement */
        u32     srtt;           /* smoothed round trip time << 3        */

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 685437a16c9731ae02700f686dfb00e46706c413..5283aa4bfa2303ca41a0095e52c2fee37cec1e33 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -500,6 +500,8 @@ extern void tcp_send_delayed_ack(struct sock *sk);
 
 /* tcp_input.c */
 extern void tcp_cwnd_application_limited(struct sock *sk);
+extern void tcp_resume_early_retransmit(struct sock *sk);
+extern void tcp_rearm_rto(struct sock *sk);
 
 /* tcp_timer.c */
 extern void tcp_init_xmit_timers(struct sock *);
@@ -805,6 +807,7 @@ static inline void tcp_enable_early_retrans(struct tcp_sock *tp)
 {
        tp->do_early_retrans = sysctl_tcp_early_retrans &&
                !sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3;
+       tp->early_retrans_delayed = 0;
 }
 
 static inline void tcp_disable_early_retrans(struct tcp_sock *tp)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e042cabb695e2bc9dfa4c1e3ed6de79f434b7e45..7096790e06bf144189094c314b1ea0c243bc0b44 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2344,6 +2344,27 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
        return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
 }
 
+static bool tcp_pause_early_retransmit(struct sock *sk, int flag)
+{
+       struct tcp_sock *tp = tcp_sk(sk);
+       unsigned long delay;
+
+       /* Delay early retransmit and entering fast recovery for
+        * max(RTT/4, 2msec) unless ack has ECE mark, no RTT samples
+        * available, or RTO is scheduled to fire first.
+        */
+       if (sysctl_tcp_early_retrans < 2 || (flag & FLAG_ECE) || !tp->srtt)
+               return false;
+
+       delay = max_t(unsigned long, (tp->srtt >> 5), msecs_to_jiffies(2));
+       if (!time_after(inet_csk(sk)->icsk_timeout, (jiffies + delay)))
+               return false;
+
+       inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, delay, TCP_RTO_MAX);
+       tp->early_retrans_delayed = 1;
+       return true;
+}
+
 static inline int tcp_skb_timedout(const struct sock *sk,
                                   const struct sk_buff *skb)
 {
@@ -2451,7 +2472,7 @@ static inline int tcp_head_timedout(const struct sock *sk)
  * Main question: may we further continue forward transmission
  * with the same cwnd?
  */
-static int tcp_time_to_recover(struct sock *sk)
+static int tcp_time_to_recover(struct sock *sk, int flag)
 {
        struct tcp_sock *tp = tcp_sk(sk);
        __u32 packets_out;
@@ -2505,7 +2526,7 @@ static int tcp_time_to_recover(struct sock *sk)
        if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
            (tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) &&
            !tcp_may_send_now(sk))
-               return 1;
+               return !tcp_pause_early_retransmit(sk, flag);
 
        return 0;
 }
@@ -3172,7 +3193,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
                if (icsk->icsk_ca_state <= TCP_CA_Disorder)
                        tcp_try_undo_dsack(sk);
 
-               if (!tcp_time_to_recover(sk)) {
+               if (!tcp_time_to_recover(sk, flag)) {
                        tcp_try_to_open(sk, flag);
                        return;
                }
@@ -3271,16 +3292,47 @@ static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 /* Restart timer after forward progress on connection.
  * RFC2988 recommends to restart timer to now+rto.
  */
-static void tcp_rearm_rto(struct sock *sk)
+void tcp_rearm_rto(struct sock *sk)
 {
-       const struct tcp_sock *tp = tcp_sk(sk);
+       struct tcp_sock *tp = tcp_sk(sk);
 
        if (!tp->packets_out) {
                inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
        } else {
-               inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-                                         inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
+               u32 rto = inet_csk(sk)->icsk_rto;
+               /* Offset the time elapsed after installing regular RTO */
+               if (tp->early_retrans_delayed) {
+                       struct sk_buff *skb = tcp_write_queue_head(sk);
+                       const u32 rto_time_stamp = TCP_SKB_CB(skb)->when + rto;
+                       s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
+                       /* delta may not be positive if the socket is locked
+                        * when the delayed ER timer fires and is rescheduled.
+                        */
+                       if (delta > 0)
+                               rto = delta;
+               }
+               inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
+                                         TCP_RTO_MAX);
        }
+       tp->early_retrans_delayed = 0;
+}
+
+/* This function is called when the delayed ER timer fires. TCP enters
+ * fast recovery and performs fast-retransmit.
+ */
+void tcp_resume_early_retransmit(struct sock *sk)
+{
+       struct tcp_sock *tp = tcp_sk(sk);
+
+       tcp_rearm_rto(sk);
+
+       /* Stop if ER is disabled after the delayed ER timer is scheduled */
+       if (!tp->do_early_retrans)
+               return;
+
+       tcp_enter_recovery(sk, false);
+       tcp_update_scoreboard(sk, 1);
+       tcp_xmit_retransmit_queue(sk);
 }
 
 /* If we get here, the whole TSO packet has not been acked. */
@@ -3729,6 +3781,9 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
        if (after(ack, tp->snd_nxt))
                goto invalid_ack;
 
+       if (tp->early_retrans_delayed)
+               tcp_rearm_rto(sk);
+
        if (after(ack, prior_snd_una))
                flag |= FLAG_SND_UNA_ADVANCED;
 

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 834e89fc541bb6529ed214886584d9373be45771..d94733009923b4ba9a4e5fd5def7e0251d6c52a4 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -78,9 +78,8 @@ static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb)
                tp->frto_counter = 3;
 
        tp->packets_out += tcp_skb_pcount(skb);
-       if (!prior_packets)
-               inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-                                         inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
+       if (!prior_packets || tp->early_retrans_delayed)
+               tcp_rearm_rto(sk);
 }
 
 /* SND.NXT, if window was not shrunk.

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 34d4a02c2f16d867cb95827ee5ebc2256d0745b3..e911e6c523eca58101f1bc044428b0b7a119e5ca 100644 (file)
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -319,6 +319,11 @@ void tcp_retransmit_timer(struct sock *sk)
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
 
+       if (tp->early_retrans_delayed) {
+               tcp_resume_early_retransmit(sk);
+               return;
+       }
+
        if (!tp->packets_out)
                goto out;