tcp: implement rb-tree based retransmit queue

Using a linear list to store all skbs in write queue has been okay
for quite a while : O(N) is not too bad when N < 500.

Things get messy when N is the order of 100,000 : Modern TCP stacks
want 10Gbit+ of throughput even with 200 ms RTT flows.

40 ns per cache line miss means a full scan can use 4 ms,
blowing away CPU caches.

SACK processing often can use various hints to avoid parsing
whole retransmit queue. But with high packet losses and/or high
reordering, hints no longer work.

Sender has to process thousands of unfriendly SACK, accumulating
a huge socket backlog, burning a cpu and massively dropping packets.

Using an rb-tree for retransmit queue has been avoided for years
because it added complexity and overhead, but now is the time
to be more resistant and say no to quadratic behavior.

1) RTX queue is no longer part of the write queue : already sent skbs
are stored in one rb-tree.

2) Since reaching the head of write queue no longer needs
sk->sk_send_head, we added an union of sk_send_head and tcp_rtx_queue

Tested:

 On receiver :
 netem on ingress : delay 150ms 200us loss 1
 GRO disabled to force stress and SACK storms.

for f in `seq 1 10`
do
 ./netperf -H lpaa6 -l30 -- -K bbr -o THROUGHPUT|tail -1
done | awk '{print $0} {sum += $0} END {printf "%7u\n",sum}'

Before patch :

323.87
351.48
339.59
338.62
306.72
204.07
304.93
291.88
202.47
176.88
   2840

After patch:

1700.83
2207.98
2070.17
1544.26
2114.76
2124.89
1693.14
1080.91
2216.82
1299.94
  18053

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 9 deletions

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 655dd8d7f064..7014cc00c74c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -156,8 +156,13 @@ static bool retransmits_timed_out(struct sock *sk,
                 return false;
 
         start_ts = tcp_sk(sk)->retrans_stamp;
-        if (unlikely(!start_ts))
-                start_ts = tcp_skb_timestamp(tcp_write_queue_head(sk));
+        if (unlikely(!start_ts)) {
+                struct sk_buff *head = tcp_rtx_queue_head(sk);
+
+                if (!head)
+                        return false;
+                start_ts = tcp_skb_timestamp(head);
+        }
 
         if (likely(timeout == 0)) {
                 linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);
@@ -304,11 +309,12 @@ static void tcp_delack_timer(unsigned long data)
 static void tcp_probe_timer(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
+        struct sk_buff *skb = tcp_send_head(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         int max_probes;
         u32 start_ts;
 
-        if (tp->packets_out || !tcp_send_head(sk)) {
+        if (tp->packets_out || !skb) {
                 icsk->icsk_probes_out = 0;
                 return;
         }
@@ -321,9 +327,9 @@ static void tcp_probe_timer(struct sock *sk)
          * corresponding system limit. We also implement similar policy when
          * we use RTO to probe window in tcp_retransmit_timer().
          */
-        start_ts = tcp_skb_timestamp(tcp_send_head(sk));
+        start_ts = tcp_skb_timestamp(skb);
         if (!start_ts)
-                tcp_send_head(sk)->skb_mstamp = tp->tcp_mstamp;
+                skb->skb_mstamp = tp->tcp_mstamp;
         else if (icsk->icsk_user_timeout &&
                  (s32)(tcp_time_stamp(tp) - start_ts) >
                  jiffies_to_msecs(icsk->icsk_user_timeout))
@@ -408,7 +414,7 @@ void tcp_retransmit_timer(struct sock *sk)
         if (!tp->packets_out)
                 goto out;
 
-        WARN_ON(tcp_write_queue_empty(sk));
+        WARN_ON(tcp_rtx_queue_empty(sk));
 
         tp->tlp_high_seq = 0;
 
@@ -441,7 +447,7 @@ void tcp_retransmit_timer(struct sock *sk)
                         goto out;
                 }
                 tcp_enter_loss(sk);
-                tcp_retransmit_skb(sk, tcp_write_queue_head(sk), 1);
+                tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1);
                 __sk_dst_reset(sk);
                 goto out_reset_timer;
         }
@@ -473,7 +479,7 @@ void tcp_retransmit_timer(struct sock *sk)
 
         tcp_enter_loss(sk);
 
-        if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk), 1) > 0) {
+        if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 0) {
                 /* Retransmission failed because of local congestion,
                  * do not backoff.
                  */
@@ -647,7 +653,7 @@ static void tcp_keepalive_timer (unsigned long data)
         elapsed = keepalive_time_when(tp);
 
         /* It is alive without keepalive 8) */
-        if (tp->packets_out || tcp_send_head(sk))
+        if (tp->packets_out || !tcp_write_queue_empty(sk))
                 goto resched;
 
         elapsed = keepalive_time_elapsed(tp);