tcp: reduce out_of_order memory use

With increasing receive window sizes, but speed of light not improved
that much, out of order queue can contain a huge number of skbs, waiting
to be moved to receive_queue when missing packets can fill the holes.

Some devices happen to use fat skbs (truesize of 4096 + sizeof(struct
sk_buff)) to store regular (MTU <= 1500) frames. This makes highly
probable sk_rmem_alloc hits sk_rcvbuf limit, which can be 4Mbytes in
many cases.

When limit is hit, tcp stack calls tcp_collapse_ofo_queue(), a true
latency killer and cpu cache blower.

Doing the coalescing attempt each time we add a frame in ofo queue
permits to keep memory use tight and in many cases avoid the
tcp_collapse() thing later.

Tested on various wireless setups (b43, ath9k, ...) known to use big skb
truesize, this patch removed the "packets collapsed in receive queue due
to low socket buffer" I had before.

This also reduced average memory used by tcp sockets.

With help from Neal Cardwell.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: H.K. Jerry Chu <hkchu@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 19 insertions, 1 deletions

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 02d61079f08b..8af0d44e4e22 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -257,6 +257,7 @@ static const struct snmp_mib snmp4_net_list[] = {
         SNMP_MIB_ITEM("TCPReqQFullDoCookies", LINUX_MIB_TCPREQQFULLDOCOOKIES),
         SNMP_MIB_ITEM("TCPReqQFullDrop", LINUX_MIB_TCPREQQFULLDROP),
         SNMP_MIB_ITEM("TCPRetransFail", LINUX_MIB_TCPRETRANSFAIL),
+        SNMP_MIB_ITEM("TCPRcvCoalesce", LINUX_MIB_TCPRCVCOALESCE),
         SNMP_MIB_SENTINEL
 };
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fa7de12c4a52..e886e2f7fa8d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4484,7 +4484,24 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
         end_seq = TCP_SKB_CB(skb)->end_seq;
 
         if (seq == TCP_SKB_CB(skb1)->end_seq) {
-                __skb_queue_after(&tp->out_of_order_queue, skb1, skb);
+                /* Packets in ofo can stay in queue a long time.
+                 * Better try to coalesce them right now
+                 * to avoid future tcp_collapse_ofo_queue(),
+                 * probably the most expensive function in tcp stack.
+                 */
+                if (skb->len <= skb_tailroom(skb1) && !tcp_hdr(skb)->fin) {
+                        NET_INC_STATS_BH(sock_net(sk),
+                                         LINUX_MIB_TCPRCVCOALESCE);
+                        BUG_ON(skb_copy_bits(skb, 0,
+                                             skb_put(skb1, skb->len),
+                                             skb->len));
+                        TCP_SKB_CB(skb1)->end_seq = end_seq;
+                        TCP_SKB_CB(skb1)->ack_seq = TCP_SKB_CB(skb)->ack_seq;
+                        __kfree_skb(skb);
+                        skb = NULL;
+                } else {
+                        __skb_queue_after(&tp->out_of_order_queue, skb1, skb);
+                }
 
                 if (!tp->rx_opt.num_sacks ||
                     tp->selective_acks[0].end_seq != seq)