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>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fa7de12..e886e2f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4484,7 +4484,24 @@
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)