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Commit c8628155 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

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: default avatarEric 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: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent e86b2919

include/linux/snmp.h

+1 −0
Original line number Diff line number Diff line
@@ -233,6 +233,7 @@ enum 
	LINUX_MIB_TCPREQQFULLDOCOOKIES,		/* TCPReqQFullDoCookies */

	LINUX_MIB_TCPREQQFULLDROP,		/* TCPReqQFullDrop */

	LINUX_MIB_TCPRETRANSFAIL,		/* TCPRetransFail */

	LINUX_MIB_TCPRCVCOALESCE,			/* TCPRcvCoalesce */

	__LINUX_MIB_MAX

};

net/ipv4/proc.c

+1 −0
Original line number Diff line number Diff line
@@ -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

};

net/ipv4/tcp_input.c

+18 −1
Original line number Diff line number Diff line
@@ -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) {

		/* 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)