netdev - [PATCH net-next 0/3] tcp: no longer keep around headers in input path

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Date:	Sun, 14 Sep 2014 19:56:17 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 0/3] tcp: no longer keep around headers in input path

Looking at tcp_try_coalesce() I was wondering why I did :

if (tcp_hdr(from)->fin)
     return false;

The answer would be to allow the aggregation, if we simply OR the FIN and PSH
flags eventually present in @from to @to packet.

Then, looking at tcp_recvmsg(), I realized we access tcp_hdr(skb)->syn
(and maybe tcp_hdr(skb)->fin) for every packet we process from socket
receive queue.

We have to understand TCP flags are cold in cpu caches most of the time
(assuming RPS/RFS is used, or that application calls recvmsg() a long
time after incoming packet was processed), and bringing a whole
cache line only to access one bit is not very nice.

It would make sense to use in TCP input path TCP_SKB_CB(skb)->tcp_flags
as we do in output path.

This saves one cache line miss, and TCP tcp_collapse() could avoid dealing
with the headers.

Eric Dumazet (3):
  tcp: use TCP_SKB_CB(skb)->tcp_flags in input path
  tcp: allow segment with FIN in tcp_try_coalesce()
  tcp: do not copy headers in tcp_collapse()

 net/ipv4/tcp.c       | 18 ++++++++++--------
 net/ipv4/tcp_input.c | 31 ++++++++-----------------------
 net/ipv4/tcp_ipv4.c  |  1 +
 net/ipv6/tcp_ipv6.c  |  1 +
 4 files changed, 20 insertions(+), 31 deletions(-)

-- 
2.1.0.rc2.206.gedb03e5

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