Commit 86c6a2c7 authored Jun 30, 2014 by Neal Cardwell Committed by David S. Miller Jul 07, 2014

tcp: switch snt_synack back to measuring transmit time of first SYNACK



Always store in snt_synack the time at which the server received the
first client SYN and attempted to send the first SYNACK.

Recent commit aa27fc50 ("tcp: tcp_v[46]_conn_request: fix snt_synack
initialization") resolved an inconsistency between IPv4 and IPv6 in
the initialization of snt_synack. This commit brings back the idea
from 843f4a55 (tcp: use tcp_v4_send_synack on first SYN-ACK), which
was going for the original behavior of snt_synack from the commit
where it was added in 9ad7c049 ("tcp: RFC2988bis + taking RTT
sample from 3WHS for the passive open side") in v3.1.

In addition to being simpler (and probably a tiny bit faster),
unconditionally storing the time of the first SYNACK attempt has been
useful because it allows calculating a performance metric quantifying
how long it took to establish a passive TCP connection.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Cc: Octavian Purdila <octavian.purdila@intel.com>
Cc: Jerry Chu <hkchu@google.com>
Acked-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 0b88e704

include/net/tcp.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -1093,7 +1093,7 @@ static inline void tcp_openreq_init(struct request_sock *req,
		req->cookie_ts = 0;
		tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
		tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
		tcp_rsk(req)->snt_synack = 0;
		tcp_rsk(req)->snt_synack = tcp_time_stamp;
		req->mss = rx_opt->mss_clamp;
		req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
		ireq->tstamp_ok = rx_opt->tstamp_ok;

net/ipv4/tcp_ipv4.c

+0 −2

Original line number	Diff line number	Diff line
		@@ -838,8 +838,6 @@ static int tcp_v4_send_synack(struct sock sk, struct dst_entry dst,
		ireq->ir_rmt_addr,
		ireq->opt);
		err = net_xmit_eval(err);
		if (!tcp_rsk(req)->snt_synack && !err)
		tcp_rsk(req)->snt_synack = tcp_time_stamp;
		}

		return err;

net/ipv6/tcp_ipv6.c

+0 −2

Original line number	Diff line number	Diff line
		@@ -498,8 +498,6 @@ static int tcp_v6_send_synack(struct sock sk, struct dst_entry dst,
		skb_set_queue_mapping(skb, queue_mapping);
		err = ip6_xmit(sk, skb, fl6, np->opt, np->tclass);
		err = net_xmit_eval(err);
		if (!tcp_rsk(req)->snt_synack && !err)
		tcp_rsk(req)->snt_synack = tcp_time_stamp;
		}

		done:

Admin message