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tcp: usec resolution SYN/ACK RTT

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Currently SYN/ACK RTT is measured in jiffies. For LAN the SYN/ACK
RTT is often measured as 0ms or sometimes 1ms, which would affect
RTT estimation and min RTT samping used by some congestion control.

This patch improves SYN/ACK RTT to be usec resolution if platform
supports it. While the timestamping of SYN/ACK is done in request
sock, the RTT measurement is carefully arranged to avoid storing
another u64 timestamp in tcp_sock.

For regular handshake w/o SYNACK retransmission, the RTT is sampled
right after the child socket is created and right before the request
sock is released (tcp_check_req() in tcp_minisocks.c)

For Fast Open the child socket is already created when SYN/ACK was
sent, the RTT is sampled in tcp_rcv_state_process() after processing
the final ACK an right before the request socket is released.

If the SYN/ACK was retransmistted or SYN-cookie was used, we rely
on TCP timestamps to measure the RTT. The sample is taken at the
same place in tcp_rcv_state_process() after the timestamp values
are validated in tcp_validate_incoming(). Note that we do not store
TS echo value in request_sock for SYN-cookies, because the value
is already stored in tp->rx_opt used by tcp_ack_update_rtt().

One side benefit is that the RTT measurement now happens before
initializing congestion control (of the passive side). Therefore
the congestion control can use the SYN/ACK RTT.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yuchung Cheng 2015-09-18 11:36:14 -07:00 • committed by David S. Miller
parent 21fe8af400
commit 0f1c28ae74
6 changed files with 20 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

29
net/ipv4/tcp_input.c
View file

@ -2953,21 +2953,21 @@ static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag,
}
/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
static void tcp_synack_rtt_meas(struct sock *sk, const u32 synack_stamp)
void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
{
struct tcp_sock *tp = tcp_sk(sk);
long seq_rtt_us = -1L;
long rtt_us = -1L;
if (synack_stamp && !tp->total_retrans)
seq_rtt_us = jiffies_to_usecs(tcp_time_stamp - synack_stamp);
if (req && !req->num_retrans && tcp_rsk(req)->snt_synack.v64) {
struct skb_mstamp now;
/* If the ACK acks both the SYNACK and the (Fast Open'd) data packets
* sent in SYN_RECV, SYNACK RTT is the smooth RTT computed in tcp_ack()
*/
if (!tp->srtt_us)
tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt_us, -1L);
skb_mstamp_get(&now);
rtt_us = skb_mstamp_us_delta(&now, &tcp_rsk(req)->snt_synack);
}
tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L);
}
static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
@ -5706,7 +5706,6 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
struct request_sock *req;
int queued = 0;
bool acceptable;
u32 synack_stamp;
tp->rx_opt.saw_tstamp = 0;
@ -5785,15 +5784,16 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
if (!acceptable)
return 1;
if (!tp->srtt_us)
tcp_synack_rtt_meas(sk, req);
/* Once we leave TCP_SYN_RECV, we no longer need req
* so release it.
*/
if (req) {
synack_stamp = tcp_rsk(req)->snt_synack;
tp->total_retrans = req->num_retrans;
reqsk_fastopen_remove(sk, req, false);
} else {
synack_stamp = tp->lsndtime;
/* Make sure socket is routed, for correct metrics. */
icsk->icsk_af_ops->rebuild_header(sk);
tcp_init_congestion_control(sk);
@ -5816,7 +5816,6 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
tcp_synack_rtt_meas(sk, synack_stamp);
if (tp->rx_opt.tstamp_ok)
tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
@ -6027,7 +6026,7 @@ static 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 = tcp_time_stamp;
skb_mstamp_get(&tcp_rsk(req)->snt_synack);
tcp_rsk(req)->last_oow_ack_time = 0;
req->mss = rx_opt->mss_clamp;
req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;