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Merge branch 'tcp-min-rtt' 

Yuchung Cheng says:

====================
tcp: do not use RTT from delayed ACKs for min-RTT

This patch set prevents TCP sender from using RTT samples from
(suspected) delayed ACKs as the minimum RTT, to avoid unbounded
over-estimation of the network path delay. This issue is common
when a connection has extended periods of one packet chit-chat
beyond the min RTT filter window. The first patch does that for TCP
general min RTT estimation. The second patch addresses specifically
the BBR congestion control's min RTT filter.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2018-01-19 15:39:31 -05:00
parent 60c2530696 e42866031f
commit 3c1693f012
3 changed files with 25 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/net/tcp.h
View File

@ -953,6 +953,7 @@ struct rate_sample {
u32 prior_in_flight; /* in flight before this ACK */
bool is_app_limited; /* is sample from packet with bubble in pipe? */
bool is_retrans; /* is sample from retransmission? */
bool is_ack_delayed; /* is this (likely) a delayed ACK? */
};
struct tcp_congestion_ops {

3
net/ipv4/tcp_bbr.c
View File

@ -766,7 +766,8 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
filter_expired = after(tcp_jiffies32,
bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
if (rs->rtt_us >= 0 &&
(rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
(rs->rtt_us <= bbr->min_rtt_us ||
(filter_expired && !rs->is_ack_delayed))) {
bbr->min_rtt_us = rs->rtt_us;
bbr->min_rtt_stamp = tcp_jiffies32;
}

24
net/ipv4/tcp_input.c
View File

@ -97,6 +97,7 @@ int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
#define FLAG_SACK_RENEGING 0x2000 /* snd_una advanced to a sacked seq */
#define FLAG_UPDATE_TS_RECENT 0x4000 /* tcp_replace_ts_recent() */
#define FLAG_NO_CHALLENGE_ACK 0x8000 /* do not call tcp_send_challenge_ack() */
#define FLAG_ACK_MAYBE_DELAYED 0x10000 /* Likely a delayed ACK */
#define FLAG_ACKED (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
@ -2857,11 +2858,18 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
*rexmit = REXMIT_LOST;
}
static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us)
static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us, const int flag)
{
u32 wlen = sock_net(sk)->ipv4.sysctl_tcp_min_rtt_wlen * HZ;
struct tcp_sock *tp = tcp_sk(sk);
if ((flag & FLAG_ACK_MAYBE_DELAYED) && rtt_us > tcp_min_rtt(tp)) {
/* If the remote keeps returning delayed ACKs, eventually
* the min filter would pick it up and overestimate the
* prop. delay when it expires. Skip suspected delayed ACKs.
*/
return;
}
minmax_running_min(&tp->rtt_min, wlen, tcp_jiffies32,
rtt_us ? : jiffies_to_usecs(1));
}
@ -2901,7 +2909,7 @@ static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
* always taken together with ACK, SACK, or TS-opts. Any negative
* values will be skipped with the seq_rtt_us < 0 check above.
*/
tcp_update_rtt_min(sk, ca_rtt_us);
tcp_update_rtt_min(sk, ca_rtt_us, flag);
tcp_rtt_estimator(sk, seq_rtt_us);
tcp_set_rto(sk);
@ -3125,6 +3133,17 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) {
seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt);
if (pkts_acked == 1 && last_in_flight < tp->mss_cache &&
last_in_flight && !prior_sacked && fully_acked &&
sack->rate->prior_delivered + 1 == tp->delivered &&
!(flag & (FLAG_CA_ALERT | FLAG_SYN_ACKED))) {
/* Conservatively mark a delayed ACK. It's typically
* from a lone runt packet over the round trip to
* a receiver w/o out-of-order or CE events.
*/
flag |= FLAG_ACK_MAYBE_DELAYED;
}
}
if (sack->first_sackt) {
sack_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->first_sackt);
@ -3614,6 +3633,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
delivered = tp->delivered - delivered; /* freshly ACKed or SACKed */
lost = tp->lost - lost; /* freshly marked lost */
rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
tcp_xmit_recovery(sk, rexmit);