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tcp: adjust tail loss probe timeout 

This patch adjusts the timeout formula to schedule the TCP loss probe
(TLP). The previous formula uses 2*SRTT or 1.5*RTT + DelayACKMax if
only one packet is in flight. It keeps a lower bound of 10 msec which
is too large for short RTT connections (e.g. within a data-center).
The new formula = 2*RTT + (inflight == 1 ? 200ms : 2ticks) which
performs better for short and fast connections.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yuchung Cheng 2017-07-19 15:41:26 -07:00 committed by David S. Miller
parent c4b2bf6b4a
commit bb4d991a28
3 changed files with 12 additions and 10 deletions
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3
include/net/tcp.h
View File

@ -139,6 +139,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
#endif
#define TCP_RTO_MAX ((unsigned)(120*HZ))
#define TCP_RTO_MIN ((unsigned)(HZ/5))
#define TCP_TIMEOUT_MIN (2U) /* Min timeout for TCP timers in jiffies */
#define TCP_TIMEOUT_INIT ((unsigned)(1*HZ)) /* RFC6298 2.1 initial RTO value */
#define TCP_TIMEOUT_FALLBACK ((unsigned)(3*HZ)) /* RFC 1122 initial RTO value, now
* used as a fallback RTO for the
@ -150,8 +151,6 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
#define TCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ/2U)) /* Maximal interval between probes
* for local resources.
*/
#define TCP_REO_TIMEOUT_MIN (2000) /* Min RACK reordering timeout in usec */
#define TCP_KEEPALIVE_TIME (120*60*HZ) /* two hours */
#define TCP_KEEPALIVE_PROBES 9 /* Max of 9 keepalive probes */
#define TCP_KEEPALIVE_INTVL (75*HZ)

15
net/ipv4/tcp_output.c
View File

@ -2377,7 +2377,6 @@ bool tcp_schedule_loss_probe(struct sock *sk)
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout, tlp_time_stamp, rto_time_stamp;
u32 rtt = usecs_to_jiffies(tp->srtt_us >> 3);
/* No consecutive loss probes. */
if (WARN_ON(icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)) {
@ -2406,15 +2405,19 @@ bool tcp_schedule_loss_probe(struct sock *sk)
tcp_send_head(sk))
return false;
/* Probe timeout is at least 1.5*rtt + TCP_DELACK_MAX to account
/* Probe timeout is 2*rtt. Add minimum RTO to account
* for delayed ack when there's one outstanding packet. If no RTT
* sample is available then probe after TCP_TIMEOUT_INIT.
*/
timeout = rtt << 1 ? : TCP_TIMEOUT_INIT;
if (tp->srtt_us) {
timeout = usecs_to_jiffies(tp->srtt_us >> 2);
if (tp->packets_out == 1)
timeout = max_t(u32, timeout,
(rtt + (rtt >> 1) + TCP_DELACK_MAX));
timeout = max_t(u32, timeout, msecs_to_jiffies(10));
timeout += TCP_RTO_MIN;
else
timeout += TCP_TIMEOUT_MIN;
} else {
timeout = TCP_TIMEOUT_INIT;
}
/* If RTO is shorter, just schedule TLP in its place. */
tlp_time_stamp = tcp_jiffies32 + timeout;

2
net/ipv4/tcp_recovery.c
View File

@ -113,7 +113,7 @@ void tcp_rack_mark_lost(struct sock *sk)
tp->rack.advanced = 0;
tcp_rack_detect_loss(sk, &timeout);
if (timeout) {
timeout = usecs_to_jiffies(timeout + TCP_REO_TIMEOUT_MIN);
timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
timeout, inet_csk(sk)->icsk_rto);
}