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Message-Id: <20180704000608.17360-1-jmaxwell37@gmail.com>
Date: Wed, 4 Jul 2018 10:06:08 +1000
From: Jon Maxwell <jmaxwell37@...il.com>
To: davem@...emloft.net
Cc: ncardwell@...gle.com, edumazet@...gle.com, kuznet@....inr.ac.ru,
yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, jmaxwell@...hat.com
Subject: [net-next,v2] tcp: Improve setsockopt() TCP_USER_TIMEOUT accuracy
v2 contains the following suggestions by Neal Cardwell:
1) Fix up units mismatch regarding msec/jiffies.
2) Address possiblility of time_remaining being negative.
3) Add a helper routine tcp_clamp_rto_to_user_timeout() to do the rto
calculation.
4) Move start_ts logic into helper routine tcp_retrans_stamp() to
validate tcp_sk(sk)->retrans_stamp.
Every time the TCP retransmission timer fires. It checks to see if there is a
timeout before scheduling the next retransmit timer. The retransmit interval
between each retransmission increases exponentially. The issue is that in order
for the timeout to occur the retransmit timer needs to fire again. If the user
timeout check happens after the 9th retransmit for example. It needs to wait for
the 10th retransmit timer to fire in order to evaluate whether a timeout has
occurred or not. If the interval is large enough then the timeout will be
inaccurate.
For example with a TCP_USER_TIMEOUT of 10 seconds without patch:
1st retransmit:
22:25:18.973488 IP host1.49310 > host2.search-agent: Flags [.]
Last retransmit:
22:25:26.205499 IP host1.49310 > host2.search-agent: Flags [.]
Timeout:
send: Connection timed out
Sun Jul 1 22:25:34 EDT 2018
We can see that last retransmit took ~7 seconds. Which pushed the total
timeout to ~15 seconds instead of the expected 10 seconds. This gets more
inaccurate the larger the TCP_USER_TIMEOUT value. As the interval increases.
Add tcp_clamp_rto_to_user_timeout() to determine if the user rto has expired.
Or whether the rto interval needs to be recalculated. Use the original interval
if user rto is not set.
Test results with the patch is the expected 10 second timeout:
1st retransmit:
01:37:59.022555 IP host1.49310 > host2.search-agent: Flags [.]
Last retransmit:
01:38:06.486558 IP host1.49310 > host2.search-agent: Flags [.]
Timeout:
send: Connection timed out
Mon Jul 2 01:38:09 EDT 2018
Signed-off-by: Jon Maxwell <jmaxwell37@...il.com>
---
net/ipv4/tcp_timer.c | 48 +++++++++++++++++++++++++++++++++++++++---------
1 file changed, 39 insertions(+), 9 deletions(-)
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 3b3611729928..d129e670d02a 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -22,6 +22,39 @@
#include <linux/gfp.h>
#include <net/tcp.h>
+unsigned int tcp_retransmit_stamp(struct sock *sk)
+{
+ unsigned int start_ts = tcp_sk(sk)->retrans_stamp;
+
+ if (unlikely(!start_ts)) {
+ struct sk_buff *head = tcp_rtx_queue_head(sk);
+
+ if (!head)
+ return false;
+ start_ts = tcp_skb_timestamp(head);
+ }
+ return start_ts;
+}
+
+static __u32 tcp_clamp_rto_to_user_timeout(struct sock *sk)
+{
+ struct inet_connection_sock *icsk = inet_csk(sk);
+ __u32 rto = icsk->icsk_rto;
+ __u32 elapsed, user_timeout;
+ unsigned int start_ts;
+
+ start_ts = tcp_retransmit_stamp(sk);
+ if (!icsk->icsk_user_timeout || !start_ts)
+ return rto;
+ elapsed = tcp_time_stamp(tcp_sk(sk)) - start_ts;
+ user_timeout = jiffies_to_msecs(icsk->icsk_user_timeout);
+ if (elapsed >= user_timeout)
+ rto = 1; /* user timeout has passed; fire ASAP */
+ else
+ rto = min(rto, (__u32)msecs_to_jiffies(user_timeout - elapsed));
+ return rto;
+}
+
/**
* tcp_write_err() - close socket and save error info
* @sk: The socket the error has appeared on.
@@ -166,14 +199,9 @@ static bool retransmits_timed_out(struct sock *sk,
if (!inet_csk(sk)->icsk_retransmits)
return false;
- start_ts = tcp_sk(sk)->retrans_stamp;
- if (unlikely(!start_ts)) {
- struct sk_buff *head = tcp_rtx_queue_head(sk);
-
- if (!head)
- return false;
- start_ts = tcp_skb_timestamp(head);
- }
+ start_ts = tcp_retransmit_stamp(sk);
+ if (!start_ts)
+ return false;
if (likely(timeout == 0)) {
linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);
@@ -407,6 +435,7 @@ void tcp_retransmit_timer(struct sock *sk)
struct tcp_sock *tp = tcp_sk(sk);
struct net *net = sock_net(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
+ __u32 rto;
if (tp->fastopen_rsk) {
WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
@@ -535,7 +564,8 @@ void tcp_retransmit_timer(struct sock *sk)
/* Use normal (exponential) backoff */
icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
}
- inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
+ rto = tcp_clamp_rto_to_user_timeout(sk);
+ inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto, TCP_RTO_MAX);
if (retransmits_timed_out(sk, net->ipv4.sysctl_tcp_retries1 + 1, 0))
__sk_dst_reset(sk);
--
2.13.6
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