Subject: [PATCH 0/3] tcp: improve setsockopt() TCP_USER_TIMEOUT accuracy

From: Jon Maxwell <[email-masked]>
Date: Wed, 18 Jul 2018 10:32:00 +1000

Based on:

https://patchwork.kernel.org/patch/10516195/

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

Jon Maxwell (3):
  tcp: convert icsk_user_timeout from jiffies to msecs
  tcp: Add tcp_retransmit_time() helper routine
  tcp: Add tcp_clamp_rto_to_user_timeout() helper to improve accuracy

 net/ipv4/tcp.c       |  4 ++--
 net/ipv4/tcp_timer.c | 51 ++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 40 insertions(+), 15 deletions(-)

-- 
2.13.6