Re: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c

[Oleksandr Natalenko <oleksandr@natalenko.name>]

Hi.

Just to note that it looks like disabling RACK and re-enabling FACK prevents 
warning from happening:

net.ipv4.tcp_fack = 1
net.ipv4.tcp_recovery = 0

Hope I get semantics of these tunables right.

On pátek 15. září 2017 21:04:36 CEST Oleksandr Natalenko wrote:
> Hello.
> 
> With net.ipv4.tcp_fack set to 0 the warning still appears:
> 
> ===
> » sysctl net.ipv4.tcp_fack
> net.ipv4.tcp_fack = 0
> 
> » LC_TIME=C dmesg -T | grep WARNING
> [Fri Sep 15 20:40:30 2017] WARNING: CPU: 1 PID: 711 at net/ipv4/tcp_input.c:
> 2826 tcp_fastretrans_alert+0x7c8/0x990
> [Fri Sep 15 20:40:30 2017] WARNING: CPU: 0 PID: 711 at net/ipv4/tcp_input.c:
> 2826 tcp_fastretrans_alert+0x7c8/0x990
> [Fri Sep 15 20:48:37 2017] WARNING: CPU: 1 PID: 711 at net/ipv4/tcp_input.c:
> 2826 tcp_fastretrans_alert+0x7c8/0x990
> [Fri Sep 15 20:48:55 2017] WARNING: CPU: 0 PID: 711 at net/ipv4/tcp_input.c:
> 2826 tcp_fastretrans_alert+0x7c8/0x990
> 
> » ps -up 711
> USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> root       711  4.3  0.0      0     0 ?        S    18:12   7:23 [irq/123-
> enp3s0]
> ===
> 
> Any suggestions?
> 
> On pátek 15. září 2017 16:03:00 CEST Neal Cardwell wrote:
> > Thanks for testing that. That is a very useful data point.
> > 
> > I was able to cook up a packetdrill test that could put the connection
> > in CA_Disorder with retransmitted packets out, but not in CA_Open. So
> > we do not yet have a test case to reproduce this.
> > 
> > We do not see this warning on our fleet at Google. One significant
> > difference I see between our environment and yours is that it seems
> > 
> > you run with FACK enabled:
> >   net.ipv4.tcp_fack = 1
> > 
> > Note that FACK was disabled by default (since it was replaced by RACK)
> > between kernel v4.10 and v4.11. And this is exactly the time when this
> > bug started manifesting itself for you and some others, but not our
> > fleet. So my new working hypothesis would be that this warning is due
> > to a behavior that only shows up in kernels >=4.11 when FACK is
> > enabled.
> > 
> > Would you be able to disable FACK ("sysctl net.ipv4.tcp_fack=0" at
> > boot, or net.ipv4.tcp_fack=0 in /etc/sysctl.conf, or equivalent),
> > reboot, and test the kernel for a few days to see if the warning still
> > pops up?
> > 
> > thanks,
> > neal
> > 
> > [ps: apologies for the previous, mis-formatted post...]