Which kernel were you testing? NixOS 18.09's Linux 4.14 seems to include BBR:

[root@hydra:~]# sysctl net.ipv4.tcp_congestion_control=bbr
net.ipv4.tcp_congestion_control = bbr

[root@hydra:~]# sysctl -a | grep cong
net.ipv4.tcp_allowed_congestion_control = bbr reno
net.ipv4.tcp_available_congestion_control = bbr reno
net.ipv4.tcp_congestion_control = bbr

[root@hydra:~]# ls -l /nix/store/jbkvclfhacpzn0xhpn1w5552zyj0892q-linux-4.14.76/lib/modules/4.14.76/kernel/net/ipv4/tcp_bbr.ko.xz
-r--r--r-- 1 root root 4272 Jan  1  1970 /nix/store/jbkvclfhacpzn0xhpn1w5552zyj0892q-linux-4.14.76/lib/modules/4.14.76/kernel/net/ipv4/tcp_bbr.ko.xz

Hmmm - 4.18 doesn't seem to have it enabled.

% uname -r ~
4.18.12
% sudo sysctl -a | grep cong ~
net.ipv4.tcp_allowed_congestion_control = reno
net.ipv4.tcp_available_congestion_control = reno
net.ipv4.tcp_congestion_control = reno
% sudo sysctl net.ipv4.tcp_congestion_control=bbr ~
sysctl: setting key "net.ipv4.tcp_congestion_control": No such file or directory
net.ipv4.tcp_congestion_control = bbr

Any update? Should I write a test to cover this?

@c00w still needed? or close and solve with going to 4.19?

I'm on 19.03 with the default kernel and I'm getting this:

uname -r                                                                                                                                                                                                                                              
4.19.34

sudo sysctl -a | grep cong
net.ipv4.tcp_allowed_congestion_control = reno
net.ipv4.tcp_available_congestion_control = reno
net.ipv4.tcp_congestion_control = reno

So, this still seems to be an issue.

I am using nixpkgs master with the hardened 4.19 kernel and

  boot.kernel.sysctl = {
    "net.core.default_qdisc"             = "fq";
    "net.ipv4.tcp_congestion_control"    = "bbr";
  };

works fine for me.

net.ipv4.tcp_allowed_congestion_control = reno bbr
net.ipv4.tcp_available_congestion_control = reno bbr
net.ipv4.tcp_congestion_control = bbr

You're right, seems like I just didn't have the module loaded, sorry.

Seems to be working with 5.0.3. I'll drop the PR

colin@akira ~ % uname -r ~
5.0.3
colin@akira ~ % modprobe tcp_bbr ~
colin@akira ~ % sudo sysctl -a | grep cong ~
net.ipv4.tcp_allowed_congestion_control = reno bbr
net.ipv4.tcp_available_congestion_control = reno bbr
net.ipv4.tcp_congestion_control = bbr