Control groups, or cgroups, are a way in Linux to control processes' hardware resource utilization by defining the resources limits, grouping them in a hierarchical structure and assigning processes to them. Cgroups can be used, in particular, to specify the skb priority of all network packets generated by a specific process. This provides a convenient way to prioritize network traffic generated in communication with the WHLE board itself (as opposed to the traffic passing through it when it’s used as a router, a case described in Ssh Prioritization (iptables)).

Cgroups perparation

The cgroups hierarchy can be defined in many ways. Instead of creating the minimal hierarchy specific for the given scenario a more generic directory tree will be used, allowing for convenient assignment of skb priority from the 0 .. 15 range, thus covering all priority levels recognized by the tc command, to all network packets generated by a process with a given PID, in a straightforward fashion like

echo ‹pid› > /sys/fs/cgroup/net_prio/prio-‹skb-priority›/cgroup.procs

for example:

echo 730 > /sys/fs/cgroup/net_prio/prio-4/cgroup.procs

The script is as follows:

mkdir /sys/fs/cgroup/net_prio
mount -t cgroup -o net_prio none /sys/fs/cgroup/net_prio
mkdir /sys/fs/cgroup/net_prio/prio-{0..15}
for p in {0..15}; do
    for if in $(cd /sys/class/net/; ls); do
        echo "${if} ${p}" > /sys/fs/cgroup/net_prio/prio-${p}/net_prio.ifpriomap
    done
done

mkdir /sys/fs/cgroup/net_prio
This command creates the root directory for network priority hierarchy inside the /sys/fs/cgroup which should already be present on the system. The name net_prio is arbitrary. It was chosen to reflect the name of the module used to mount the cgroups filesystem there.
mount -t cgroup -o net_prio none /sys/fs/cgroup/net_prio
This command mounts the virtual filesystem used to communicate to the kernel the PIDs priority assignments. The -t cgroup signifies the cgroups V1. Unfortunately the more modern cgroups V2 cannot be used in this case as the net_prio module is not defined for it yet. Upon mounting the system the following listing should appear:
```
root@whle-ls1046a:~# ls -1 /sys/fs/cgroup/net_prio
cgroup.clone_children
cgroup.procs
cgroup.sane_behavior
net_prio.ifpriomap
net_prio.prioidx
notify_on_release
release_agent
tasks
```
Of these files only the following are relevant in further discussion:
- net_prio.ifpriomap
  The default priorities per network interface. More details below.
- cgroups.procs
  List of all PIDs whose packets priority isn’t modified in any way.
mkdir /sys/fs/cgroup/net_prio/prio-{0..15}
Create directories prio-0, prio-1, …, prio-15 inside the /sys/fs/cgroup/net_prio. Each of them will be automatically populated with files:
```
root@whle-ls1046a:~# ls -1 /sys/fs/cgroup/net_prio/prio-13
cgroup.clone_children
cgroup.procs
net_prio.ifpriomap
net_prio.prioidx
notify_on_release
tasks
```
Again, only two are of concern here:
- net_prio.ifpriomap
  The mapping of network interfaces to skb priorities, like
```
root@whle-ls1046a:~# cat /sys/fs/cgroup/net_prio/prio-13/net_prio.ifpriomap
lo 0
eth0 0
eth1 4
eth2 4
eth3 8
eth4 0
eth5 0
```
  While the initial discussion of cgroups mentioned assigning skb priority to PIDs, the actual priority assignment subject is the (PID, interface) pair. This file covers the second part.
- cgroups.procs
  List of all PIDs whose packets are assigned the priority according to the map given in net_prio.ifpriomap.
echo "${if} ${p}" > /sys/fs/cgroup/net_prio/prio-${p}/net_prio.ifpriomap
This line, executed for each network interface if, results in a uniform mapping in prio-‹p›/net_prio.ifpriomap like
```
eth0 ‹p›
eth1 ‹p›
eth2 ‹p›
eth3 ‹p›
eth4 ‹p›
eth5 ‹p›
```
for example:
```
root@whle-ls1046a:~# cat /sys/fs/cgroup/net_prio/prio-13/net_prio.ifpriomap
lo 0
eth0 13
eth1 13
eth2 13
eth3 13
eth4 13
eth5 13
```
This allows for abstracting over the interface prioritization granularity which isn’t needed.

Direct Connection (cgroups)

Cgroups perparation