How we use cgroups in rkt

Iago López Galeiras iago at
Wed Jun 17 11:09:10 UTC 2015

Hi everyone,

We are working on rkt[1] and we want to ask for feedback about the way we use
cgroups to implement isolation in containers. rkt uses systemd-nspawn internally
so I guess the best way to start is explaining how this is handled in

The approach taken by nspawn is mounting the cgroup controllers read-only inside
the container except the part that corresponds to it inside the systemd
controller. It is done this way because allowing the container to modify the
other controllers is considered unsafe[2].

This is how bind mounts look like:

/sys/fs/cgroup/devices RO
/sys/fs/cgroup/memory RO
/sys/fs/cgroup/systemd RO
/sys/fs/cgroup/systemd/machine.slice/machine-a.scope RW

In rkt we have a concept called pod[3] which is a list of apps that run inside a
container, each running in its own chroot. To implement this concept, we start a
systemd-nspawn container with a minimal systemd installation that starts each
app as a service.

We want to be able to apply different restrictions to each app of a pod using
cgroups and the straightforward way we thought was delegating to systemd inside
the container. Initially, this didn't work because, as mentioned earlier, the
cgroup controllers are mounted read-only.

The way we solved this problem was mounting the cgroup hierarchy (with the
directories expected by systemd) outside the container. The difference with
systemd-nspawn’s approach is that we don’t mount everything read-only; instead,
we leave the knobs we need in each of the application’s subcgroups read-write.

For example, if we want to restrict the memory usage of an application we leave
read-write so systemd inside the container can set the appropriate restrictions
but the rest of /sys/fs/cgroup/memory/ is still read-only.

We know this doesn’t provide perfect isolation but we assume non-malicious
applications. We also know we’ll have to rework this when systemd starts using
the unified hierarchy.

What do you think about our approach?




Iago López Galeiras

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