How can I create and use Linux cgroups as a non-root user?

Question

How can I create and use cgroups as a non-root user?

For example, can I, entirely as a non-root user:

• create a cgroup with access to one CPU
• create a new process in that cgroup

?

I first asked here but I didn't receive a complete answer. I also asked on stackoverflow, but the question was closed as off topic.

Solution 1

You can't do that as a normal user. But you can setup a cgroup as root, and make it configurable by your user.

If you do not already have the default cgroups controllers mounted by e.g. systemd:

$ sudo mount -t tmpfs cgroup_root /sys/fs/cgroup
$ sudo mkdir /sys/fs/cgroup/cpuset
$ sudo mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset

Create a cgroup:

$ sudo mkdir /sys/fs/cgroup/cpuset/${USER}
$ sudo chown -R ${USER} /sys/fs/cgroup/cpuset/${USER}

You can now modify the configuration of your cgroup as a normal user:

$ echo 0-3 > /sys/fs/cgroup/cpuset/${USER}/cpuset.cpus

Add a process to that group:

$ ./my_task &
$ echo $! > /sys/fs/cgroup/cpuset/${USER}/tasks

Or create a subgroup:

$ mkdir /sys/fs/cgroup/cpuset/${USER}/subgroup
$ echo 0-1 > /sys/fs/cgroup/cpuset/${USER}/subgroup/cpuset.cpus
$ ./my_other_task &
$ echo $! > /sys/fs/cgroup/cpuset/${USER}/subgroup/tasks

Solution 2

If you're using Ubuntu you (the root user) can install cgroup-lite and add what you need to /etc/cgconfig.conf, including which user(s) can change the cgroup's configuration. It runs on boot.

Failing that you (the root user) could add your own script to run during boot.

Comments

[chris]

That link for part 1 should probably point to lwn.net/Articles/531114 instead.

[hbogert]

how would you delete the subgroup? rm -r fails for me

[Dennis B.]

The linked LWN article is about namespaces, lwn.net/Articles/604609 is about cgroups.

[Mike S]

You cannot rmdir until the tasks file is empty. root would need to look at the tasks file in the subgroup, and echo each pid there into the root cgroup's tasks file (/dev/cpuset/tasks on my RHEL 6.7 [kernel 2.6.32-358] box, but it looks like /sys/fs/cgroup/cpuset/tasks might by appropriate here).

[Michael Hampton]

Interesting. What distributions did this fail on?

[Mikko Rantalainen]

I'm currently running Ubuntu 18.04 LTS. I know it used to fail with older versions, too, but I haven't tested if it works in more recent version. It appears that it's missing fixes to bug github.com/systemd/systemd/issues/9512 – note that without sudo you probably don't want --scope but --pty.

[Michael Hampton]

Works for me on 248, no sudo or --pty needed. I wonder why Ubuntu hasn't backported it? Did nobody open a bug in launchpad?

[Mikko Rantalainen]

It seems that the problem was the hack called snap by Canonical. It didn't support cgroupv2 until recently so Ubuntu postponed the switch until snap could work with newer API. This should work with Ubuntu 21.10 or greater according to documentation.

[Mikko Rantalainen]

If mount | grep cgroupv2 outputs something, you're running recent enough system.