[systemd-devel] running systemd in a cgroup: future compatibility

[Josh Snyder]

Hi all,

I'm working on a use-case where I want to impose memory limits on the system in
aggregate, with one process (e.g. memcached) opted-out of the limit. With a
typical distribution's setup (Ubuntu, in my case), I would be able to impose a
memory limit on each systemd slice (viz. user, machine, system) separately, but
not in aggregate.

I've found a solution for this by launching systemd in a cgroup of its own. In
the initramfs, I have a script (snippet included below) which mounts the
cgroupfs, puts all of the extant processes into a new cgroup, and then umounts
the cgroupfs.

This works for my needs: all of the processes in the system live in one cgroup,
except memcached in a separate cgroup. systemd seems perfectly happy to boot in
this configuration, correctly sensing that it's operating in a cgroup and
nesting the processes it is responsible for under the existing cgroup. With
memcached running separately, the resulting hierarchy looks like:

/sys/fs/cgroup/
├── memcached
└── root
    ├── init.scope
    ├── system.slice
    ...

And /proc/1/cgroup shows that the systemd (and more importantly, its
descendants) lives in the memory cgroup:

11:net_cls,net_prio:/
10:cpuset:/
9:freezer:/
8:hugetlb:/
7:perf_event:/
6:pids:/root/init.scope
5:devices:/root/init.scope
4:cpu,cpuacct:/
3:blkio:/
2:memory:/root
1:name=systemd:/root/init.scope

I've tested against both legacy and unified cgroup hierarchies. The
functionality to detect the current cgroups and nest processes underneath them
appears to be in manager_setup_cgroup (src/core/cgroup.c:2033). My question for
the list is what motivated adding this awesome feature to systemd in the first
place, and (more importantly to me) is it likely to continue to exist in the
future?

Josh

---
#!/bin/sh

move_tasks()
{
  cd "${MOUNTPOINT}"
  mkdir root
  exec 3< "${CGROUP_FILE}"
  set +e
  while read task; do
      echo $task > "root/${CGROUP_FILE}" 2>/dev/null
  done <&3
  exec 3<&-
  set -e
  cd /
  umount "${MOUNTPOINT}"

}

MOUNTPOINT=/sys/fs/cgroup
CGROUP_FILE=tasks
mount -t cgroup -o none,name=systemd none "${MOUNTPOINT}"
move_tasks

mount -t cgroup -o memory none "${MOUNTPOINT}"
cd "${MOUNTPOINT}"
echo 1 > memory.use_hierarchy
move_tasks

mount -t cgroup2 none "${MOUNTPOINT}"
CGROUP_FILE=cgroup.procs
move_tasks