Mounting of /sys/fs/cgroup is not necessary in Docker backend.

[jieyu]

This allows each dc/os container to see the host cgroups. For the sake of isolation, this is not the ideal way. I don't see a reason why /sys/fs/cgroup needs to be bind mounted into the DC/OS container.

By default, in privileged mode, Docker gives you write access to the cgroup of the container itself.

There might be some tweak needed to make Mesos agent happy. This is definitely doable.

[adamtheturtle]

FWIW I also don't know why this is there and it comes from the port of DC/OS Docker - see https://github.com/dcos/dcos-docker/blob/feed8d98a88262b7023c0ed98f454644360318aa/Makefile#L58.

[adamtheturtle]

This is the original commit which added the mounting - mesosphere-backup/dcos-docker@bb56822. We have no context except the commit message "mount systemd all the things".

[adamtheturtle]

The comment by the code is "# Mount cgroups into agents for Mesos DRF." from @timaa2k . Any thoughts on whether removing this would be an issue @jieyu or how to maybe get around it?

Prompted by @vishnu2kmohan 's comment "Just wanted to let you know that it doesn't work on non-systemd distributions like Gentoo with OpenRC and I need to fake mount the systemd cgroup on my machine, just to get started."

[adamtheturtle]

I removed the mount in #1601 - let's see if this comes back to bite us! At least if it does we will know why we had this mount.

[adamtheturtle]

When we remove the mount, and do no tinkering, we see the following when attempting to deploy Kafka.

Feb 14 15:13:40 dcos-e2e-default-4d773-agent-2 mesos-agent[1873]: E0214 15:13:40.545511  1882 cgroups.cpp:712] Failed to assign container cd4b727a-885d-4674-9f8b-8c3c39aa8d81 pid 2850 to cgroup at '/sys/fs/cgroup/cpuset/docker/6c77bd70585c35ba8c7ccdd785f15278d29fbc2a025616a5359df66f3f24e7cb/mesos/cd4b727a-885d-4674-9f8b-8c3c39aa8d81': No space left on device

@jieyu , @Gilbert88 - any ideas what tinkering we need to do.

[adamtheturtle]

Because of the above, @mhrabovcin and I added "Mount cgroups by default, adding an option to not mount #1611". We would like to be able to offer users an option to not mount /sys/fs/cgroups while allowing them to deploy applications such as Kafka.