cgroup: systemd: properly expand systemd slice names

[cyphar]

Rather than using '/' to denote hierarchy in slice names, systemd uses
'-' in an odd way. This results in runC incorrectly assuming that
certain kernel features are missing (and using inconsistent paths for
the cgroups not supported by systemd), because the "subsystem path" used
is not the one that systemd has created. Fix all of this by properly
expanding slice names.

Signed-off-by: Aleksa Sarai asarai@suse.com

/cc @LK4D4 @mrunalp @dqminh @crosbymichael @hqhq

[cyphar]

You can notice the issue if you do something like this:

Before this patch:

% docker run --cgroup-parent=test-a-b-c.slice alpine cat /proc/self/cgroup
11:memory:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
10:blkio:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
9:perf_event:/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
8:pids:/
7:cpuset:/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
6:devices:/test.slice
5:net_cls,net_prio:/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
4:freezer:/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
3:cpu,cpuacct:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
2:hugetlb:/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope
1:name=systemd:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope

After this patch:

% docker run --cgroup-parent=test-a-b-c.slice alpine cat /proc/self/cgroup
11:memory:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
10:blkio:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
9:perf_event:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
8:pids:/
7:cpuset:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
6:devices:/test.slice
5:net_cls,net_prio:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
4:freezer:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
3:cpu,cpuacct:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
2:hugetlb:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope
1:name=systemd:/test.slice/test-a.slice/test-a-b.slice/test-a-b-c.slice/docker-9916d763e97c6f6ae45412cb06353b226a5ec1386dfd1506e0bd357a19246a3c.scope

Now, the main reason why this is a big issue is because runC assumes that the cgroup path is the same as with cgroupfs (which it clearly isn't). This results in things like OOM notifications not being set because Docker uses the cgroup path that runC tells it is correct (which then doesn't actually affect the container). Note that the devices slice isn't /wrong/ it's just that systemd is being clever about what cgroup it attaches transient units to.

You'll notice this by observing the warnings in the Docker daemon:

WARN[0004] Your kernel does not support OOM notifications: open /sys/fs/cgroup/memory/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope/memory.oom_control: no such file or directory 
WARN[0004] Your kernel does not support OOM notifications: open /sys/fs/cgroup/memory/test-a-b-c.slice/docker-b8c2d0aebc73e7683ae31f14037ed647d60591fe79d6026fe35ff78d2d991d02.scope/memory.oom_control: no such file or directory 

[mrunalp]

LGTM

[hqhq]

LGTM