cgroups v2 support

[dtrudg]

Is your feature request related to a problem? Please describe.

SingularityCE will not work completely out of the box (OCI mode / cgroups settings) on distributions that are using the cgroups v2 hierarchy.

Fedora is using cgroups v2. Also Debian bullseye (the next release, currently testing). We can reasonably expect the next Ubuntu LTS and the next EL release to do so also.

Describe the solution you'd like

SingularityCE should support the cgroups v2 hierarchy.

[dtrudg]

We use a TOML cgroups config format that is defined here: https://github.com/sylabs/singularity/blob/master/internal/pkg/cgroups/config_linux.go

This is closely related to the opencontainers/runtime-spec/specs-go constructs: https://github.com/opencontainers/runtime-spec/blob/master/specs-go/config.go

At present, the v1 cgroups config is applied via containerd/cgroups routines that directly accept LinuxResources (s below):

singularity/internal/pkg/cgroups/cgroups_linux.go

Line 79 in 1fbbe63

m.cgroup, err = cgroups.New(cgroups.V1, path, s)

For cgroups v2 a v2.LoadManager followed by a v2.NewManager call will setup resource limits. The resources type is now v2.Resources (https://github.com/imilchev/cgroups/blob/dd572fc860afa53ef84ec0a8cd040b3e86f78bb4/v2/manager.go#L59) - we cannot simply pass the specs-go LinuxResources type, even though it has had a 'Unified' attribute added to support representing v2.

We need to investigate a way of moving forward... how the existing toml structure maps into the v2.Resources etc.