[menage]

It's true that a better userspace API for cgroups might have involved something more like ioctl(), rather than a filesystem with text-based control files. But people had been trying to get such APIs into the kernel for years with no success. Which is why we tried to piggyback on something existing.

Cpusets, which had already been accepted into the kernel a year or two previously, provided the basic userspace API for cgroups. It was just expanded to support control files for more resource types. (And multiple resource hierarchies, although that's gone in cgroups v2 I think).