I spent a while trying to debug a runc problem where it would always get an EACCES error writing the cpu.cfs_period_us file in a cpu cgroup.

The problem turned out to be that I had not enabled CONFIG_CFS_BANDWIDTH in my kernel build. Presumably, when runc tries to write the file, it passes O_CREAT and cgroupfs doesn’t let it create a new file, which leads to the somewhat surprising error.

So, if you get this error, just turn on CONFIG_CFS_BANDWIDTH :)