Writing "+pids" to cgroup.subtree_control flie yields EINVAL

[Michael Kerrisk (man-pages)]

Hello Tejun,

I was trying to do some simple testing ot the CPU controller
that is merged into 4.15, and ran immediately into some confusion.
In the root cgroup on a freshly booted 4.150-rc1, I try the following:

# pwd
/sys/fs/cgroup/unified
# echo '+cpu' > cgroup.subtree_control 
sh: echo: write error: Invalid argument

What am I missing> I presume I'm missing something obvious, although
nothing jumped out at me as I read the cgroups-v2.txt file.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/