Unexpected EINVAL when enabling cpuset in subtree_control when io_uring threads are running

[Daniel Dao <dqminh@cloudflare.com>]

Hi all,

We encountered EINVAL when enabling cpuset in cgroupv2 when io_uring
worker threads are running. Here are the steps to reproduce the failure
on kernel 6.1.14:

1. Remove cpuset from subtree_control

  > for d in $(find /sys/fs/cgroup/ -maxdepth 1 -type d); do echo
'-cpuset' | sudo tee -a $d/cgroup.subtree_control; done
  > cat /sys/fs/cgroup/cgroup.subtree_control
  cpu io memory pids

2. Run any applications that utilize the uring worker thread pool. I used
   https://github.com/cloudflare/cloudflare-blog/tree/master/2022-02-io_uring-worker-pool

  > cargo run -- -a -w 2 -t 2

3. Enabling cpuset will return EINVAL

  > echo '+cpuset' | sudo tee -a /sys/fs/cgroup/cgroup.subtree_control
  +cpuset
  tee: /sys/fs/cgroup/cgroup.subtree_control: Invalid argument

We traced this down to task_can_attach that will return EINVAL when it
encounters
kthreads with PF_NO_SETAFFINITY, which io_uring worker threads have.

This seems like an unexpected interaction when enabling cpuset for the subtrees
that contain kthreads. We are currently considering a workaround to try to
enable cpuset in root subtree_control before any io_uring applications
can start,
hence failure to enable cpuset is localized to only cgroup with
io_uring kthreads.
But this is cumbersome.

Any suggestions would be very much appreciated.

Thanks,
Daniel.