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

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On 3/8/23 09:26, Jens Axboe wrote:
On 3/8/23 7:20?AM, Waiman Long wrote:
On 3/8/23 06:42, Daniel Dao wrote:
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

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

3. Enabling cpuset will return EINVAL

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

We traced this down to task_can_attach that will return EINVAL when it
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.
Anytime you echo "+cpuset" to cgroup.subtree_control to enable cpuset,
the tasks within the child cgroups will do an implicit move from the
parent cpuset to the child cpusets. However, that move will fail if
any task has the PF_NO_SETAFFINITY flag set due to task_can_attach()
function which checks for this. One possible solution is for the
cpuset to ignore tasks with PF_NO_SETAFFINITY set for implicit move.
IOW, allowing the implicit move without touching it, but not explicit
one using cgroup.procs.
I was pondering this too as I was typing my reply, but at least for
io-wq, this report isn't the first to be puzzled or broken by the fact
that task threads might have PF_NO_SETAFFINITY set. So while it might be
worthwhile to for cpuset to ignore PF_NO_SETAFFINITY as a separate fix,
I think it's better to fix io-wq in general. Not sure we have other
cases where it's even possible to have PF_NO_SETAFFINITY set on
userspace threads?

Changing current cpuset behavior is an alternative solution. It is a problem anytime a task (user or kthread) has PF_NO_SETAFFINITY set but not in the root cgroup. Besides io_uring, I have no idea if there is other use cases out there. It is just a change we may need to do in the future if there are other similar cases. Since you are fixing it on the io-wq side, it is not an urgent issue that needs to be addressed from the cpuset side.


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