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

[Jens Axboe <axboe@kernel.dk>]

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
>>     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.
> 
> 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?

-- 
Jens Axboe