Re: [PATCH v1 0/2] io_uring/io-wq: let workers exit when unused

[Jens Axboe]

On 2/2/26 7:37 AM, Li Chen wrote:
> io_uring uses io-wq to offload regular file I/O. When that happens, the kernel
> creates per-task iou-wrk-<tgid> workers (PF_IO_WORKER) via create_io_thread(),
> so the worker is part of the process thread group and shows up under
> /proc/<pid>/task/.
> 
> io-wq shrinks the pool on idle, but it intentionally keeps the last worker
> around indefinitely as a keepalive to avoid churn. Combined with io_uring's
> per-task context lifetime (tctx stays attached to the task until exit), a
> process may permanently retain an idle iou-wrk thread even after it has closed
> its last io_uring instance and has no active rings.
> 
> The keepalive behavior is a reasonable default(I guess): workloads may have
> bursty I/O patterns, and always tearing down the last worker would add thread
> churn and latency. Creating io-wq workers goes through create_io_thread()
> (copy_process), which is not cheap to do repeatedly.
> 
> However, CRIU currently doesn't cope well with such workers being part of the
> checkpointed thread group. The iou-wrk thread is a kernel-managed worker
> (PF_IO_WORKER) running io_wq_worker() on a kernel stack, rather than a normal
> userspace thread executing application code. In our setup, if the iou-wrk
> thread remains present after quiescing and closing the last io_uring instance,
> criu dump may hang while trying to stop and dump the thread group.
> 
> Besides the resource overhead and surprising userspace-visible threads, this is
> a problem for checkpoint/restore. CRIU needs to freeze and dump all threads in
> the thread group. With a lingering iou-wrk thread, we observed criu dump can
> hang even after the ring has been quiesced and the io_uring fd closed, e.g.:
> 
>   criu dump -t $PID -D images -o dump.log -v4 --shell-job
>   ps -T -p $PID -o pid,tid,comm | grep iou-wrk
> 
> This series is a kernel-side enabler for checkpoint/restore in the current
> reality where userspace needs to quiesce and close io_uring rings before dump.
> It is not trying to make io_uring rings checkpointable, nor does it change what
> CRIU can or cannot restore (e.g. in-flight SQEs/CQEs, SQPOLL, SQE128/CQE32,
> registered resources). Even with userspace gaining limited io_uring support,
> this series only targets the specific "no active io_uring contexts left, but an
> idle iou-wrk keepalive thread remains" case.
> 
> This series adds an explicit exit-on-idle mode to io-wq, and toggles it from
> io_uring task context when the task has no active io_uring contexts
> (xa_empty(&tctx->xa)). The mode is cleared on subsequent io_uring usage, so the
> default behavior for active io_uring users is unchanged.
> 
> Tested on x86_64 with CRIU 4.2.
> With this series applied, after closing the ring iou-wrk exited within ~200ms
> and criu dump completed.

Applied with the mentioned commit message and IO_WQ_BIT_EXIT_ON_IDLE test
placement.

-- 
Jens Axboe