io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx

[Christoph Hellwig]

On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote:
> On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote:
> > On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote:
> > > On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote:
> > > > As Thomas clarified, workqueue hasn't such issue any more, and only other
> > > > per CPU kthreads can run until the CPU clears the online bit.
> > > > 
> > > > So the question is if IO can be submitted from such kernel context?
> > > 
> > > What other per-CPU kthreads even exist?
> > 
> > I don't know, so expose to wider audiences.
> 
> One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see
> io_sq_offload_start(), and it is a IO submission kthread.

As far as I can tell that code is buggy, as it still needs to migrate
the thread away when the cpu is offlined.  This isn't a per-cpu kthread
in the sene of having one for each CPU.

Jens?