Re: 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

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
> > On 5/20/20 1:41 PM, Thomas Gleixner wrote:
> >> Jens Axboe <axboe@kernel.dk> writes:
> >>> On 5/20/20 8:45 AM, Jens Axboe wrote:
> >>>> It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure
> >>>> they just break affinity if that CPU goes offline.
> >>>
> >>> Just checked, and it works fine for me. If I create an SQPOLL ring with
> >>> SQ_AFF set and bound to CPU 3, if CPU 3 goes offline, then the kthread
> >>> just appears unbound but runs just fine. When CPU 3 comes online again,
> >>> the mask appears correct.
> >> 
> >> When exactly during the unplug operation is it unbound?
> >
> > When the CPU has been fully offlined. I check the affinity mask, it
> > reports 0. But it's still being scheduled, and it's processing work.
> > Here's an example, PID 420 is the thread in question:
> >
> > [root@archlinux cpu3]# taskset -p 420
> > pid 420's current affinity mask: 8
> > [root@archlinux cpu3]# echo 0 > online 
> > [root@archlinux cpu3]# taskset -p 420
> > pid 420's current affinity mask: 0
> > [root@archlinux cpu3]# echo 1 > online 
> > [root@archlinux cpu3]# taskset -p 420
> > pid 420's current affinity mask: 8
> >
> > So as far as I can tell, it's working fine for me with the goals
> > I have for that kthread.
> 
> Works for me is not really useful information and does not answer my
> question:
> 
> >> When exactly during the unplug operation is it unbound?
> 
> The problem Ming and Christoph are trying to solve requires that the
> thread is migrated _before_ the hardware queue is shut down and
> drained. That's why I asked for the exact point where this happens.
> 
> When the CPU is finally offlined, i.e. the CPU cleared the online bit in
> the online mask is definitely too late simply because it still runs on
> that outgoing CPU _after_ the hardware queue is shut down and drained.

IMO, the patch in Christoph's blk-mq-hotplug.2 still works for percpu
kthread.

It is just not optimal in the retrying, but it should be fine. When the
percpu kthread is scheduled on the CPU to be offlined:

- if the kthread doesn't observe the INACTIVE flag, the allocated request
will be drained.

- otherwise, the kthread just retries and retries to allocate & release,
and sooner or later, its time slice is consumed, and migrated out, and the
cpu hotplug handler will get chance to run and move on, then the cpu is
shutdown.

- After the cpu is shutdown, the percpu kthread becomes unbound, and
the allocation from new online cpu will succeed.

Thanks,
Ming