Re: [PATCH for-next v3 4/7] io_uring: add IORING_SETUP_DEFER_TASKRUN

[Dylan Yudaken <dylany@fb.com>]

On Mon, 2022-08-29 at 14:32 +0800, Hao Xu wrote:
> > > @@ -3289,17 +3409,29 @@ static __cold int
> > > io_uring_create(unsigned 
> > > entries, struct io_uring_params *p,
> > >       if (ctx->flags & IORING_SETUP_SQPOLL) {
> > >           /* IPI related flags don't make sense with SQPOLL */
> > >           if (ctx->flags & (IORING_SETUP_COOP_TASKRUN |
> > > -                  IORING_SETUP_TASKRUN_FLAG))
> > > +                  IORING_SETUP_TASKRUN_FLAG |
> > > +                  IORING_SETUP_DEFER_TASKRUN))
> > 
> > Sounds like we should also fail if SQPOLL is set, especially with
> > the task check on the waiting side.
> 
> sqpoll as a natural single issuer case, shouldn't we support this
> feature for it? And surely, in that case, don't do local task work
> check
> in cqring wait time and be careful in other places like
> io_uring_register

I think there is definitely scope for that - but it's less obvious how
to do it.
i.e. in it's current form DEFER_TASKRUN requires the GETEVENTS to be
submitted on the same task as the initial submission, but with SQPOLL
the submission task is a kernel thread so would have to have some
difference in the API.

As an idea for a later patch set - perhaps the semantics should be to
keep task work local but only run it once submissions have been
processed for a ctx. I suspect that will require some care to ensure
the wakeup flag is set correctly and that it cleans up properly.

Dylan