Re: [PATCH] io_uring/uring_cmd: push IRQ based completions through task_work

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

On Mon, Mar 20, 2023 at 08:36:15PM +0530, Kanchan Joshi wrote:
> On Sun, Mar 19, 2023 at 8:51 PM Jens Axboe <axboe@kernel.dk> wrote:
> >
> > This is similar to what we do on the non-passthrough read/write side,
> > and helps take advantage of the completion batching we can do when we
> > post CQEs via task_work. On top of that, this avoids a uring_lock
> > grab/drop for every completion.
> >
> > In the normal peak IRQ based testing, this increases performance in
> > my testing from ~75M to ~77M IOPS, or an increase of 2-3%.
> >
> > Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >
> > ---
> >
> > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> > index 2e4c483075d3..b4fba5f0ab0d 100644
> > --- a/io_uring/uring_cmd.c
> > +++ b/io_uring/uring_cmd.c
> > @@ -45,18 +45,21 @@ static inline void io_req_set_cqe32_extra(struct io_kiocb *req,
> >  void io_uring_cmd_done(struct io_uring_cmd *ioucmd, ssize_t ret, ssize_t res2)
> >  {
> >         struct io_kiocb *req = cmd_to_io_kiocb(ioucmd);
> > +       struct io_ring_ctx *ctx = req->ctx;
> >
> >         if (ret < 0)
> >                 req_set_fail(req);
> >
> >         io_req_set_res(req, ret, 0);
> > -       if (req->ctx->flags & IORING_SETUP_CQE32)
> > +       if (ctx->flags & IORING_SETUP_CQE32)
> >                 io_req_set_cqe32_extra(req, res2, 0);
> > -       if (req->ctx->flags & IORING_SETUP_IOPOLL)
> > +       if (ctx->flags & IORING_SETUP_IOPOLL) {
> >                 /* order with io_iopoll_req_issued() checking ->iopoll_complete */
> >                 smp_store_release(&req->iopoll_completed, 1);
> > -       else
> > -               io_req_complete_post(req, 0);
> > +               return;
> > +       }
> > +       req->io_task_work.func = io_req_task_complete;
> > +       io_req_task_work_add(req);
> >  }
> 
> Since io_uring_cmd_done itself would be executing in task-work often
> (always in case of nvme), can this be further optimized by doing
> directly what this new task-work (that is being set up here) would
> have done?
> Something like below on top of your patch -

But we have io_uring_cmd_complete_in_task() already, just wondering why
not let driver decide if explicit running in task-work is taken?

Thanks,
Ming