Re: [PATCH 7/8] io_uring/rw: add write support for IOCB_DIO_DEFER

Darrick J. Wong
2023-07-21T15:51:40Z

On Thu, Jul 20, 2023 at 12:13:09PM -0600, Jens Axboe wrote:
> If the filesystem dio handler understands IOCB_DIO_DEFER, we'll get
> a kiocb->ki_complete() callback with kiocb->dio_complete set. In that
> case, rather than complete the IO directly through task_work, queue
> up an intermediate task_work handler that first processes this
> callback and then immediately completes the request.
>
> For XFS, this avoids a punt through a workqueue, which is a lot less
> efficient and adds latency to lower queue depth (or sync) O_DIRECT
> writes.
>
> Only do this for non-polled IO, as polled IO doesn't need this kind
> of deferral as it always completes within the task itself. This then
> avoids a check for deferral in the polled IO completion handler.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Seems pretty obvious to me, though I'm famous for not being an
experienced io_uring user yet...

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> io_uring/rw.c | 27 ++++++++++++++++++++++++---
> 1 file changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/io_uring/rw.c b/io_uring/rw.c
> index 1bce2208b65c..f4f700383b4e 100644
> --- a/io_uring/rw.c
> +++ b/io_uring/rw.c
> @@ -285,6 +285,14 @@ static inline int io_fixup_rw_res(struct io_kiocb *req, long res)
>
> void io_req_rw_complete(struct io_kiocb *req, struct io_tw_state *ts)
> {
> + struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw);
> +
> + if (rw->kiocb.dio_complete) {
> + long res = rw->kiocb.dio_complete(rw->kiocb.private);
> +
> + io_req_set_res(req, io_fixup_rw_res(req, res), 0);
> + }
> +
> io_req_io_end(req);
>
> if (req->flags & (REQ_F_BUFFER_SELECTED|REQ_F_BUFFER_RING)) {
> @@ -300,9 +308,11 @@ static void io_complete_rw(struct kiocb *kiocb, long res)
> struct io_rw *rw = container_of(kiocb, struct io_rw, kiocb);
> struct io_kiocb *req = cmd_to_io_kiocb(rw);
>
> - if (__io_complete_rw_common(req, res))
> - return;
> - io_req_set_res(req, io_fixup_rw_res(req, res), 0);
> + if (!rw->kiocb.dio_complete) {
> + if (__io_complete_rw_common(req, res))
> + return;
> + io_req_set_res(req, io_fixup_rw_res(req, res), 0);
> + }
> req->io_task_work.func = io_req_rw_complete;
> __io_req_task_work_add(req, IOU_F_TWQ_LAZY_WAKE);
> }
> @@ -916,6 +926,17 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
> }
> kiocb->ki_flags |= IOCB_WRITE;
>
> + /*
> + * For non-polled IO, set IOCB_DIO_DEFER, stating that our handler
> + * groks deferring the completion to task context. This isn't
> + * necessary and useful for polled IO as that can always complete
> + * directly.
> + */
> + if (!(kiocb->ki_flags & IOCB_HIPRI)) {
> + kiocb->ki_flags |= IOCB_DIO_DEFER;
> + kiocb->dio_complete = NULL;
> + }
> +
> if (likely(req->file->f_op->write_iter))
> ret2 = call_write_iter(req->file, kiocb, &s->iter);
> else if (req->file->f_op->write)
> --
> 2.40.1