On 23/05/2020 21:57, Jens Axboe wrote:
> If the file is flagged with FMODE_BUF_RASYNC, then we don't have to punt
> the buffered read to an io-wq worker. Instead we can rely on page
> unlocking callbacks to support retry based async IO. This is a lot more
> efficient than doing async thread offload.
>
> The retry is done similarly to how we handle poll based retry. From
> the unlock callback, we simply queue the retry to a task_work based
> handler.
>
> Signed-off-by: Jens Axboe <[email protected]>
> ---
> fs/io_uring.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 99 insertions(+)
>
...
> +
> + init_task_work(&rw->task_work, io_async_buf_retry);
> + /* submit ref gets dropped, acquire a new one */
> + refcount_inc(&req->refs);
> + tsk = req->task;
> + ret = task_work_add(tsk, &rw->task_work, true);
> + if (unlikely(ret)) {
> + /* queue just for cancelation */
> + init_task_work(&rw->task_work, io_async_buf_cancel);
> + tsk = io_wq_get_task(req->ctx->io_wq);
IIRC, task will be put somewhere around io_free_req(). Then shouldn't here be
some juggling with reassigning req->task with task_{get,put}()?
> + task_work_add(tsk, &rw->task_work, true);
> + }
> + wake_up_process(tsk);
> + return 1;
> +}
...
> static int io_read(struct io_kiocb *req, bool force_nonblock)
> {
> struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
> @@ -2601,6 +2696,7 @@ static int io_read(struct io_kiocb *req, bool
> force_nonblock)
> if (!ret) {
> ssize_t ret2;
>
> +retry:
> if (req->file->f_op->read_iter)
> ret2 = call_read_iter(req->file, kiocb, &iter);
> else
> @@ -2619,6 +2715,9 @@ static int io_read(struct io_kiocb *req, bool
> force_nonblock)
> if (!(req->flags & REQ_F_NOWAIT) &&
> !file_can_poll(req->file))
> req->flags |= REQ_F_MUST_PUNT;
> + if (io_rw_should_retry(req))
It looks like a state machine with IOCB_WAITQ and gotos. Wouldn't it be cleaner
to call call_read_iter()/loop_rw_iter() here directly instead of "goto retry" ?
BTW, can this async stuff return -EAGAIN ?
> + goto retry;
> + kiocb->ki_flags &= ~IOCB_WAITQ;
> return -EAGAIN;
> }
> }
>
--
Pavel Begunkov
