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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH 12/12] io_uring: support true async buffered reads, if file provides it
Date: Mon, 25 May 2020 10:29:25 +0300
Message-ID: <8d429d6b-81ee-0a28-8533-2e1d4faa6b37@gmail.com> (raw)
In-Reply-To: <20200523185755.8494-13-axboe@kernel.dk>

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 <axboe@kernel.dk>
> ---
>  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

  reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-23 18:57 [PATCHSET v2 0/12] Add support for async buffered reads Jens Axboe
2020-05-23 18:57 ` [PATCH 01/12] block: read-ahead submission should imply no-wait as well Jens Axboe
2020-05-23 18:57 ` [PATCH 02/12] mm: allow read-ahead with IOCB_NOWAIT set Jens Axboe
2020-05-23 18:57 ` [PATCH 03/12] mm: abstract out wake_page_match() from wake_page_function() Jens Axboe
2020-05-23 18:57 ` [PATCH 04/12] mm: add support for async page locking Jens Axboe
2020-05-23 18:57 ` [PATCH 05/12] mm: support async buffered reads in generic_file_buffered_read() Jens Axboe
2020-05-24 14:05   ` Trond Myklebust
2020-05-24 16:30     ` Jens Axboe
2020-05-24 16:40       ` Jens Axboe
2020-05-24 17:11         ` Trond Myklebust
2020-05-24 17:12           ` Jens Axboe
2020-05-23 18:57 ` [PATCH 06/12] fs: add FMODE_BUF_RASYNC Jens Axboe
2020-05-23 18:57 ` [PATCH 07/12] ext4: flag as supporting buffered async reads Jens Axboe
2020-05-23 18:57 ` [PATCH 08/12] block: flag block devices as supporting IOCB_WAITQ Jens Axboe
2020-05-23 18:57 ` [PATCH 09/12] xfs: flag files as supporting buffered async reads Jens Axboe
2020-05-23 18:57 ` [PATCH 10/12] btrfs: " Jens Axboe
2020-05-23 18:57 ` [PATCH 11/12] mm: add kiocb_wait_page_queue_init() helper Jens Axboe
2020-05-23 18:57 ` [PATCH 12/12] io_uring: support true async buffered reads, if file provides it Jens Axboe
2020-05-25  7:29   ` Pavel Begunkov [this message]
2020-05-25 19:59     ` Jens Axboe
2020-05-26  7:44       ` Pavel Begunkov
2020-05-26 13:50         ` Jens Axboe
2020-05-26  7:38   ` Pavel Begunkov
2020-05-26 13:47     ` Jens Axboe
2020-05-23 19:20 ` [PATCHSET v2 0/12] Add support for async buffered reads Jens Axboe
2020-05-24  9:46   ` Chris Panayis
2020-05-24 19:24     ` Jens Axboe
2020-05-24 19:21 [PATCHSET v4 " Jens Axboe
2020-05-24 19:22 ` [PATCH 12/12] io_uring: support true async buffered reads, if file provides it Jens Axboe
2020-05-26 19:51 [PATCHSET v5 0/12] Add support for async buffered reads Jens Axboe
2020-05-26 19:51 ` [PATCH 12/12] io_uring: support true async buffered reads, if file provides it Jens Axboe

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