Re: [PATCH 08/15] io_uring: don't sleep when polling for I/O

Sagi Grimberg
2021-05-12T21:56:32Z

On 5/12/21 6:15 AM, Christoph Hellwig wrote:
> There is no point in sleeping for the expected I/O completion timeout
> in the io_uring async polling model as we never poll for a specific
> I/O. Split the boolean spin argument to blk_poll into a set of flags
> to control sleeping and the oneshot behavior separately.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> block/blk-mq.c | 18 ++++++++----------
> drivers/nvme/host/core.c | 2 +-
> fs/block_dev.c | 8 ++++----
> fs/io_uring.c | 14 +++++++-------
> fs/iomap/direct-io.c | 6 +++---
> include/linux/blkdev.h | 6 +++++-
> include/linux/fs.h | 2 +-
> include/linux/iomap.h | 2 +-
> mm/page_io.c | 2 +-
> 9 files changed, 31 insertions(+), 29 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ac0b517c5503..164e39d34bf6 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3873,7 +3873,7 @@ static bool blk_mq_poll_hybrid(struct request_queue *q, blk_qc_t qc)
> }
>
> static int blk_mq_poll_classic(struct request_queue *q, blk_qc_t cookie,
> - bool spin)
> + unsigned int flags)
> {
> struct blk_mq_hw_ctx *hctx = blk_qc_to_hctx(q, cookie);
> long state = current->state;
> @@ -3896,7 +3896,7 @@ static int blk_mq_poll_classic(struct request_queue *q, blk_qc_t cookie,
> if (current->state == TASK_RUNNING)
> return 1;
>
> - if (ret < 0 || !spin)
> + if (ret < 0 || (flags & BLK_POLL_ONESHOT))
> break;
> cpu_relax();
> } while (!need_resched());
> @@ -3909,15 +3909,13 @@ static int blk_mq_poll_classic(struct request_queue *q, blk_qc_t cookie,
> * blk_poll - poll for IO completions
> * @q: the queue
> * @cookie: cookie passed back at IO submission time
> - * @spin: whether to spin for completions
> + * @flags: BLK_POLL_* flags that control the behavior
> *
> * Description:
> * Poll for completions on the passed in queue. Returns number of
> - * completed entries found. If @spin is true, then blk_poll will continue
> - * looping until at least one completion is found, unless the task is
> - * otherwise marked running (or we need to reschedule).
> + * completed entries found.
> */
> -int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> +int blk_poll(struct request_queue *q, blk_qc_t cookie, unsigned int flags)
> {
> if (cookie == BLK_QC_T_NONE ||
> !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> @@ -3926,12 +3924,12 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> if (current->plug)
> blk_flush_plug_list(current->plug, false);
>
> - /* If specified not to spin, we also should not sleep. */
> - if (spin && q->poll_nsec != BLK_MQ_POLL_CLASSIC) {
> + if (!(flags & BLK_POLL_NOSLEEP) &&
> + q->poll_nsec != BLK_MQ_POLL_CLASSIC) {
> if (blk_mq_poll_hybrid(q, cookie))
> return 1;
> }
> - return blk_mq_poll_classic(q, cookie, spin);
> + return blk_mq_poll_classic(q, cookie, flags);

I think that the combination of oneshot and nosleep flags to replace
a boolen spin is a little hard to follow (especially that spin doesn't
mean spinning without sleeping).

Maybe we should break it to:
1. replace spin to flags with ONESHOT passed from io_uring (direct
replacement)
2. add NOSLEEP passed from io_uring as there is no need for it.

Just a suggestion though that would help (me at least) to follow
this more easily.