Re: [dm-devel] [PATCH v5 03/12] block: add poll method to support bio-based IO polling

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Mar 03 2021 at  6:57am -0500,
Jeffle Xu <jefflexu@linux.alibaba.com> wrote:

> ->poll_fn was introduced in commit ea435e1b9392 ("block: add a poll_fn
> callback to struct request_queue") to support bio-based queues such as
> nvme multipath, but was later removed in commit 529262d56dbe ("block:
> remove ->poll_fn").
> 
> Given commit c62b37d96b6e ("block: move ->make_request_fn to struct
> block_device_operations") restore the possibility of bio-based IO
> polling support by adding an ->poll method to gendisk->fops.
> 
> Make blk_mq_poll() specific to blk-mq, while blk_bio_poll() is
> originally a copy from blk_mq_poll(), and is specific to bio-based
> polling. Currently hybrid polling is not supported by bio-based polling.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  block/blk-core.c       | 58 ++++++++++++++++++++++++++++++++++++++++++
>  block/blk-mq.c         | 22 +---------------
>  include/linux/blk-mq.h |  1 +
>  include/linux/blkdev.h |  1 +
>  4 files changed, 61 insertions(+), 21 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index fc60ff208497..6d7d53030d7c 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1119,6 +1119,64 @@ blk_qc_t submit_bio(struct bio *bio)
>  }
>  EXPORT_SYMBOL(submit_bio);
>  
> +

Minor nit: Extra empty new line here? ^

Otherwise, looks good (I like the end result of blk-mq and bio-based
polling being decoupled like hch suggested).

Reviewed-by: Mike Snitzer <snitzer@redhat.com>

> +static int blk_bio_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> +{
> +	long state;
> +	struct gendisk *disk = queue_to_disk(q);
> +
> +	state = current->state;
> +	do {
> +		int ret;
> +
> +		ret = disk->fops->poll(q, cookie);
> +		if (ret > 0) {
> +			__set_current_state(TASK_RUNNING);
> +			return ret;
> +		}
> +
> +		if (signal_pending_state(state, current))
> +			__set_current_state(TASK_RUNNING);
> +
> +		if (current->state == TASK_RUNNING)
> +			return 1;
> +		if (ret < 0 || !spin)
> +			break;
> +		cpu_relax();
> +	} while (!need_resched());
> +
> +	__set_current_state(TASK_RUNNING);
> +	return 0;
> +}
> +
> +/**
> + * blk_poll - poll for IO completions
> + * @q:  the queue
> + * @cookie: cookie passed back at IO submission time
> + * @spin: whether to spin for completions
> + *
> + * 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).
> + */
> +int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> +{
> +	if (!blk_qc_t_valid(cookie) ||
> +	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> +		return 0;
> +
> +	if (current->plug)
> +		blk_flush_plug_list(current->plug, false);
> +
> +	if (queue_is_mq(q))
> +		return blk_mq_poll(q, cookie, spin);
> +	else
> +		return blk_bio_poll(q, cookie, spin);
> +}
> +EXPORT_SYMBOL_GPL(blk_poll);
> +
>  /**
>   * blk_cloned_rq_check_limits - Helper function to check a cloned request
>   *                              for the new queue limits
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index d4d7c1caa439..214fa30b460a 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3852,30 +3852,11 @@ static bool blk_mq_poll_hybrid(struct request_queue *q,
>  	return blk_mq_poll_hybrid_sleep(q, rq);
>  }
>  
> -/**
> - * blk_poll - poll for IO completions
> - * @q:  the queue
> - * @cookie: cookie passed back at IO submission time
> - * @spin: whether to spin for completions
> - *
> - * 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).
> - */
> -int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> +int blk_mq_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
>  {
>  	struct blk_mq_hw_ctx *hctx;
>  	long state;
>  
> -	if (!blk_qc_t_valid(cookie) ||
> -	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> -		return 0;
> -
> -	if (current->plug)
> -		blk_flush_plug_list(current->plug, false);
> -
>  	hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
>  
>  	/*
> @@ -3917,7 +3898,6 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
>  	__set_current_state(TASK_RUNNING);
>  	return 0;
>  }
> -EXPORT_SYMBOL_GPL(blk_poll);
>  
>  unsigned int blk_mq_rq_cpu(struct request *rq)
>  {
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 2c473c9b8990..6a7b693b9917 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -615,6 +615,7 @@ static inline void blk_rq_bio_prep(struct request *rq, struct bio *bio,
>  }
>  
>  blk_qc_t blk_mq_submit_bio(struct bio *bio);
> +int blk_mq_poll(struct request_queue *q, blk_qc_t cookie, bool spin);
>  void blk_mq_hctx_set_fq_lock_class(struct blk_mq_hw_ctx *hctx,
>  		struct lock_class_key *key);
>  
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index b81a9fe015ab..9dc83c30e7bc 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1866,6 +1866,7 @@ static inline void blk_ksm_unregister(struct request_queue *q) { }
>  
>  struct block_device_operations {
>  	blk_qc_t (*submit_bio) (struct bio *bio);
> +	int (*poll)(struct request_queue *q, blk_qc_t cookie);
>  	int (*open) (struct block_device *, fmode_t);
>  	void (*release) (struct gendisk *, fmode_t);
>  	int (*rw_page)(struct block_device *, sector_t, struct page *, unsigned int);
> -- 
> 2.27.0
> 

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