Re: [PATCH v5 10/12] block: fastpath for bio-based polling

[Ming Lei <ming.lei@redhat.com>]

On Wed, Mar 03, 2021 at 07:57:38PM +0800, Jeffle Xu wrote:
> Offer one fastpath for bio-based polling when bio submitted to dm
> device is not split.
> 
> In this case, there will be only one bio submitted to only one polling
> hw queue of one underlying mq device, and thus we don't need to track
> all split bios or iterate through all polling hw queues. The pointer to
> the polling hw queue the bio submitted to is returned here as the
> returned cookie. In this case, the polling routine will call
> mq_ops->poll() directly with the hw queue converted from the input
> cookie.
> 
> If the original bio submitted to dm device is split to multiple bios and
> thus submitted to multiple polling hw queues, the polling routine will
> fall back to iterating all hw queues (in polling mode) of all underlying
> mq devices.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  block/blk-core.c          | 73 +++++++++++++++++++++++++++++++++++++--
>  include/linux/blk_types.h | 66 +++++++++++++++++++++++++++++++++--
>  include/linux/types.h     |  2 +-
>  3 files changed, 135 insertions(+), 6 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 6d7d53030d7c..e5cd4ff08f5c 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -947,14 +947,22 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
>  {
>  	struct bio_list bio_list_on_stack[2];
>  	blk_qc_t ret = BLK_QC_T_NONE;
> +	struct request_queue *top_q;
> +	bool poll_on;
>  
>  	BUG_ON(bio->bi_next);
>  
>  	bio_list_init(&bio_list_on_stack[0]);
>  	current->bio_list = bio_list_on_stack;
>  
> +	top_q = bio->bi_bdev->bd_disk->queue;
> +	poll_on = test_bit(QUEUE_FLAG_POLL, &top_q->queue_flags) &&
> +		  (bio->bi_opf & REQ_HIPRI);
> +
>  	do {
> -		struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> +		blk_qc_t cookie;
> +		struct block_device *bdev = bio->bi_bdev;
> +		struct request_queue *q = bdev->bd_disk->queue;
>  		struct bio_list lower, same;
>  
>  		if (unlikely(bio_queue_enter(bio) != 0))
> @@ -966,7 +974,23 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
>  		bio_list_on_stack[1] = bio_list_on_stack[0];
>  		bio_list_init(&bio_list_on_stack[0]);
>  
> -		ret = __submit_bio(bio);
> +		cookie = __submit_bio(bio);
> +
> +		if (poll_on && blk_qc_t_valid(cookie)) {

In patch 8, dm_submit_bio() is changed to return BLK_QC_T_NONE always,
so the returned cookie may be BLK_QC_T_NONE for DM device, such as, in
case of DM_MAPIO_SUBMITTED returned from ->map(), and underlying bios
can be submitted from another context, then nothing is fed to blk_poll().


thanks, 
Ming