Re: [PATCH 3/7] blk-mq: add a new queue sysfs attribute async_depth

[Nilay Shroff]

On 9/30/25 12:41 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> 
> Add a new field async_depth to request_queue and related APIs, this is
> currently not used, following patches will convert elevators to use
> this instead of internal async_depth.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  block/blk-core.c       |  1 +
>  block/blk-mq.c         |  4 ++++
>  block/blk-sysfs.c      | 47 ++++++++++++++++++++++++++++++++++++++++++
>  block/elevator.c       |  1 +
>  include/linux/blkdev.h |  1 +
>  5 files changed, 54 insertions(+)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index dd39ff651095..76df70cfc103 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -463,6 +463,7 @@ struct request_queue *blk_alloc_queue(struct queue_limits *lim, int node_id)
>  	fs_reclaim_release(GFP_KERNEL);
>  
>  	q->nr_requests = BLKDEV_DEFAULT_RQ;
> +	q->async_depth = BLKDEV_DEFAULT_RQ;
>  
>  	return q;
>  
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 09f579414161..260e54fa48f0 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -529,6 +529,8 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
>  			data->rq_flags |= RQF_USE_SCHED;
>  			if (ops->limit_depth)
>  				ops->limit_depth(data->cmd_flags, data);
> +			else if (!blk_mq_sched_sync_request(data->cmd_flags))
> +				data->shallow_depth = q->async_depth;
>  		}

In the subsequent patches, I saw that ->limit_depth is still used for the
BFQ scheduler. Given that, it seems more consistent to also retain ->limit_depth
for the mq-deadline and Kyber schedulers, and set data->shallow_depth within their
respective ->limit_depth methods. If we take this approach, the additional 
blk_mq_sched_sync_request() check above becomes unnecessary.

So IMO:
- Keep ->limit_depth for all schedulers (bfq, mq-deadline, kyber).
- Remove the extra blk_mq_sched_sync_request() check from the core code.

Thanks,
--Nilay