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

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Message-ID: <1b55e01b-64bc-94d9-c0cc-9850b9ee6582@huaweicloud.com>
Date: Thu, 9 Oct 2025 08:48:41 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Yu Kuai <hailan@...uai.org.cn>, Nilay Shroff <nilay@...ux.ibm.com>,
 Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, bvanassche@....org,
 ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 3/7] blk-mq: add a new queue sysfs attribute async_depth

Hi,

在 2025/10/06 9:57, Yu Kuai 写道:
> Hi,
> 
> 在 2025/10/2 23:10, Nilay Shroff 写道:
>>
>> On 9/30/25 12:41 PM, Yu Kuai wrote:
>>> From: Yu Kuai <yukuai3@...wei.com>
>>>
>>> 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@...wei.com>
>>> ---
>>>   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.
> 
> I was thinking to save a function call for deadline and kyber, however, 
> I don't
> have preference here and I can do this in the next version.

How abount following, I feel this is better while cooking the new
version. Consider only bfq have specail handling for async request.

static void blk_mq_sched_limit_async_depth(struct blk_mq_alloc_data *data)
{
	if (blk_mq_sched_sync_request(data->cmd_flags))
		return;

	data->shallow_depth = q->async_depth;
	if (ops->limit_async_depth)
		ops->limit_async_depth(data);
}

Thanks,
Kuai

> 
> Thanks,
> Kuai
> 
>> Thanks,
>> --Nilay
>>
> .
>