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

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Message-ID: <f7fd8fa3-6368-48c1-93b2-942d9d0f75c7@yukuai.org.cn>
Date: Mon, 6 Oct 2025 09:57:40 +0800
From: Yu Kuai <hailan@...uai.org.cn>
To: 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,
 yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com
Subject: Re: [PATCH 3/7] blk-mq: add a new queue sysfs attribute async_depth

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.

Thanks,
Kuai

> Thanks,
> --Nilay
>