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

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Message-ID: <cfefdf5c-984f-1505-be15-0cc9e79df31b@huaweicloud.com>
Date: Thu, 9 Oct 2025 14:42:00 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Yu Kuai <yukuai1@...weicloud.com>, Yu Kuai <hailan@...uai.org.cn>,
 Nilay Shroff <nilay@...ux.ibm.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 2025/10/09 8:48, Yu Kuai 写道:
> 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);
> }
> 

Just realize I forgot that bfq can limit sync requests as well due to
bfq cgroup policy, so this is not good.

Please ignore this :)

Thanks,
Kuai

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