linux-kernel - Re: [RFC V5 01/16] blk: make the whole blk_mq_submit_bio under q_usage

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Date:   Fri, 25 Feb 2022 10:11:29 +0800
From:   Wang Jianchao <jianchao.wan9@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <jbacik@...com>,
        Tejun Heo <tj@...nel.org>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V5 01/16] blk: make the whole blk_mq_submit_bio under
 q_usage_counter



On 2022/2/24 9:38 下午, Christoph Hellwig wrote:
> On Thu, Feb 24, 2022 at 05:06:39PM +0800, Wang Jianchao (Kuaishou) wrote:
>> This is to protect the rqos list against the rqos open/close. We
>> need to drain all of the caller of blk_mq_submit_bio() before
>> we can operate the rqos list.
> 
> This means ever I/O now has to d an extra refcount roundtrip.

If we want to save cpu cycles for the very fast device, why not introduce
another blk_mq_submit_bio_fast() which can reduce the code drastically.

Thanks
Jianchao