linux-kernel - Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and modular

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Date:   Wed, 23 Feb 2022 16:07:51 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Wang Jianchao <jianchao.wan9@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <jbacik@...com>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and
 modular

Hello,

On Thu, Feb 24, 2022 at 09:51:04AM +0800, Wang Jianchao wrote:
> The initial version of this patchset has two targets：
> (1) Add a sysfs interface to open/close the policy per device. Then we needn't
>     waste cpu cycles and memory if the device doesn't need the policy.
> (2) Make the policies modular, then it easy to maintain the code of policy in
>     production environment as we only need to close the policy and replace the
>     .ko file.
> 
> The loading module when open policy in sysfs interface is just to avoid modprobe
> manually. There is similar operation when switch io scheduler.

Each rq-qos mechanism already needs and has a way to turn off itself.
There's no reason to add another layer on top. If the current way of
disabling isn't efficient, we should improve that instead of adding a new
layer of interface on top.

And please don't add a custom interface to avoid modprobing. All it adds is
unnecessary deviation. There's no benefit to echoing a selector to a custom
sysfs file compared to explicitly modprobing it.

Thanks.

-- 
tejun