linux-kernel - Re: [PATCH 1/1] block, bfq: delete "bfq" prefix from cgroup filenames

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Date:   Mon, 8 Apr 2019 09:05:19 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Johannes Thumshirn <jthumshirn@...e.de>,
        Paolo Valente <paolo.valente@...aro.org>
Cc:     linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@...glegroups.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Angelo Ruocco <angeloruocco90@...il.com>,
        lennart@...ttering.net, mkoutny@...e.cz
Subject: Re: [PATCH 1/1] block, bfq: delete "bfq" prefix from cgroup filenames

On 4/8/19 9:04 AM, Johannes Thumshirn wrote:
> [+Cc Michal ]
> On Mon, Apr 08, 2019 at 04:54:39PM +0200, Paolo Valente wrote:
>>
>>
>>> Il giorno 8 apr 2019, alle ore 16:49, Johannes Thumshirn <jthumshirn@...e.de> ha scritto:
>>>
>>> On Mon, Apr 08, 2019 at 04:39:35PM +0200, Paolo Valente wrote:
>>>> From: Angelo Ruocco <angeloruocco90@...il.com>
>>>>
>>>> When bfq was merged into mainline, there were two I/O schedulers that
>>>> implemented the proportional-share policy: bfq for blk-mq and cfq for
>>>> legacy blk. bfq's interface files in the blkio/io controller have the
>>>> same names as cfq. But the cgroups interface doesn't allow two
>>>> entities to use the same name for their files, so for bfq we had to
>>>> prepend the "bfq" prefix to each of its files. However no legacy code
>>>> uses these modified file names. This naming also causes confusion, as,
>>>> e.g., in [1].
>>>>
>>>> Now cfq has gone with legacy blk, so there is no need any longer for
>>>> these prefixes in (the never used) bfq names. In view of this fact, this
>>>> commit removes these prefixes, thereby enabling legacy code to truly
>>>> use the proportional share policy in blk-mq.
>>>>
>>>> [1] https://github.com/systemd/systemd/issues/7057
>>>
>>> Hmm, but isn't this a user-space facing interface and thus some sort of ABI?
>>> Do you know what's using it and what breaks due to this conversion?
>>>
>>
>> Yep, but AFAIK, the problem is exactly the opposite: nobody uses these
>> names for the proportional-share policy, or wants to use these names.  I'm
>> CCing Lennart too, in case he has some improbable news on this.
>>
>> So the idea is to align names to what people expect, possibly before
>> more confusion arises.
> 
> OK, crazy idea, not sure if Jens and Tejun will beat me for this, but
> symlinks?
> 
> This way we can a) keep the old files and b) have them point to the new (a.k.a
> cfq style) files.

I did consider that, and that would be doable. But honestly, I'm having a
hard time seeing what issue we are attempting to fix by doing this.

-- 
Jens Axboe