linux-kernel - Re: [PATCH 2/2] block, bfq: delete "bfq" prefix from cgroup filenames

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Date:   Wed, 18 Sep 2019 07:18:50 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org, ulf.hansson@...aro.org,
        linus.walleij@...aro.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, cgroups@...r.kernel.org,
        Angelo Ruocco <angeloruocco90@...il.com>
Subject: Re: [PATCH 2/2] block, bfq: delete "bfq" prefix from cgroup filenames



> Il giorno 17 set 2019, alle ore 23:32, Tejun Heo <tj@...nel.org> ha scritto:
> 
> Hello,
> 
> On Tue, Sep 17, 2019 at 06:51:48PM +0200, Paolo Valente wrote:
>> 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.
> 
> So, I wrote the iocost switching patch and don't have a strong
> interest in whether bfq prefix should get dropped or not.  However, I
> gotta point out that flipping interface this way is way out of the
> norm.
> 
> In the previous release cycle, the right thing to do was dropping the
> bfq prefix but that wasn't possible because bfq's interface wasn't
> compatible at that point and didn't made to be compatible in time.
> Non-obviously different interface with the same name is a lot worse
> than giving it a new name, so the only acceptable course of action at
> that point was keeping the bfq prefix.
> 
> Now that the interface has already been published in a released
> kernel, dropping the prefix would be something extremely unusual as
> there would already be users who will be affected by the interface
> flip-flop.  We sometimes do change interfaces but I'm having a
> difficult time seeing the overriding rationales in this case.
> 

This issue is a nightmare :)

Userspace wants the weight to be called weight (I'm not reporting
links to threads again).  *Any* solution that gets to this is ok for me.

A solution that both fulfills userspace request and doesn't break
anything for hypothetical users of the current interface already made
it to mainline, and Linus liked it too.  It is:
19e9da9e86c4 ("block, bfq: add weight symlink to the bfq.weight cgroup parameter")

But it was then reverted on Tejun's request to do exactly what we
don't want do any longer now:
cf8929885de3 ("cgroup/bfq: revert bfq.weight symlink change")

So, Jens, Tejun, can we please just revert that revert?

Thanks,
Paolo

> Thanks.
> 
> -- 
> tejun