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

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Date:   Fri, 20 Sep 2019 07:05:05 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Paolo Valente <paolo.valente@...aro.org>, Tejun Heo <tj@...nel.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>,
        noreply-spamdigest via bfq-iosched 
        <bfq-iosched@...glegroups.com>,
        Oleksandr Natalenko <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

On 9/20/19 12:58 AM, Paolo Valente wrote:
> 
> 
>> Il giorno 18 set 2019, alle ore 18:19, Paolo Valente <paolo.valente@...aro.org> ha scritto:
>>
>>
>>
>>> Il giorno 18 set 2019, alle ore 17:19, Tejun Heo <tj@...nel.org> ha scritto:
>>>
>>> Hello,
>>>
>>> On Wed, Sep 18, 2019 at 07:18:50AM +0200, Paolo Valente wrote:
>>>> 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:
>>>
>>> Linus didn't like it.  The implementation was a bit nasty.  That was
>>> why it became a subject in the first place.
>>>
>>>> 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")
>>>
>>> Note that the interface was wrong at the time too.
>>>
>>>> So, Jens, Tejun, can we please just revert that revert?
>>>
>>> I think presenting both io.weight and io.bfq.weight interfaces are
>>> probably the best course of action at this point but why does it have
>>> to be a symlink?  What's wrong with just creating another file with
>>> the same backing function?
>>>
>>
>> I think a symlink would be much clearer for users, given the confusion
>> already caused by two names for the same parameter.  But let's hear
>> others' opinion too.
>>
> 
> Jens, could you express your opinion on this?  Any solution you and
> Tejun agree on is ok for me.  Also this new (fourth) possible
> implementation of this fix, provided that then it is definitely ok for
> both of you.

Retaining both interfaces is arguably the right solution. It would be
nice if we didn't have to, but the first bfq variant was incompatible
with the in-kernel one, so we'll always have that out in the wild.
Adding everything to stable doesn't work, as we still have existing
kernels out there with the interface. In fact, in some ways that's
worse, as you definitely don't want interfaces to change between two
stable kernels.

I know it's not ideal, and some better initial planning would have
made it better, but we have to deal with the situation as it stands
now.

-- 
Jens Axboe