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

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Date:   Sat, 21 Sep 2019 08:55:53 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Tejun Heo <tj@...nel.org>,
        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



> Il giorno 20 set 2019, alle ore 15:05, Jens Axboe <axboe@...nel.dk> ha scritto:
> 
> 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.

So you also are voting for BFQ to create two files, instead of having a
symlink, aren't you?  I just want to be certain before submitting one
more solution.

Looking forward to your confirmation,
Paolo

> 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