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Subject: Re: [PATCH 2/2] block, bfq: delete "bfq" prefix from cgroup filenames
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <de7664b1-6f47-8a7b-b231-727336c0ef85@kernel.dk>
Date:   Sat, 21 Sep 2019 08:55:53 +0200
Cc:     Tejun Heo <tj@kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        noreply-spamdigest via bfq-iosched 
        <bfq-iosched@googlegroups.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        cgroups@vger.kernel.org, Angelo Ruocco <angeloruocco90@gmail.com>
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Message-Id: <FE1AEF80-F07F-42DE-A979-BC5AE72B25A1@linaro.org>
References: <20190917165148.19146-1-paolo.valente@linaro.org>
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 <20190917213209.GK3084169@devbig004.ftw2.facebook.com>
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 <de7664b1-6f47-8a7b-b231-727336c0ef85@kernel.dk>
To:     Jens Axboe <axboe@kernel.dk>
Sender: linux-kernel-owner@vger.kernel.org
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> Il giorno 20 set 2019, alle ore 15:05, Jens Axboe <axboe@kernel.dk> ha =
scritto:
>=20
> On 9/20/19 12:58 AM, Paolo Valente wrote:
>>=20
>>=20
>>> Il giorno 18 set 2019, alle ore 18:19, Paolo Valente =
<paolo.valente@linaro.org> ha scritto:
>>>=20
>>>=20
>>>=20
>>>> Il giorno 18 set 2019, alle ore 17:19, Tejun Heo <tj@kernel.org> ha =
scritto:
>>>>=20
>>>> Hello,
>>>>=20
>>>> 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:
>>>>=20
>>>> Linus didn't like it.  The implementation was a bit nasty.  That =
was
>>>> why it became a subject in the first place.
>>>>=20
>>>>> 19e9da9e86c4 ("block, bfq: add weight symlink to the bfq.weight =
cgroup parameter")
>>>>>=20
>>>>> 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")
>>>>=20
>>>> Note that the interface was wrong at the time too.
>>>>=20
>>>>> So, Jens, Tejun, can we please just revert that revert?
>>>>=20
>>>> 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?
>>>>=20
>>>=20
>>> 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.
>>>=20
>>=20
>> 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.
>=20
> 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.
>=20
> 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.
>=20
> --=20
> Jens Axboe