linux-kernel - Re: [PATCH] target/file: don't zero iter before iov_iter

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Date:   Mon, 11 Jan 2021 05:31:00 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>
Cc:     "Martin K . Petersen" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] target/file: don't zero iter before iov_iter_bvec

On 11/01/2021 05:23, Chaitanya Kulkarni wrote:
> On 1/10/21 18:32, Pavel Begunkov wrote:
>> On 11/01/2021 02:06, Chaitanya Kulkarni wrote:
>>> On 1/9/21 13:29, Pavel Begunkov wrote:
>>>> On 09/01/2021 20:52, Chaitanya Kulkarni wrote:
>>>>> On 1/9/21 12:40, Pavel Begunkov wrote:
>>>>>> I expect you won't find any, but such little things can pile up
>>>>>> into a not-easy-to-spot overhead over time.
>>>>> That is what I suspected with the resulting assembly. The commit log
>>>>> needs to document that there is no direct impact on the performance
>>>> It's obvious that 3-4 extra mov $0 off(%reg) won't change performance
>>>> but still hasn't been formally confirmed ...
>>> This is obvious for you and me since we spent time into looking into
>>> resulting assembly not every reviewer is expected to do that see [1].
>>>>> which can be seen with this patch, but this is nice to have
>>>> ... so if you don't mind, I won't be resending just for that.
>>> As per commit log guidelines [1] you have to quantify the optimization.
>>>
>>> Since you cannot quantify the optimization modify the commit log explaining
>> And then you see "Optimizations usually aren’t free but trade-offs
>> between", and the patch doesn't fall under it.
> First part applies to all the optimizations with and without tradeoffs
> "Quantify optimizations and trade-offs."
> The later part doesn't mean optimizations without trade-offs should be
> allowed without having any supportive data.
>>
>> Let me be frank, I see it more like as a whim. If the maintainer agrees
>> with that strange requirement of yours and want to bury it under
>> bureaucracy, fine by me, don't take it, I don't care, but I haven't
>> ever been asked here to do that for patches as this.
> I didn't write the commit log guidelines, as a reviewer I'm following them.
> The patch commit log claims optimization with neither having any data nor
> having the supporting fact ("possibly no observable difference but in the
> long term it matters") for the completeness.
>> It's not "I cannot" but rather "I haven't even tried to and expect...".
>> Don't mix, there is a huge difference between.
> Then provide the numbers to support your claim.

What claim? I didn't make any regarding performance, you may want to
re-read the commit message.

Anyway, I'll halt replying to this topic. Nothing personal, but it's
getting annoying.

-- 
Pavel Begunkov