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

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Message-ID: <68887abc-2ef4-035e-625b-84569c1b5b24@gmail.com>
Date:   Mon, 11 Jan 2021 02:28:52 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Cc:     "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 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.

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.

> that there is not significant performance benefit observe.

It's not "I cannot" but rather "I haven't even tried to and expect...".
Don't mix, there is a huge difference between.

-- 
Pavel Begunkov