linux-kernel - Re: build reproducibility

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Date:   Sun, 17 Oct 2021 11:32:38 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Julia Lawall <julia.lawall@...ia.fr>, linux-kernel@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org
Subject: Re: build reproducibility

On 10/17/21 11:12 AM, Julia Lawall wrote:
> Hello,
> 
> If I do the following:
> 
> git clean -dfx
> cp saved_config .config
> make olddefconfig && make && make modules_install && make install
> 
> Should I always end up with the same kernel, regardless of the kernel that
> is currently running on the machine?
> 
> I see a large performance difference between Linux 5.10 and all versions
> afterwards for a particular benchmark.  I am unable to bisect the problem
> eg between 5.10 and 5.11, because as soon as I come to a kernel that gives
> the bad performance, all of the kernels that I generate subsequently in
> the bisecting process (using the above commands) also have the bad
> performance.
> 
> It could of course be that I have completely misinterpreted the problem,
> and it has nothing to do with the kernel.  But I have tested the program a
> lot when only working on variants of Linux 5.9.  I only start to have
> problems when I use versions >= 5.11.

Hi,

My "guess" is that this has something to do with the build
reusing some current file(s) that need to be rebuilt.
I.e., adding a "make clean" or "make proper" might be needed.

I say this only because sometimes I cannot even reproduce
a build that has errors or warnings unless I prefix it with
make clean or mrproper. (i.e., nothing to do with booting
and running the new kernel)
Even though the .config file has changed and I do
"make olddefconfig", the same build errors do not show up
unless I do the clean or mrproper step also.


-- 
~Randy