linux-kernel - build reproducibility

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Date:   Sun, 17 Oct 2021 20:12:00 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     linux-kernel@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org
Subject: build reproducibility

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.

thanks,
julia