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Message-ID: <12e09497-a848-b767-88f4-7dabd8360c5e@leemhuis.info>
Date:   Wed, 2 Feb 2022 12:46:27 +0100
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To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        workflows@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Randy Dunlap <rdunlap@infradead.org>,
        regressions@lists.linux.dev,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>
References: <cover.1643110442.git.linux@leemhuis.info>
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 <CAMuHMdXRoVt_zRBNvugJjYhJnyYbABeCWv9fFRM0r_=s7FYvJQ@mail.gmail.com>
From:   Thorsten Leemhuis <linux@leemhuis.info>
Subject: Re: [PATCH v3 1/2] docs: add a document about regression handling
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Hi! I noticed I forgot to reply here:

On 26.01.22 15:28, Geert Uytterhoeven wrote:
> On Tue, Jan 25, 2022 at 5:45 PM Thorsten Leemhuis <linux@leemhuis.info> wrote:
>> +How to create a configuration similar to the one of an older kernel?
>> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +Start a known-good kernel and configure the newer Linux version with ``make
>> +olddefconfig``. This makes the kernel's build scripts pick up the configuration
>> +file (the `.config` file) from the running kernel as base for the new one you
>> +are about to compile; afterwards they set all new configuration options to their
>> +default value, which should disable new features that might cause regressions.
> 
> Doing so may actually cause mutations to appear in your .config
> when going back and forth (i.e. when bisecting), interfering with
> the bisection process.

Good point, I knew about this, but hadn't thought of this when writing
the text.

> To avoid that, I usually start bisecting with
> "cp .config <src>/arch/<arch>/configs/bisect_defconfig", and use
> "make bisect_defconfig" in every bisection step. That way all steps
> are reproducible, and unaffected by config mutations.

That's a really cool trick, thx for mentioning it. But I think it's not
needed in the text about regressions and instead better be mentioned in
Documentation/admin-guide/bug-bisect.rst. I hope to sooner or later
improve (rewrite?) that document anyway and will make sure to keep it in
mind for that time.

I wonder if there is a way to make this work without messing in the
source tree? Took a quick look at the sources. It seems to me that it's
possible to "cp .config ~/working.config" and then using "make
KBUILD_DEFCONFIG=~/working.config defconfig" at every bisection step. It
seems to do the trick as well -- but I only tried briefly, so maybe I
might have missed something.

Ciao, Thorsten