linux-kernel - Re: [PATCH 0/3] bitmap: convert self-test to KUnit

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Message-ID: <CAJ-ks9=f8d7jh=15bHc28Z37p9rA-Kg4J2mQ++VBcsesVvezUA@mail.gmail.com>
Date: Sat, 8 Feb 2025 13:42:02 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Yury Norov <yury.norov@...il.com>
Cc: David Gow <davidgow@...gle.com>, John Hubbard <jhubbard@...dia.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Naveen N Rao <naveen@...nel.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Shuah Khan <shuah@...nel.org>, Kees Cook <kees@...nel.org>, 
	Muhammad Usama Anjum <usama.anjum@...labora.com>, linux-kernel@...r.kernel.org, 
	linux-m68k@...ts.linux-m68k.org, linuxppc-dev@...ts.ozlabs.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 0/3] bitmap: convert self-test to KUnit

On Sat, Feb 8, 2025 at 12:53 PM Yury Norov <yury.norov@...il.com> wrote:
>
> [...]
>
> Take over means that you'd at least add the Co-developed-by tag.

I didn't use their code - the thing being "taken over" is the work of
having these debates with the maintainers.

> [...]
>
> KUNIT is disabled in defconfig, at least on x86_64. It is also disabled
> on my Ubuntu 24.04 machine. If I take your patches, I'll be unable to
> boot-test bitmaps. Even worse, I'll be unable to build the standalone
> test from sources as a module and load it later.
>
> Or I misunderstand it, and there's a way to build some particular KUNIT
> test without enabling KUNIT in config and/or re-compiling the whole kernel?
> Please teach me, if so
>
> Unless you give me a way to build and run the test in true
> production environment, I'm not going with KUNITs. Sorry.

This is a question for David -- I don't know if this is possible.

> [...]
>
> This is my evidence: sometimes people report performance or whatever
> issues on their systems, suspecting bitmaps guilty. I ask them to run
> the bitmap or find_bit test to narrow the problem. Sometimes I need to
> test a hardware I have no access to, and I have to (kindly!) ask people
> to build a small test and run it. I don't want to ask them to rebuild
> the whole kernel, or even to build something else.
>
> https://lore.kernel.org/all/YuWk3titnOiQACzC@yury-laptop/

This is compelling evidence, and it was not previously raised. Thank you.

I notice that two things are true about the performance test part of
test_bitmap.c:
- It's a minority of the code in the file (48 lines out of 1462).
- There are no assertions in it.

Do you also find value in running the testing portion on other
people's machines, to which you don't have access?

> [...]
>
> Nice summary for the discussion. Unfortunately you missed my concerns.
> Which are:
>
> Pros:
>  - Now we switch to KUNITs because KUNITs are so good
>
> Cons:
>  - Wipes git history;

I was very careful to minimize churn, and the result is 249 lines on
which I'd now own the blame (228 with `-w`). Still, it's a valid con.

>  - Bloats the test's source code;

The test is 74 lines shorter after this series.

>  - Adds dependencies;
>  - Doesn't run on most popular distros and defconfig;

Yep, I understand your concerns much better now - and I'm grateful for
your having taken the time to explain and show receipts. Still, I
wonder if we can get the best of both worlds - either by finding what
you need in KUnit, or by moving the testing bit to KUnit and keeping
the performance bit where it is.

Thanks.
Tamir