linux-kernel - Re: [PATCH/RFC] headers: drop 2 #included headers from <linux/interrupt.h>

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Date:   Mon, 26 Feb 2018 00:19:56 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arch\@vger.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH/RFC] headers: drop 2 #included headers from <linux/interrupt.h>

On Fri, Feb 16 2018, Ingo Molnar <mingo@...nel.org> wrote:

> * Randy Dunlap <rdunlap@...radead.org> wrote:
>
>> Does anyone know or see why <linux/interrupt.h> needs <linux/linkage.h> or
>> <linux/preempt.h>?  There are no direct uses in <linux/interrupt.h> AFAICT.
>
> These are likely historical, plus it's a classic 'tragedy of the commons' 
> scenario: it's easy to take away from the "public good" of "clean header 
> dependencies" and very hard to add to it:
>
>  - it's very easy to add a new #include line
>
>  - it's very hard to remove one, not just due to the 2^10000 complex Kconfig
>    space, but due to 99% of the developers not testing cross-arch builds
>    (and we don't want to require them to do it)
>
>  - the cost of getting the removal wrong is high (the build breaks for many 
>    people who will be unhappy at the person removing the header), while the 
>    benefit of the removal is low in isolation
>
> Then apply this dynamic for more than 20 years and we get a spaghetti:
>
[snip]
>
> I'd argue that due to the basic human dynamics here this calls for tooling help.

I started trying to write some scripts a few years ago, but only got as
far as doing a pass over lib/ (e.g. b8b6db17) - but doing individual .c
files is never going to be enough; one really needs to eliminate
#includes from header files. But then I decided that before trying to
systematically attack that much bigger problem, I'd want some objective
metrics to see that it actually worked. Now I finally got around to
writing some scripts for that, so here's a visualization of
(the evolution of) the problem:

https://wildmoose.dk/header-bloat/

I hope to find some time soonish to polish the old programs and see if
they actually still work.

Rasmus