Re: [PATCH/RFC] headers: drop 2 #included headers from

Rasmus Villemoes
2018-02-25T23:20:02Z

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

> * Randy Dunlap <rdunlap@infradead.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