linux-kernel - Re: mmotm 2009-02-02-17-12 uploaded (x86/nopmd etc.)

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Date:	Wed, 4 Feb 2009 22:29:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	jeremy@...p.org, jaswinderrajput@...il.com,
	randy.dunlap@...cle.com, linux-kernel@...r.kernel.org,
	x86@...nel.org, righi.andrea@...il.com
Subject: Re: mmotm 2009-02-02-17-12 uploaded (x86/nopmd etc.)


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 4 Feb 2009 21:08:46 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > > > This is getting painful.
> > > > 
> > > > the include file spaghetti is ... interesting there, and it's historic.
> > > > 
> > > > I could blame it on highmem, PAE or paravirt - but i'll only blame it on 
> > > > paravirt for now because those developers are still around! ;-)
> > > > 
> > > > Jeremy, any ideas how to reduce the historic dependency mess in that area?
> > > > I think we should go on three routes at once:
> > > > 
> > > >  - agressive splitup and separation of type definitions from method
> > > >    declaration (+ inline definitions). The spinlock_types.h / spinlock.h 
> > > >    splitup was really nice in solving such dependency problems.
> > > 
> > > I like this one.  The mixing up of declare-something with use-something
> > > is often the source of our woes.
> > 
> > yes. I mapped this problem area once and this is how the include file 
> > spaghetti gets generated in practice:
> > 
> >   - type A gets declared
> >   - type A gets _used_ in the same file in an inline method, BUT,
> > 
> >       that usage also brings in instantiated use of type X1, X2 and X3.
> > 
> > if all types are declared like that everywhere, it can be seen (and it's a 
> > mathematical certainty) that the only conflict-free way of doing this is to:
> > 
> >   - initially add random #include lines to bring in type X1, X2 and X3. 
> >     Which brings in recursive dependencies from those X1 X2 and X3 files.
> > 
> >   - when the stuff hits the fan then folks are in a big mess already and 
> >     only a deep restructuring could gets them out of it - which they rarely 
> >     do in an iterative environment. So they work it around iteratively: 
> >     instead of new nice inline methods [which we really prefer] they delay 
> >     all the 'usage' instantiation to .c file via the use of CPP macros 
> >     [which we hate because they hide bugs and cause bugs].
> 
> None of which would happen if we didn't also have an inlining fetish.

inlining is a nice and convenient tool that helps us do better code in many 
cases. It has this long-term dependency-deteriorating effect though.

	Ingo
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