linux-kernel - Re: [RFC PATCH 00/50] tools/perf: Speed up the build system

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Message-ID: <20131007164156.GA12790@gmail.com>
Date:	Mon, 7 Oct 2013 18:41:56 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC PATCH 00/50] tools/perf: Speed up the build system

* David Ahern <dsahern@...il.com> wrote:

> I also noticed that the dwarf test is still run even with the NO_DWARF 
> option passed in:
> 
> [daahern@...s-vdc-dev1 perf]$ make O=/tmp/junk LDFLAGS=-static
> NO_DWARF=1 -j 4
>     BUILD: Doing 'make -j16' parallel build
> 
> Auto-detecting system features:
> 
> ...                     backtrace: [ on  ]
> ...                         dwarf: [ on  ]
> ...                fortify-source: [ on  ]
> 
> Note the dwarf test shows 'on'.

Hm, yes.

This is just the print-out though - the actual feature logic should still 
follow the NO_DWARF=1 setting (modulo bugs).

So I'm wondering, should we solve this by adding extra logic linking the 
feature flags with their legacy names. It would get unwieldy rather 
quickly I think.

Another solution would be to introduce a new method to disable features, 
via something like:

	make FEATURE_dwarf=0

Where the pattern would follow the auto-detected naming. This would 
simplify the printout logic and would simplify the feature support / flags 
decision tree as well.

Furthermore, it would unify the various flags we have today, which is 
rather mixed: for example there's NO_DWARF which is a name that shows 
negated logic, but there's also HAVE_CPLUS_DEMANGLE_SUPPORT is is a name 
with positive logic. We'd have one uniform naming scheme permeating the 
whole build system.

And that brings in your [K]config patches: which would make sense in that 
context as well, as they'd allow the permanent configuration of features 
with 3 states for each feature flag:

	off
	auto-detect
	on

Your scheme I think makes a lot of sense on top of my bits. Packagers 
would likely want to use a .config to build perf, most users would likely 
be fine with a default of everything on auto-detect. Specialized users 
would want to use their own .config's.

Thanks,

	Ingo
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