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Date: Thu, 12 Sep 2013 22:43:13 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: David Ahern <dsahern@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] perf fixes
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* Ingo Molnar <mingo@kernel.org> wrote:

> Instead of having 20 testcases crammed into a single Makefile:
> 
> comet:~/tip/tools/perf> git grep -w main config/feature-tests.mak 
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(int argc, char *argv[])
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> config/feature-tests.mak:int main(void)
> 
> Why not put those into 20x standalone .c files and try to build them as 
> an explicit make command attempt from within the main Makefile? If the 
> build fails then the feature flag is set to off, if it succeeds it's 
> defined.
> 
> That makes feature tests generally easier to debug, easier to understand 
> and generally easier to extend as well.
> 
> 'Feature test binaries' that are already built will be 'cached' 
> automatically, and the result of their build attempt will be a quick 
> success.

A successfull 'make' attempt of a target file within a separate, simple 
Makefile is very, very fast:

 comet:~/tip/tools/power/x86/turbostat> perf stat --null --sync --repeat 10 
 make turbostat
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.
 make: `turbostat' is up to date.

  Performance counter stats for 'make turbostat' (10 runs):

        0.002146182 seconds time elapsed                                          ( +-  0.35% )

That's 2 msecs only. 20 such tests will take less than 50 msecs in the 
'cached' case.

Thanks,

	Ingo