linux-kernel - Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

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Date:	Fri, 28 Jun 2013 16:09:38 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Wedson Almeida Filho <wedsonaf@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

On Fri, Jun 28, 2013 at 01:19:48PM +0200, Ingo Molnar wrote:
> 
> * Wedson Almeida Filho <wedsonaf@...il.com> wrote:
> 
> > The new implementation allows the compiler to better optimize the code; the
> > original implementation is still used when the kernel is compiled with older
> > versions of gcc that don't support asm-goto.
> > 
> > Compiling with gcc 4.7.3, the original mutex_lock() is 60 bytes with the fast
> > path taking 16 instructions; the new mutex_lock() is 42 bytes, with the fast
> > path taking 12 instructions.
> > 
> > The original mutex_unlock() is 24 bytes with the fast path taking 7
> > instructions; the new mutex_unlock() is 25 bytes (because the compiler used
> > a 2-byte ret) with the fast path taking 4 instructions.
> > 
> > The two versions of the functions are included below for reference.

Btw, do we have any perf data showing any improvements from this patch?

[ … ]

> One detail:
> 
> > +#if defined(CC_HAVE_ASM_GOTO)
> > +#if defined(CC_HAVE_ASM_GOTO)
> 
> Please change these to #ifdef.

Cool - so we have a scripts/gcc-goto.sh which checks for support
for asm goto.

Our testing for asm goto otherwise is a bit more, hmm, hands-on in
arch/x86/include/asm/cpufeature.h:

#if __GNUC__ > 4 || __GNUC_MINOR__ >= 5

Maybe I should change that to the more explicit CC_HAVE_ASM_GOTO then.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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