linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Message-ID: <alpine.LFD.2.00.0901081943150.6528@localhost.localdomain>
Date:	Thu, 8 Jan 2009 19:46:30 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Ingo Molnar <mingo@...e.hu>, Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Thu, 8 Jan 2009, H. Peter Anvin wrote:
> 
> Right.   gcc simply doesn't have any way to know how heavyweight an
> asm() statement is

I don't think that's relevant.

First off, gcc _does_ have a perfectly fine notion of how heavy-weight an 
"asm" statement is: just count it as a single instruction (and count the 
argument setup cost that gcc _can_ estimate).

That would be perfectly fine. If people use inline asms, they tend to use 
it for a reason.

However, I doubt that it's the inline asm that was the biggest reason why 
gcc decided not to inline - it was probably the constant "switch()" 
statement. The inline function actually looks pretty large, if it wasn't 
for the fact that we have a constant argument, and that one makes the 
switch statement go away.

I suspect gcc has some pre-inlining heuristics that don't take constant 
folding and simplifiation into account - if you look at just the raw tree 
of the function without taking the optimization into account, it will look 
big.

			Linus
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