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

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Date:	Thu, 8 Jan 2009 19:33:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Unrelated:
> 
> On Thu, 8 Jan 2009, Chris Mason wrote:
> >
> > RIP: 0010:[<ffffffff8024f4de>]  [<ffffffff8024f4de>] __cmpxchg+0x36/0x3f
> 
> Ouch. HOW THE HELL DID THAT NOT GET INLINED?
> 
> cmpxchg() is a _single_ instruction if it's inlined, but it's a horrible 
> mess of dynamic conditionals on the (constant - per call-site) size 
> argument if it isn't.
> 
> It looks like you probably enabled the "let gcc mess up inlining" config 
> option.
> 
> Ingo - I think we need to remove that crap again. Because gcc gets the 
> inlining horribly horribly wrong. As usual.

Apparently it messes up with asm()s: it doesnt know the contents of the 
asm() and hence it over-estimates the size [based on string heuristics] 
...

Which is bad - asm()s tend to be the most important entities to inline - 
all over our fastpaths .

Despite that messup it's still a 1% net size win:

      text    data     bss     dec     hex filename
   7109652 1464684  802888 9377224  8f15c8 vmlinux.always-inline
   7046115 1465324  802888 9314327  8e2017 vmlinux.optimized-inlining

That win is mixed in slowpath and fastpath as well.

I see three options:

 - Disable CONFIG_OPTIMIZE_INLINING=y altogether (it's already 
   default-off)

 - Change the asm() inline markers to something new like asm_inline, which
   defaults to __always_inline.

 - Just mark all asm() inline markers as __always_inline - realizing that 
   these should never ever be out of line.

We might still try the second or third options, as i think we shouldnt go 
back into the business of managing the inline attributes of ~100,000 
kernel functions.

I'll try to annotate the inline asms (there's not _that_ many of them), 
and measure what the size impact is.

	Ingo
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