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

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-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