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

Ingo Molnar
2009-01-08T18:33:48Z

* 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