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

H. Peter Anvin
2009-01-08T18:41:11Z

Ingo Molnar wrote:
>
> 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]
> ...
>

Right. gcc simply doesn't have any way to know how heavyweight an
asm() statement is, and it WILL do the wrong thing in many cases --
especially the ones which involve an out-of-line recovery stub. This is
due to a fundamental design decision in gcc to not integrate the
compiler and assembler (which some compilers do.)

> 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.

The good part here is that the assembly ones really don't have much
subtlety -- a function call is at least five bytes, usually more once
you count in the register spill penalties -- so __always_inline-ing them
should still end up with numbers looking very much like the above.

> 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.

The main reason to do #2 over #3 would be for programmer documentation.
There simply should be no reason to ever out-of-lining these. However,
documenting the reason to the programmer is a valuable thing in itself.

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.