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

["H. Peter Anvin" <hpa@zytor.com>]

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.