Re: [patch 2/3] mutex subsystem: fastpath inlining

[Ingo Molnar]

* Nicolas Pitre <nico@xxxxxxx> wrote:

> This is with all mutex patches applied and CONFIG_DEBUG_MUTEX_FULL=n, 
> therefore using the current semaphore code:
> 
>    text	   data	    bss	    dec	    hex	filename
> 1821108	 287792	  88264	2197164	 2186ac	vmlinux
> 
> Now with CONFIG_DEBUG_MUTEX_FULL=y to substitute semaphores with 
> mutexes:
> 
>    text	   data	    bss	    dec	    hex	filename
> 1797108	 287568	  88172	2172848	 2127b0	vmlinux
> 
> Finally with CONFIG_DEBUG_MUTEX_FULL=y and fast paths inlined:
> 
>    text	   data	    bss	    dec	    hex	filename
> 1807824	 287136	  88172	2183132	 214fdc	vmlinux
> 
> This last case is not the smallest, but it is the fastest.

i.e. 1.3% text savings from going to mutexes, and inlining them again 
gives up 0.5% of that. We've uninlined stuff for a smaller gain in the 
past ...

> > Note that x86 went to a non-inlined fastpath _despite_ 
> > having a compact CISC semaphore fastpath.
> 
> The function call overhead on x86 is less significant than the ARM 
> one, so always calling out of line code might be sensible in that 
> case.

i'm highly doubtful we should do that. The spinlock APIs are 4 times 
more frequent than mutexes are ever going to be, still they too are 
mostly out of line. (and we only inline the unlock portions that are a 
space win!) Can you measure any significant difference in performance?  
(e.g. lat_pipe triggers the mutex fastpath, in DEBUG_MUTEX_FULL=y mode) 

the performance won by inlining is often offset by the performance cost 
of the higher icache footprint. (and ARM CPUs dont have that large 
caches to begin with)

	Ingo
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