Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"

[Steven Rostedt]

On Sat, 6 Oct 2018 14:12:11 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, Oct 05, 2018 at 09:51:11PM -0400, Steven Rostedt wrote:
> > +#define arch_dynfunc_trampoline(name, def)	\
> > +	asm volatile (				\
> > +	".globl dynfunc_" #name "; \n\t"	\
> > +	"dynfunc_" #name ": \n\t"		\
> > +	"jmp " #def " \n\t"			\
> > +	".balign 8 \n \t"			\
> > +	: : : "memory" )  
> 
> Bah, what is it with you people and trampolines. Why can't we, just like
> jump_label, patch the call directly?
> 
> The whole call+jmp thing is silly, don't do that. It just wrecks I$ and
> is slower for no real reason afaict.

My first attempt was to do just that. But to add a label at the
call site required handling all the parameters too. See my branch:
 ftrace/jump_function-v1 for how ugly it got (and it didn't work).

> 
> Steve, also see:
> 
>   https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@xxxxxxxxxx

Interesting. I don't have time to look at it at the moment to see what
was done, but will do so in the near future.

Remember, this was a proof of concept and even with the trampolines, it
showed a great level of improvement. One thought was to do a
"recordmcount.c" type of action to find where the calls were and patch
them directly at boot up. I tried to keep the API the same where this
could actually be done as an improvement later.

Perhaps a gcc plugin might work too.

I'll have to see what Ard did to handle the function parameters.

-- Steve