On Sat, 6 Oct 2018 14:12:11 +0200 Peter Zijlstra <pet...@infradead.org> 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.biesheu...@linaro.org 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
