Re: [PATCH v7 04/17] tracing: add static function tracer supportfor MIPS

[David Daney]

Wu Zhangjin wrote:
> Hi,
>
> On Mon, 2009-11-09 at 16:26 -0800, David Daney wrote: 
> > Wu Zhangjin wrote:
> >
> > >  
> > > +ifndef CONFIG_FUNCTION_TRACER
> > >  cflags-y := -ffunction-sections
> > > +else
> > > +cflags-y := -mlong-calls
> > > +endif
> > >  cflags-y += $(call cc-option, -mno-check-zero-division)
> > >  
> > That doesn't make sense to me.  Modules are already compiled with 
> > -mlong-calls.  The only time you would need the entire kernel compiled 
> > with -mlong-calls is if the tracer were in a module.  The logic here 
> > doesn't seem to reflect that.
>
> Yes, I knew the module have gotten the -mlong-calls, Here we just want
> to use -mlong-calls for the whole kernel, and then we get the same
> _mcount stuff in the whole kernel, at last, we can use the same
> scripts/recordmcount.pl and ftrace_make_nop & ftrace_make_call for the
> dynamic ftracer.

-mlong-calls really degrades performance.  I have seen things like 6% 
drop in network packet forwarding rates with -mlong-calls.

It would be better to fix all the tools so that they could handle both 
-mlong-calls and -mno-long-calls code.


David Daney
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