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SubjectRe: [PATCH 2/2] jump label: disable due to compiler bug
* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Fri, 2010-10-29 at 14:22 +0200, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > > On Fri, 2010-10-29 at 08:34 +0200, Ingo Molnar wrote:
>
> > > Do you mean to have it dependent on !EMBEDDED, or perhaps !CC_OPTIMIZE_FOR_SIZE ?
> >
> > CC_OPTIMIZE_FOR_SIZE dependency probably not. Might even leave out !EMBEDDED, to
> > make it generally configurable.
> >
> > Also, we should make it default off - we need to see whether there are any compiler
> > bugs lurking.
>
> Ah, so we should then add a:
>
> config JUMP_LABEL
> prompt "Optimize trace point call sites"
> depends on HAVE_ARCH_JUMP_LABEL
> help
> If it is detected that the compiler has support for
> "asm goto", the kernel will compile trace points locations
> with just a nop instruction. When trace points are enabled
> the nop will be converted to a jump to the trace function.
> This technique lowers overhead and stress on the branch
> prediction of the processor.
>
> On i386, the options added to make this work may increase
> the size of the kernel slightly.
>
>
> Then in the code have:
>
> -#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_HAVE_ARCH_JUMP_LABEL)
> +#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_HAVE_ARCH_JUMP_LABEL) \

Technically, you could remove the dependency on
"defined(CONFIG_HAVE_ARCH_JUMP_LABEL)", because it is now dealt with at
the Kconfig level (CONFIG_JUMP_LABEL depends on it).

> + && defined(JUMP_LABEL)

this would be defined(CONFIG_JUMP_LABEL)

The rest looks fine,

Thanks,

Mathieu

> # include <asm/jump_label.h>
> # define HAVE_JUMP_LABEL
> #endif
>
>
> If you agree, I can write up a patch.
>
> -- Steve
>
> >
> > > I can see people making a big stink about adding back a gcc option that makes the
> > > kernel bigger.
> >
> > Depends on exactly how much .text we are talking about here. Plus it would be
> > default-off.
> >
> > Thanks,
> >
> > Ingo
>
>

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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