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SubjectRe: [PATCH] tracing: Cleanup the convoluted softirq tracepoints
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On Tue, 2010-10-19 at 14:48 -0700, H. Peter Anvin wrote:
> On 10/19/2010 02:23 PM, Steven Rostedt wrote:
> >
> > But it seemed that gcc for you inlined the code in the wrong spot.
> > Perhaps it's not a good idea to have the something like h - softirq_vec
> > in the parameter of the tracepoint. Not saying that your change is not
> > worth it. It is, because h - softirq_vec is used by others now too.
> >
>
> OK, first of all, there are some serious WTFs here:
>
> # define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
>
> A jump instruction is one of the worst possible NOPs. Why are we doing
> this?


Good question. Safety? Jason?

This is the initial jumps and are converted on boot up to a better nop.

>
> The second thing that I found when implementing static_cpu_has() was
> that it is actually better to encapsulate the asm goto in a small inline
> which returns bool (true/false) -- gcc will happily optimize out the
> variable and only see it as a flow of control thing. I would be very
> curious if that wouldn't make gcc generate better code in cases like that.
>
> gcc 4.5.0 has a bug in that there must be a flowthrough case in the asm
> goto (you can't have it unconditionally branch one way or the other), so
> that should be the likely case and accordingly it should be annotated
> likely() so that gcc doesn't reorder. I suspect in the end one ends up
> with code like this:
>
> static __always_inline __pure bool __switch_point(...)
> {
> asm goto("1: " JUMP_LABEL_INITIAL_NOP
> /* ... patching stuff */
> : : : : t_jump);
> return false;
> t_jump:
> return true;
> }
>
> #define SWITCH_POINT(x) unlikely(__switch_point(x))
>
> I *suspect* this will resolve the need for hot/cold labels just fine.

Interesting, we could try this.

Thanks!

-- Steve




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