Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

[Steven Rostedt]

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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