Messages in this thread | | | Subject | Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints | From | Steven Rostedt <> | Date | Tue, 19 Oct 2010 18:23:25 -0400 |
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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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