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    SubjectRe: [PATCH] tracing: Cleanup the convoluted softirq tracepoints
    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.


    -- Steve

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