lkml.org 
[lkml]   [2010]   [Oct]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH] tracing: Cleanup the convoluted softirq tracepoints
    From
    Date
    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




    \
     
     \ /
      Last update: 2010-10-20 00:25    [W:0.024 / U:64.252 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site