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    SubjectRe: [PATCH] tracing/function-branch-tracer: support for x86-64
    2008/12/2 Ingo Molnar <mingo@elte.hu>:
    > it's this bit:
    >
    >> +#ifdef CONFIG_X86_64
    >> + "1: movq (%[parent_old]), %[old]\n"
    >> + "2: movq %[return_hooker], (%[parent_replaced])\n"
    >> +#else
    >> "1: movl (%[parent_old]), %[old]\n"
    >> "2: movl %[return_hooker], (%[parent_replaced])\n"
    >> +#endif
    >> " movl $0, %[faulted]\n"
    >>
    >> ".section .fixup, \"ax\"\n"
    >> @@ -476,8 +481,13 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
    >> ".previous\n"
    >>
    >> ".section __ex_table, \"a\"\n"
    >> +#ifdef CONFIG_X86_64
    >> + " .quad 1b, 3b\n"
    >> + " .quad 2b, 3b\n"
    >> +#else
    >> " .long 1b, 3b\n"
    >> " .long 2b, 3b\n"
    >> +#endif
    >
    > i think we might want to introduce a few assembly helpers/defines to
    > standardize such constructs - they are quite frequent. Something like:
    >
    > " .ip_ptr 1b, 3b\n"
    > " .ip_ptr 2b, 3b\n"


    Yeah. Note that in this particular case I could use EX_TABLE() macro
    too (if it is defined for x86, I can't verify yet).

    > (Cc:-ed Alexander and Cyrill who have done work in this area recently)
    >
    > we might also introduce instruction helpers:
    >
    > "1: mov_ptr (%[parent_old]), %[old]\n"
    > "2: mov_ptr %[return_hooker], (%[parent_replaced])\n"
    >
    > and avoid the #ifdefs altogether.


    Good idea.

    >
    >> Note that arch/x86/process_64.c is not traced, as in X86-32. I first
    >> thought __switch_to() was responsible of crashes during tracing because
    >> I believed current task were changed inside but that's actually not the
    >> case (actually yes, but not the "current" pointer).
    >>
    >> So I will have to investigate to find the functions that harm here, to
    >> enable tracing of the other functions inside (but there is no issue at
    >> this time, while process_64.c stays out of -pg flags).
    >
    > ok. You should take a look at arch/x86/include/asm/system.h's switch_to()
    > macros - it has special stack switching smarts for context-switching.
    >
    > the other special stack layout case is the starting of kernel threads -
    > ret_from_fork and its details in process*.c.


    Ok, I will have a look inside.

    >> A little possible race condition is fixed inside this patch too. When
    >> the tracer allocate a return stack dynamically, the current depth is
    >> not initialized before but after. An interrupt could occur at this time
    >> and, after seeing that the return stack is allocated, the tracer could
    >> try to trace it with a random uninitialized depth. It's a prevention,
    >> even if I hadn't problems with it.
    >
    >> index 08b536a..1e9379d 100644
    >> --- a/kernel/trace/ftrace.c
    >> +++ b/kernel/trace/ftrace.c
    >> @@ -1673,8 +1673,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
    >> }
    >>
    >> if (t->ret_stack == NULL) {
    >> - t->ret_stack = ret_stack_list[start++];
    >> t->curr_ret_stack = -1;
    >> + t->ret_stack = ret_stack_list[start++];
    >> atomic_set(&t->trace_overrun, 0);
    >> }
    >> } while_each_thread(g, t);
    >
    > okay - the (optimization-)safe way to tell the compiler about such local
    > CPU ordering information is:
    >
    > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
    > index 08b536a..f724996 100644
    > --- a/kernel/trace/ftrace.c
    > +++ b/kernel/trace/ftrace.c
    > @@ -1673,8 +1673,10 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
    > }
    >
    > if (t->ret_stack == NULL) {
    > - t->ret_stack = ret_stack_list[start++];
    > t->curr_ret_stack = -1;
    > + /* Make sure IRQs see the -1 first: */
    > + barrier();
    > + t->ret_stack = ret_stack_list[start++];
    > atomic_set(&t->trace_overrun, 0);
    > }
    > } while_each_thread(g, t);
    >
    > i changed the patch to do that.


    Oops, I forgot it one more time.

    That reminds me: I don't know why I always confuse the name of this
    tracer: function-branch-tracer instead of function-graph-tracer.
    Weird :-)


    > All in one, great stuff!


    Thanks :-)


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