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    SubjectRe: [PATCH] tracing/function-branch-tracer: support for x86-64

    Hmm, I had issues with my mail server so I just received this. I was
    porting it to x86-64 last night too.

    On Tue, 2 Dec 2008, Ingo Molnar wrote:

    >
    > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >
    > > This patch implements the support for function branch tracer under x86-64.
    > > Both static and dynamic tracing are supported.
    >
    > Fantastic stuff! :-)
    >
    > > Small note: Ingo, I have only one test box and I had to install a 64
    > > bits distro to make this patch. So I can't verify if it breaks
    > > something in x86-32. I don't know what could be broken here but we
    > > never know. For further patches, I will use a virtual machine to test
    > > under 32.
    >
    > that's OK. The patch looks fairly safe on the 32-bit side.
    >
    > > This causes some small CPP conditional asm on arch/x86/kernel/ftrace.c
    > > I wanted to use probe_kernel_read/write to make the return address
    > > saving/patching code more generic but it causes tracing recursion.
    >
    > 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"
    >
    > (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.

    I fixed this in my last patch queue.

    >
    > > 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.

    I'm hitting some crashes but it does not seem to be related to this.
    I'm still investigating, but it looks like it is due to some strange race
    because I can run for hours sometimes and other times it crashes right
    away.

    >
    > > 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.
    >
    > All in one, great stuff!

    Agree, this is really awesome. I'm also working on a way to trigger
    specific functions to trace instead of tracing all functions.

    -- Steve



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