Messages in this thread Patch in this message | | | Date | Tue, 2 Dec 2008 10:02:05 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] tracing/function-branch-tracer: support for x86-64 |
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* 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.
> 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.
> 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!
Ingo
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