On Thu, 13 Nov 2014 09:23:45 +0000
Markos Chandras <Markos.Chandras@xxxxxxxxxx> wrote:
> Hi,
>
> I am trying to understand why ftrace function graph doesn't work when
> using static ftrace on MIPS. So, what happens when I do 'echo
> function_graph > current_tracer' is that the ftrace_graph_caller from
> mcount.S is executed once. The function that called it is
> 'core_kernel_data()' from __register_ftrace_function in
> kernel/trace/ftrace.c
>
> However, this is the only function that is reported in the trace file
>
> # cat trace
> # tracer: function_graph
> #
> # CPU DURATION FUNCTION CALLS
> # | | | | | | |
> 0) | core_kernel_data() {
> 0) 0.000 us | } /* core_kernel_data */
>
> The reason that the ftrace_graph_caller is never executed after that is
> that the following as far as I understand:
>
> NESTED(_mcount, PT_SIZE, ra)
> PTR_LA t1, ftrace_stub
> PTR_L t2, ftrace_trace_function /* Prepare t2 for (1) */
> bne t1, t2, static_trace
> nop
>
> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> PTR_L t3, ftrace_graph_return
> bne t1, t3, ftrace_graph_caller
> nop
> PTR_LA t1, ftrace_graph_entry_stub
> PTR_L t3, ftrace_graph_entry
> bne t1, t3, ftrace_graph_caller
> nop
> #endif
>
> The previous 3 conditionals exists in arch/mips/kernel/mcount.S.
> Originally, ftrace_trace_function == ftrace_stub, so the first
> conditional is not taken and we end up executed the ftrace_graph_caller.
> All good.
> However, later on, ftrace_trace_function is set to 'ftrace_ops_no_ops',
> so the first 'bne' is taken and the ftrace_graph_caller is never
> executed after that. It is not clear to me if this behaviour is expected
> so I used QEMU to get a backtrace when ftrace_trace_function is set to
> ftrace_ops_no_ops.
Looks like the static tracing code in wrong. The function graph code
should be tested every time.
-- Steve
