arm64: ftrace: function_graph tracer not working without dynamic instrumentation

[Steven Rostedt]

On Thu, 26 Oct 2017 09:50:13 +0100
Julien Thierry <julien.thierry at arm.com> wrote:

> Hi,
> 
> On arm64, I noticed that when CONFIG_DYNAMIC_FTRACE is not enabled, 
> using the function_graph tracer in ftrace alway gives empty traces.
> 
> The issues seems to be that the ftrace_trace_function is set to a 
> function different than ftrace_stub when running mcount. If I allow 
> mcount to continue beyond that check, I get the graph traces.
> 
> Guidelines given in Documentation/trace/ftrace-design.txt suggest that

Heh, I haven't looked at that document since I reviewed it.
 
> ftrace_trace_function != ftrace_stub should be checked before any 
> function_graph related values and if the check is true then we shouldn't 
> try to call ftrace_trace_function.
> 
> Investigating a bit more, ftrace_trace_function gets set 
> ftrace_ops_no_ops in update_ftrace_function called when setting up the 
> function_graph tracer.
> 
> The issue does not occur when dynamic ftrace is supported because we 
> just replace some nops at known locations and replace them with branches.
> 
> It seemed to me that the arm64 side of things is following the 
> guidelines in ftrace-design, so I am unsure whether it is intended that 
> the ftrace_trace_function is not set to ftrace_stub when using the 
> function_graph tracer.

With all honesty, I didn't look at the "static" part too much. Yeah,
what's in the document is wrong. Just follow what x86 does. I'll have
to make a note to myself to update that document.

Thanks!

-- Steve