On Thu, 26 Oct 2017 09:50:13 +0100 Julien Thierry <julien.thie...@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
