Re: 4.8.6-rt5 ftrace function_graph hangs

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 18 Nov 2016 14:23:24 +0800
He Zhe <zhe.he@windriver.com> wrote:

> Hi,
> 
> I'm using 4.8.6-rt5 and find the following issue on nxp-t4xxx board/qemuppc/qemux86-64.
> When executing "echo function_graph > /sys/kernel/debug/tracing/current_tracer" the system hangs.
> I traced the kernel to where it dies but don't know how to fix. Please help. Thank you.
> 
> (gdb) b ftrace_startup
> 
> ...
> 
> Run till exit from #0 ftrace_startup (ops=0xc0cf1704 <graph_ops>, command=8) at /PATH/TO/kernel/trace/ftrace.c:2653
> 0xc00eb768 in register_ftrace_graph (retfunc=0xc010186c <trace_graph_return>, entryfunc=0xc01014c4 <trace_graph_entry>)
>     at /PATH/TO/kernel-source/kernel/trace/ftrace.c:5908
> 5908 ret = ftrace_startup(&graph_ops, FTRACE_START_FUNC_RET);
> Value returned is $1 = 0
> (gdb) l
> 5903 */
> 5904 __ftrace_graph_entry = entryfunc;
> 5905 ftrace_graph_entry = ftrace_graph_entry_test;
> 5906 update_function_graph_func();
> 5907
> 5908 ret = ftrace_startup(&graph_ops, FTRACE_START_FUNC_RET);
> 5909 out:
> 5910 mutex_unlock(&ftrace_lock);
> 5911 return ret;
> 5912 }
> (gdb) n
> 5910 mutex_unlock(&ftrace_lock);
> (gdb) n
> 
> hangs...
> 

Can't you get a backtrace from this? It's running under qemu right? So
it should be able to break it and see where it is at.

Note, my PowerPC box has just died, so I'm no longer able to work on
it, and don't have time to set up a qemu ppc image at the moment.

-- Steve