Crashes and anomaly with debug/tracing

[Xen]

I am on a 16.04 Kubuntu system installed on USB 2.0 with kernel 
4.4.0-21-generic #37-Ubuntu SMP on x86_64.

Previously I was running the same system in a live session.

When I echo "function" to /sys/kernel/debug/tracing/current_tracer, the 
system reboots.

Only the first trace I can get after reboot is helpful. I do not seem to 
be able to get the output I want after.

I might have a lot of output but my function call is not actually in 
there.

My method is to turn the tracer on (1 > tracing_on) and then run my 
program, and then turn it off again.

If I echo 1 > free_buffer the output is botched and I get only very 
meager output (say 40 lines) and not what I want. "function_graph" does 
work and does not reboot the system.

While inserting a module that did nothing special and worked as intended 
(very small test module so to speak), if the tracer was on 
(function_graph) and I did insmod, the kernel would panic and I think 
also freeze.

So I could not trace anything happening in my module or its calls.

Before I installed the system and I was running from DVD with a cache 
partition on harddisk, the tracer seemed to have worked better and 
free_buffers seemed not to have caused that aberrant behaviour.

Currently though I cannot free the buffer or the output gets botched 
(severely limited output) and without clearing it apparently I also do 
not get the output I want, so it comes down to running one trace per 
reboot and that's it.

I don't know if this needs to be said anywhere else but they told me to 
go here first.

I have not filed a bug.

Regards.