Yes, you need to turn on tracing. See my blog on here on this topic.
[root@mylinux1 tracing]# pwd
/sys/kernel/debug/tracing
[root@mylinux1 tracing]# echo pci_bus_write_config_byte > set_ftrace_filter
[root@mylinux1 tracing]# echo function > current_tracer
[root@mylinux1 tracing]# echo 1 > tracing_on
Checkout more detailed info on ftrace,perf at Brendan Gregg's blog