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  • I turned on tracing, but that didn't do anything. # echo SyS_read >set_graph_function ; # echo function_graph > current_tracer ; # echo 1 > tracing_on Then I cat trace or trace_pipe, nothing is displayed out. I am specifically trying to track the pci_bus_write_config_byte function, but I get the same result. Commented Mar 13, 2017 at 3:16
  • See my updated answer. You'll see any output, only when that function get's called. you can test with other simple syscalls, by echoing them into set_ftrace_filter Commented Mar 13, 2017 at 11:27
  • Makes sense, so I did exactly that. I'm also testing systemtap which works quite well. It seems to be the superior system, similar'ish to dtrace. Commented Mar 13, 2017 at 16:01
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    What kernel version is this? As SyS_read isn't available anymore on x86 kernels. It's now something like: __x64_sys_read. And tracing_on should be on by default unless you did something to turn it off. Commented May 27, 2019 at 23:50
  • same issue with me. On 5.4 LTS kernel and fed up of ftrace. The function that I am trying to trace indeed gets called because I get dmesg logs from it. But ftrace does not capture it. Commented Feb 9, 2022 at 8:01