title: No such device when I use adb to show /sys/kernel/debug/tracing/available_filter_functions
body: <p>Guys.<br/></p>
  <p>I am trying to study virtual memory management on Nexus 6P(An Old Android Smartphone)<br/></p>
  <p>Because the default kernel in AOSP didn't turn CONFIG_FUNCITON_TRACER on, and I can't use function tracer to study memory management related functions. <br/></p>
  <p>So I downloaded android kernel source code, turn on config_function_tracer and the other configs its depends, rebuilt kernel. Then I put the rebuilt kernel image file into aosp, rebuilt boot image , and flash the building output images into the phone.<br/></p>
  <p>the phone can boot normally, everything is ok but I can't 'cat   /sys/kernel/debug/tracing/available_filter_functions' normally, it prompts &quot;no such device&quot;.<br/></p>
  <p>I checked the selinux label, and I turn off the selinux by command 'setenforce permissive', it can't help.<br/></p>
  <p>can anyone help me to fix this problem?<br/></p>
  <pre><code>C:\Users\dvdface&gt;adb shell
  angler:/ # cd /sys/kernel/debug/tracing/
  angler:/sys/kernel/debug/tracing # ls
  README                     dyn_ftrace_total_info per_cpu            set_ftrace_pid     tracing_cpumask
  available_events           enabled_functions     printk_formats     set_graph_function tracing_on
  available_filter_functions events                saved_cmdlines     trace              tracing_thresh
  available_tracers          free_buffer           saved_tgids        trace_clock
  buffer_size_kb             instances             set_event          trace_marker
  buffer_total_size_kb       max_graph_depth       set_ftrace_filter  trace_options
  current_tracer             options               set_ftrace_notrace trace_pipe
  angler:/sys/kernel/debug/tracing # cat available_filter_functions
  cat: available_filter_functions: No such device
  </code></pre>