假设debugfs已经挂载到了/sys/kernel/debug目录下，下面的小脚本用来抓取unlink系统调用的耗时：
cd /sys/kernel/debug/tracing
echo function_graph > current_tracer
echo > tracing_on
echo do_unlinkat > set_graph_function
echo do_unlinkat > set_ftrace_filter
echo > trace
cat trace_pipe > /tmp/ftrace.unlink.out &
sleep
#cat trace > /tmp/ftrace.unlink.out
echo nop > current_tracer
echo > tracing_on