4.3.2. Call Graph Tracing

4.3.2. Call Graph Tracing

This section describes how to trace incoming and outgoing function calls.

para-callgraph-simple.stp

function trace(entry_p) {
  if(tid() in trace)
    printf("%s%s%s\n",thread_indent(entry_p),
           (entry_p>0?"->":"<-"),
           probefunc())
}

global trace
probe kernel.function(@1).call {
  if (execname() == "stapio") next # skip our own helper process
  trace[tid()] = 1
  trace(1)
}
probe kernel.function(@1).return {
  trace(-1)
  delete trace[tid()]
}

probe kernel.function(@2).call { trace(1) }
probe kernel.function(@2).return { trace(-1) }
function trace(entry_p) {
  if(tid() in trace)
    printf("%s%s%s\n",thread_indent(entry_p),
           (entry_p>0?"->":"<-"),
           probefunc())
}

global trace
probe kernel.function(@1).call {
  if (execname() == "stapio") next # skip our own helper process
  trace[tid()] = 1
  trace(1)
}
probe kernel.function(@1).return {
  trace(-1)
  delete trace[tid()]
}

probe kernel.function(@2).call { trace(1) }
probe kernel.function(@2).return { trace(-1) }

para-callgraph-simple.stp takes two command-line arguments:

A trigger function (@1), which enables or disables tracing on a per-thread basis. Tracing in each thread will continue as long as the trigger function has not exited yet.
The kernel function/s whose entry/exit call you'd like to trace (@2).

para-callgraph-simple.stp uses thread_indent(); as such, its output contains the timestamp, process name, and thread ID of @2 (i.e. the probe function you are tracing). For more information about thread_indent(), refer to its entry in SystemTap Functions.

The following example contains an excerpt from the output for stap para-callgraph.stp sys_read '*@fs/*.c':

[...]
     0 klogd(1391):->sys_read
    14 klogd(1391): ->fget_light
    22 klogd(1391): <-fget_light
    27 klogd(1391): ->vfs_read
    35 klogd(1391):  ->rw_verify_area
    43 klogd(1391):  <-rw_verify_area
    49 klogd(1391):  ->kmsg_read
     0 sendmail(1696):->sys_read
    17 sendmail(1696): ->fget_light
    26 sendmail(1696): <-fget_light
    34 sendmail(1696): ->vfs_read
    44 sendmail(1696):  ->rw_verify_area
    52 sendmail(1696):  <-rw_verify_area
    58 sendmail(1696):  ->proc_file_read
    70 sendmail(1696):   ->loadavg_read_proc
    84 sendmail(1696):    ->proc_calc_metrics
    92 sendmail(1696):    <-proc_calc_metrics
    95 sendmail(1696):   <-loadavg_read_proc
   101 sendmail(1696):  <-proc_file_read
   106 sendmail(1696):  ->dnotify_parent
   115 sendmail(1696):  <-dnotify_parent
   119 sendmail(1696):  ->inotify_dentry_parent_queue_event
   127 sendmail(1696):  <-inotify_dentry_parent_queue_event
   133 sendmail(1696):  ->inotify_inode_queue_event
   141 sendmail(1696):  <-inotify_inode_queue_event
   146 sendmail(1696): <-vfs_read
   151 sendmail(1696):<-sys_read

Example 4.14. para-callgraph-simple.stp Sample Output