Starting with clone, waitid and wait4:

  # trace -e waitid,wait4
     1.385 ( 1.385 ms): bash/12122 wait4(upid: -1, stat_addr: 0x7ffe0cee1720, options: UNTRACED|CONTINUED) = 1210 (ls)
     1.426 ( 0.002 ms): bash/12122 wait4(upid: -1, stat_addr: 0x7ffe0cee1150, options: NOHANG|UNTRACED|CONTINUED) = 0
     3.293 ( 0.604 ms): bash/1211 wait4(upid: -1, stat_addr: 0x7ffe0cee0560                             ) = 1214 (sed)
     3.342 ( 0.002 ms): bash/1211 wait4(upid: -1, stat_addr: 0x7ffe0cee01d0, options: NOHANG            ) = -1 ECHILD No child processes
     3.576 ( 0.016 ms): bash/12122 wait4(upid: -1, stat_addr: 0x7ffe0cee0550, options: NOHANG|UNTRACED|CONTINUED) = 1211 (bash)
  ^C# trace -e clone
     0.027 ( 0.000 ms): systemd/1  ... [continued]: clone()) = 1227 (systemd)
     0.050 ( 0.000 ms): systemd/1227  ... [continued]: clone()) = 0
  ^C[root@jouet ~]#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-lyf5d3y5j15wikjb6pe6ukoi@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>