The kernel parts are not that useful:

  # trace -m 512 -e nanosleep --call dwarf  usleep 1
     0.065 ( 0.065 ms): usleep/18732 nanosleep(rqtp: 0x7ffc4ee4e200) = 0
                                       syscall_slow_exit_work ([kernel.kallsyms])
                                       do_syscall_64 ([kernel.kallsyms])
                                       return_from_SYSCALL_64 ([kernel.kallsyms])
                                       __nanosleep (/usr/lib64/libc-2.22.so)
                                       usleep (/usr/lib64/libc-2.22.so)
                                       main (/usr/bin/usleep)
                                       __libc_start_main (/usr/lib64/libc-2.22.so)
                                       _start (/usr/bin/usleep)
  #

So lets just use perf_event_attr.exclude_callchain_kernel to avoid
collecting it in the ring buffer:

  # trace -m 512 -e nanosleep --call dwarf  usleep 1
     0.063 ( 0.063 ms): usleep/19212 nanosleep(rqtp: 0x7ffc3df10fb0) = 0
                                       __nanosleep (/usr/lib64/libc-2.22.so)
                                       usleep (/usr/lib64/libc-2.22.so)
                                       main (/usr/bin/usleep)
                                       __libc_start_main (/usr/lib64/libc-2.22.so)
                                       _start (/usr/bin/usleep)
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-qctu3gqhpim0dfbcp9d86c91@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>