It's handier than an <unknown> entry.
Also replace the kernel/user column with a more compact version:

  0.52              cc1  [k]  page_fault
  0.57               :0  [k]   _spin_lock
  0.59            :7506  [.]  <unknown>
  0.69               as  [.]  /usr/bin/as: <unknown>
  0.76              cc1  [.]  /lib64/libc-2.8.so: _int_free
  0.92              cc1  [k]  clear_page_c
  1.00            :7465  [.]  <unknown>
  1.43              cc1  [.]  /lib64/libc-2.8.so: memset
  1.86              cc1  [.]  /lib64/libc-2.8.so: _int_malloc
 70.33              cc1  [.]  /usr/libexec/gcc/x86_64-redhat-linux/4.3.2/cc1: <unknown>
Signed-off-by: Ingo Molnar <mingo@elte.hu>