When no target cpu/user/task option is given, perf trace will do its job
system wide for all online cpus.  Make it explicit to reduce possible
confusion when reading code.

No functional changes intended.
Signed-off-by: NNamhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1349413336-26936-3-git-send-email-namhyung@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>