There were some combinations of these switches that are not so
appropriate IMHO.

Since there are implicit priorities between them and they worked well
anyway, but it ends up opening useless duplicated events.

For example, 'perf stat -t <pid> -a' will open multiple events for the
thread instead of one.

Add explicit checks and warn user in perf_target__validate().
Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1335417327-11796-7-git-send-email-namhyung.kim@lge.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>