The 'trace' sort key is to show tracepoint event output using either
print fmt or plugin.  For example sched_switch event (using plugin) will
show output like below:

  # perf record -e sched:sched_switch -a usleep 10
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.197 MB perf.data (69 samples) ]
  #

  $ perf report -s trace --stdio
  ...
  # Overhead  Trace output
  # ........  ...................................................
  #
       9.48%  swapper/0:0 [120] R ==> transmission-gt:17773 [120]
       9.48%  transmission-gt:17773 [120] S ==> swapper/0:0 [120]
       9.04%  swapper/2:0 [120] R ==> transmission-gt:17773 [120]
       8.92%  transmission-gt:17773 [120] S ==> swapper/2:0 [120]
       5.25%  swapper/0:0 [120] R ==> kworker/0:1H:109 [100]
       5.21%  kworker/0:1H:109 [100] S ==> swapper/0:0 [120]
       1.78%  swapper/3:0 [120] R ==> transmission-gt:17773 [120]
       1.78%  transmission-gt:17773 [120] S ==> swapper/3:0 [120]
       1.53%  Xephyr:6524 [120] S ==> swapper/0:0 [120]
       1.53%  swapper/0:0 [120] R ==> Xephyr:6524 [120]
       1.17%  swapper/2:0 [120] R ==> irq/33-iwlwifi:233 [49]
       1.13%  irq/33-iwlwifi:233 [49] S ==> swapper/2:0 [120]

Note that the 'trace' sort key works only for tracepoint events.  If
it's used to other type of events, just "N/A" will be printed.
Suggested-and-acked-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1450804030-29193-8-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>