perf evsel: Add comment for 'idx' member in 'struct perf_sample_id

The 'idx' member was added as preparation for AUX area sampling. Add a
comment to describe why.
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/83ff264f-84c3-5372-8976-dd9293d20c6f@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.h

@@ -23,6 +23,13 @@ struct perf_sample_id {
 	struct hlist_node 	node;
 	u64		 	id;
 	struct evsel		*evsel;
+       /*
+	* 'idx' will be used for AUX area sampling. A sample will have AUX area
+	* data that will be queued for decoding, where there are separate
+	* queues for each CPU (per-cpu tracing) or task (per-thread tracing).
+	* The sample ID can be used to lookup 'idx' which is effectively the
+	* queue number.
+	*/
 	int			idx;
 	int			cpu;
 	pid_t			tid;