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: Adrian 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.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
index 9cd6e3ae479a719896a7907fc5148bcad553e537..efe08065838f0c8b2423d0641bcffcde706ce85f 100644 (file)
--- a/tools/perf/util/evsel.h
+++ b/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;