[PATCH] perf tools: Ensure event leader stays at head of evlist after sorting

[Chun-Tse Shao <ctshao@google.com>]

From: Ian Rogers <irogers@google.com>

For evlist of a certain event/metric, the HEAD should be the event
leader. In some scenarios where uncore_xxx_0 does not exist, the event
leader is not the first element after sorting. For example, on my test
machine uncore_iio_0 does not exist, the event leader is uncore_iio_2.

However, in `evlist__cmp`, it was reordered based on the PMU name, which
makes uncore_iio_1 the HEAD of evlist, breaking the following merge
logic in `evsel__merge_aliases`.

The patch adds a loop at the end of
`parse_events__sort_events_and_fix_groups` to make sure the first
wildcard match is the earliest entry in the list, updating pointers
accordingly without breaking reordering detection.

Tested on DUT lacks uncore_iio_0, and `perf test` looks good.

Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
 tools/perf/util/parse-events.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
index 1497e1f2a08c..be825617321d 100644
--- a/tools/perf/util/parse-events.c
+++ b/tools/perf/util/parse-events.c
@@ -2251,6 +2251,33 @@ static int parse_events__sort_events_and_fix_groups(struct list_head *list)
 		}
 		last_event_was_forced_leader = (force_grouped_leader == pos);
 	}
+
+	/*
+	 * Make sure the first wildcard match is the earliest entry in the list.
+	 * Since list_sort might have reordered the aliases, the original leader
+	 * might not be at the head of the list anymore. We find the first
+	 * alias in the sorted list and make it the new leader, and redirect
+	 * all other aliases to it.
+	 */
+	list_for_each_entry(pos, list, core.node) {
+		struct evsel *l = pos->first_wildcard_match;
+
+		if (!l)
+			continue;
+
+		if (l->first_wildcard_match) {
+			/* Original leader was redirected to a new leader */
+			pos->first_wildcard_match = l->first_wildcard_match;
+		} else if (pos->core.idx < l->core.idx) {
+			/*
+			 * We are earlier than the original leader in sorted order,
+			 * and no earlier alias has claimed leadership yet.
+			 */
+			l->first_wildcard_match = pos;
+			pos->first_wildcard_match = NULL;
+		}
+	}
+
 	list_for_each_entry(pos, list, core.node) {
 		struct evsel *pos_leader = evsel__leader(pos);
 
-- 
2.54.0.545.g6539524ca2-goog