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

[Ian Rogers]

On Thu, Apr 30, 2026 at 9:35 PM Chun-Tse Shao <ctshao@xxxxxxxxxx> wrote:
>
> From: Ian Rogers <irogers@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>

Fixes: 347c2f0a0988 ("perf parse-events: Sort and group parsed events")

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks!
Ian

> ---
>  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
>