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

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Apr 30, 2026 at 09:35:10PM -0700, Chun-Tse Shao wrote:
> 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.

DUT?

> 
> Signed-off-by: Chun-Tse Shao <ctshao@google.com>

If this commit is from Ian, there should be Ian's sign-off before yours.

> ---
>  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;

Why 'l'?  It's not conventional and can be confusing with capital 'I'.
Can it be just 'evsel', 'first' or 'leader'?

Thanks,
Namhyung

> +
> +		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
>