[PATCH] perf sched: Suppress latency table output when trace samples are missing

[Aaron Tomlin <atomlin@atomlin.com>]

When 'perf sched latency' is executed on a perf.data file that lacks
tracepoint samples (i.e., a file recorded without the -R flag or
containing only non-tracepoint events), perf_session__has_traces()
correctly outputs an error message. However, perf_sched__read_events()
subsequently falls through and returns 0 (success).

Consequently, caller functions such as perf_sched__lat() assume event
processing succeeded and proceed to render empty latency header tables
and total summary statistics.

Fix this behaviour by ensuring perf_sched__read_events() aborts early
and returns a suitable error code when perf_session__has_traces()
evaluates to false. This prevents superfluous table output from being
generated when no trace data is available.

Fixes: 27295592c22e ("perf session: Share the common trace sample_check routine as perf_session__has_traces")
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 tools/perf/builtin-sched.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 7fd63a9db457..b3cf678573e0 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -2001,7 +2001,7 @@ static int perf_sched__read_events(struct perf_sched *sched)
 		.mode  = PERF_DATA_MODE_READ,
 		.force = sched->force,
 	};
-	int rc = -1;
+	int rc = -1, err;
 
 	session = perf_session__new(&data, &sched->tool);
 	if (IS_ERR(session)) {
@@ -2018,18 +2018,19 @@ static int perf_sched__read_events(struct perf_sched *sched)
 	if (perf_session__set_tracepoints_handlers(session, handlers))
 		goto out_delete;
 
-	if (perf_session__has_traces(session, "record -R")) {
-		int err = perf_session__process_events(session);
-		if (err) {
-			pr_err("Failed to process events, error %d", err);
-			goto out_delete;
-		}
+	if (!perf_session__has_traces(session, "record -R"))
+		goto out_delete;
 
-		sched->nr_events      = session->evlist->stats.nr_events[0];
-		sched->nr_lost_events = session->evlist->stats.total_lost;
-		sched->nr_lost_chunks = session->evlist->stats.nr_events[PERF_RECORD_LOST];
+	err = perf_session__process_events(session);
+	if (err) {
+		pr_err("Failed to process events, error %d", err);
+		goto out_delete;
 	}
 
+	sched->nr_events      = session->evlist->stats.nr_events[0];
+	sched->nr_lost_events = session->evlist->stats.total_lost;
+	sched->nr_lost_chunks = session->evlist->stats.nr_events[PERF_RECORD_LOST];
+
 	rc = 0;
 out_delete:
 	perf_session__delete(session);
-- 
2.54.0