[PATCH v3 13/16] perf test trace_summary: Improve error diagnostics

[Ian Rogers <irogers@google.com>]

When pattern matching fails in test_perf_trace(), print the command
that failed along with the actual match count, the matching lines
found, and the last 20 lines of output to aid debugging.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/tests/shell/trace_summary.sh | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/perf/tests/shell/trace_summary.sh b/tools/perf/tests/shell/trace_summary.sh
index b80dea77cec6..4589b4581419 100755
--- a/tools/perf/tests/shell/trace_summary.sh
+++ b/tools/perf/tests/shell/trace_summary.sh
@@ -28,10 +28,16 @@ test_perf_trace() {
 
     count=$(grep -E -c -m 3 "${search}" ${OUTPUT})
     if [ "${count}" != "3" ]; then
-	echo "Error: cannot find enough pattern ${search} in the output"
-	cat ${OUTPUT}
-	rm -f ${OUTPUT}
-	exit 1
+        echo "Error: cannot find enough pattern ${search} (count=${count}) in output of:"
+        echo "Error:   perf trace ${args} -- ${workload}"
+        echo "Error: matched lines:"
+        grep -E "${search}" ${OUTPUT} || echo "none"
+        echo "Error: last 20 lines of output:"
+        # The summary is printed after any trace output, so show the end of
+        # the file: with -S the head of it is the trace, not what is matched.
+        tail -n 20 ${OUTPUT}
+        rm -f ${OUTPUT}
+        exit 1
     fi
 }
 
-- 
2.55.0.1082.g2b9226bbc0-goog