[PATCH v8 51/58] perf wakeup-latency: Port wakeup-latency to use python module

Ian Rogers
2026-04-28T07:21:01Z

Port the legacy Perl script wakeup-latency.pl to a python script using
the perf module in tools/perf/python.

The new script uses a class-based architecture and leverages the
perf.session API for event processing.

It measures wakeup latency by tracking timestamps of
sched:sched_wakeup and sched:sched_switch events.

Complications:
- Used min() and max() built-in functions instead of if blocks to
satisfy pylint recommendations.
- pylint warns about the module name not being snake_case, but it is
kept for consistency with the original script name.

Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
---
v2:

- Fixed Wakeup Latency Logic: Modified the script to track wakeup
timestamps per task (using sample.pid as the key) instead of per
CPU. This ensures that context switches are correctly paired with
the specific task that was woken up, even if multiple tasks are
woken up on the same CPU or if a task is migrated to a different
CPU before running.

- Prevented Memory Growth: Added del self.last_wakeup[next_pid] after
successful latency calculation to prevent the dictionary from
growing unbounded over time.

- Added Error Tracking: Added try-except blocks around tracepoint
field access in process_event() and tracked missing fields in self.
unhandled instead of ignoring them.
---
tools/perf/python/wakeup-latency.py | 88 +++++++++++++++++++++++++++++
1 file changed, 88 insertions(+)
create mode 100755 tools/perf/python/wakeup-latency.py

diff --git a/tools/perf/python/wakeup-latency.py b/tools/perf/python/wakeup-latency.py
new file mode 100755
index 000000000000..1b0db115abcf
--- /dev/null
+++ b/tools/perf/python/wakeup-latency.py
@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-only
+"""Display avg/min/max wakeup latency."""
+
+import argparse
+from collections import defaultdict
+import sys
+from typing import Optional, Dict
+import perf
+
+class WakeupLatency:
+ """Tracks and displays wakeup latency statistics."""
+ def __init__(self) -> None:
+ self.last_wakeup: Dict[int, int] = defaultdict(int)
+ self.max_wakeup_latency = 0
+ self.min_wakeup_latency = 1000000000
+ self.total_wakeup_latency = 0
+ self.total_wakeups = 0
+ self.unhandled: Dict[str, int] = defaultdict(int)
+ self.session: Optional[perf.session] = None
+
+ def process_event(self, sample: perf.sample_event) -> None:
+ """Process events."""
+ event_name = str(sample.evsel)
+ sample_time = sample.sample_time
+
+ if "sched:sched_wakeup" in event_name:
+ try:
+ pid = sample.pid
+ self.last_wakeup[pid] = sample_time
+ except AttributeError:
+ self.unhandled[event_name] += 1
+ elif "sched:sched_switch" in event_name:
+ try:
+ next_pid = sample.next_pid
+ wakeup_ts = self.last_wakeup.get(next_pid, 0)
+ if wakeup_ts:
+ latency = sample_time - wakeup_ts
+ self.max_wakeup_latency = max(self.max_wakeup_latency, latency)
+ self.min_wakeup_latency = min(self.min_wakeup_latency, latency)
+ self.total_wakeup_latency += latency
+ self.total_wakeups += 1
+ del self.last_wakeup[next_pid]
+ except AttributeError:
+ self.unhandled[event_name] += 1
+ else:
+ self.unhandled[event_name] += 1
+
+ def print_totals(self) -> None:
+ """Print summary statistics."""
+ print("wakeup_latency stats:\n")
+ print(f"total_wakeups: {self.total_wakeups}")
+ if self.total_wakeups:
+ avg = self.total_wakeup_latency // self.total_wakeups
+ print(f"avg_wakeup_latency (ns): {avg}")
+ else:
+ print("avg_wakeup_latency (ns): N/A")
+ print(f"min_wakeup_latency (ns): {self.min_wakeup_latency}")
+ print(f"max_wakeup_latency (ns): {self.max_wakeup_latency}")
+
+ if self.unhandled:
+ print("\nunhandled events:\n")
+ print(f"{'event':<40s} {'count':>10s}")
+ print(f"{'-'*40} {'-'*10}")
+ for event_name, count in self.unhandled.items():
+ print(f"{event_name:<40s} {count:10d}")
+
+ def run(self, input_file: str) -> None:
+ """Run the session."""
+ self.session = perf.session(perf.data(input_file), sample=self.process_event)
+ self.session.process_events()
+ self.print_totals()
+
+def main() -> None:
+ """Main function."""
+ parser = argparse.ArgumentParser(description="Trace wakeup latency")
+ parser.add_argument("-i", "--input", default="perf.data", help="Input file")
+ args = parser.parse_args()
+
+ analyzer = WakeupLatency()
+ try:
+ analyzer.run(args.input)
+ except IOError as e:
+ print(e, file=sys.stderr)
+ sys.exit(1)
+
+if __name__ == "__main__":
+ main()
--
2.54.0.545.g6539524ca2-goog