Date:   Mon, 16 Aug 2021 17:21:33 -0700
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH] perf script python: Allow reporting [un]throttle

perf_events may sometimes throttle an event due to creating too many
samples during a given timer tick. As of now, the perf tool will not
report on throttling, which means this is a silent error. Implement a
callback for the throttle and unthrottle events within the Python
scripting engine, which can allow scripts to detect and report when
events may have been lost due to throttling.

A python script could simply define throttle() and unthrottle()
functions to begin receiving them, e.g.:

```
from __future__ import print_function

def process_event(param_dict):
    print("event cpu={} time={}".format(
        param_dict["sample"]["cpu"], param_dict["sample"]["time"]))

def throttle(*args):
    print("throttle(time={}, cpu={}, pid={}, tid={})".format(*args))

def unthrottle(*args):
    print("unthrottle(time={}, cpu={}, pid={}, tid={})".format(*args))
```

Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
---

Since this mailing list thread[1] I've been wondering about ways to
detect and handle throttling. Perf will warn when events are missed
because the ring buffer filled up, but it will not give any indication
about the throttling -- except for the throttle message logged by the
kernel. Ideally, I'd like to also extend the other perf tools to give
a warning, but detecting it after the fact via a script was easiest to
implement, and most flexible for me to use. I'd appreciate feedback on
this change as well as what such a warning in perf record/report would
look like. For example:

[ perf record: WARNING: samples were throttled %u times for %u seconds ]

[1]: https://lore.kernel.org/linux-perf-users/87lf6rclcm.fsf@stepbren-lnx.us.oracle.com/T/#u

 tools/perf/builtin-script.c                   | 13 ++++++++
 .../scripting-engines/trace-event-python.c    | 30 +++++++++++++++++++
 tools/perf/util/trace-event.h                 |  3 ++
 3 files changed, 46 insertions(+)

diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
index 064da7f3618d..072869a35f85 100644
--- a/tools/perf/builtin-script.c
+++ b/tools/perf/builtin-script.c
@@ -2492,6 +2492,17 @@ process_lost_event(struct perf_tool *tool,
 			   sample->tid);
 }
 
+static int
+process_throttle_event(struct perf_tool *tool __maybe_unused,
+		       union perf_event *event,
+		       struct perf_sample *sample,
+		       struct machine *machine)
+{
+	if (scripting_ops && scripting_ops->process_throttle)
+		scripting_ops->process_throttle(event, sample, machine);
+	return 0;
+}
+
 static int
 process_finished_round_event(struct perf_tool *tool __maybe_unused,
 			     union perf_event *event,
@@ -3652,6 +3663,8 @@ int cmd_script(int argc, const char **argv)
 			.stat_config	 = process_stat_config_event,
 			.thread_map	 = process_thread_map_event,
 			.cpu_map	 = process_cpu_map_event,
+			.throttle	 = process_throttle_event,
+			.unthrottle	 = process_throttle_event,
 			.ordered_events	 = true,
 			.ordering_requires_timestamps = true,
 		},
diff --git a/tools/perf/util/scripting-engines/trace-event-python.c b/tools/perf/util/scripting-engines/trace-event-python.c
index 69129e2aa7a1..5ef1ba5e29bb 100644
--- a/tools/perf/util/scripting-engines/trace-event-python.c
+++ b/tools/perf/util/scripting-engines/trace-event-python.c
@@ -1422,6 +1422,35 @@ static void python_process_event(union perf_event *event,
 	}
 }
 
+static void python_process_throttle(union perf_event *event,
+				    struct perf_sample *sample,
+				    struct machine *machine)
+{
+	const char *handler_name;
+	PyObject *handler, *t;
+
+	if (event->header.type == PERF_RECORD_THROTTLE)
+		handler_name = "throttle";
+	else
+		handler_name = "unthrottle";
+	handler = get_handler(handler_name);
+	if (!handler)
+		return;
+
+	t = tuple_new(4);
+	if (!t)
+		return;
+
+	tuple_set_u64(t, 0, sample->time);
+	tuple_set_s32(t, 1, sample->cpu);
+	tuple_set_s32(t, 2, sample->pid);
+	tuple_set_s32(t, 3, sample->tid);
+
+	call_object(handler, t, handler_name);
+
+	Py_DECREF(t);
+}
+
 static void python_do_process_switch(union perf_event *event,
 				     struct perf_sample *sample,
 				     struct machine *machine)
@@ -2079,5 +2108,6 @@ struct scripting_ops python_scripting_ops = {
 	.process_auxtrace_error	= python_process_auxtrace_error,
 	.process_stat		= python_process_stat,
 	.process_stat_interval	= python_process_stat_interval,
+	.process_throttle	= python_process_throttle,
 	.generate_script	= python_generate_script,
 };
diff --git a/tools/perf/util/trace-event.h b/tools/perf/util/trace-event.h
index 54aadeedf28c..640981105788 100644
--- a/tools/perf/util/trace-event.h
+++ b/tools/perf/util/trace-event.h
@@ -90,6 +90,9 @@ struct scripting_ops {
 	void (*process_stat)(struct perf_stat_config *config,
 			     struct evsel *evsel, u64 tstamp);
 	void (*process_stat_interval)(u64 tstamp);
+	void (*process_throttle)(union perf_event *event,
+				 struct perf_sample *sample,
+				 struct machine *machine);
 	int (*generate_script) (struct tep_handle *pevent, const char *outfile);
 };
 
-- 
2.30.2

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