[RFC] perf evlist: Warn if event group has mixed sw/hw events

[Namhyung Kim]

I found that order of events in a group impacts performance during the
open.  If a group has a software event as a leader and has other
hardware events, the lead needs to be moved to a hardware context.
This includes RCU synchronization which takes about 20 msec on my
system.  And this is just for a single group, so total time increases
in proportion to the number of event groups and the number of cpus.

On my 36 cpu system, opening 3 groups system-wide takes more than 2
seconds.  You can see and compare it easily with the following:

  $ time ./perf stat -a -e '{cs,cycles},{cs,cycles},{cs,cycles}' sleep 1
  ...
       1.006333430 seconds time elapsed

  real	0m3.969s
  user	0m0.089s
  sys	0m0.074s

  $ time ./perf stat -a -e '{cycles,cs},{cycles,cs},{cycles,cs}' sleep 1
  ...
       1.006755292 seconds time elapsed

  real	0m1.144s
  user	0m0.067s
  sys	0m0.083s

This patch just added a warning before running it.  I'd really want to
fix the kernel if possible but don't have a good idea.  Thoughts?

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 tools/perf/builtin-record.c |  2 +
 tools/perf/builtin-stat.c   |  2 +
 tools/perf/builtin-top.c    |  2 +
 tools/perf/util/evlist.c    | 78 +++++++++++++++++++++++++++++++++++++
 tools/perf/util/evlist.h    |  1 +
 5 files changed, 85 insertions(+)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index adf311d15d3d..c0b08cacbae0 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -912,6 +912,8 @@ static int record__open(struct record *rec)
 
 	perf_evlist__config(evlist, opts, &callchain_param);
 
+	evlist__warn_mixed_group(evlist);
+
 	evlist__for_each_entry(evlist, pos) {
 try_again:
 		if (evsel__open(pos, pos->core.cpus, pos->core.threads) < 0) {
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index b01af171d94f..d5d4e02bda69 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -738,6 +738,8 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
 	if (affinity__setup(&affinity) < 0)
 		return -1;
 
+	evlist__warn_mixed_group(evsel_list);
+
 	evlist__for_each_cpu (evsel_list, i, cpu) {
 		affinity__set(&affinity, cpu);
 
diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 7c64134472c7..9ad319cea948 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -1027,6 +1027,8 @@ static int perf_top__start_counters(struct perf_top *top)
 
 	perf_evlist__config(evlist, opts, &callchain_param);
 
+	evlist__warn_mixed_group(evlist);
+
 	evlist__for_each_entry(evlist, counter) {
 try_again:
 		if (evsel__open(counter, top->evlist->core.cpus,
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 8bdf3d2c907c..02cff39e509e 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -28,6 +28,7 @@
 #include <unistd.h>
 #include <sched.h>
 #include <stdlib.h>
+#include <dirent.h>
 
 #include "parse-events.h"
 #include <subcmd/parse-options.h>
@@ -1980,3 +1981,80 @@ struct evsel *evlist__find_evsel(struct evlist *evlist, int idx)
 	}
 	return NULL;
 }
+
+static int *sw_types;
+static int nr_sw_types;
+
+static void collect_software_pmu_types(void)
+{
+	const char *known_sw_pmu[] = {
+		"software", "tracepoint", "breakpoint", "kprobe", "uprobe", "msr"
+	};
+	DIR *dir;
+	struct dirent *d;
+	char path[PATH_MAX];
+	int i;
+
+	if (sw_types != NULL)
+		return;
+
+	nr_sw_types = ARRAY_SIZE(known_sw_pmu);
+	sw_types = calloc(nr_sw_types, sizeof(int));
+	if (sw_types == NULL) {
+		pr_err("Memory allocation failed!\n");
+		return;
+	}
+
+	dir = opendir("/sys/bus/event_source/devices");
+	while ((d = readdir(dir)) != NULL) {
+		for (i = 0; i < nr_sw_types; i++) {
+			if (strcmp(d->d_name, known_sw_pmu[i]))
+				continue;
+
+			snprintf(path, sizeof(path), "%s/%s/type",
+				 "bus/event_source/devices", d->d_name);
+			sysfs__read_int(path, &sw_types[i]);
+		}
+	}
+	closedir(dir);
+}
+
+static bool is_software_event(struct evsel *evsel)
+{
+	int i;
+
+	for (i = 0; i < nr_sw_types; i++) {
+		if (evsel->core.attr.type == (unsigned)sw_types[i])
+			return true;
+	}
+	return false;
+}
+
+void evlist__warn_mixed_group(struct evlist *evlist)
+{
+	struct evsel *leader, *evsel;
+	bool warn = true;
+
+	collect_software_pmu_types();
+
+	/* Warn if an event group has a sw leader and hw siblings */
+	evlist__for_each_entry(evlist, leader) {
+		if (!evsel__is_group_event(leader))
+			continue;
+
+		if (!is_software_event(leader))
+			continue;
+
+		for_each_group_member(evsel, leader) {
+			if (is_software_event(evsel))
+				continue;
+			if (!warn)
+				continue;
+
+			pr_warning("WARNING: Event group has mixed hw/sw events.\n"
+				   "This will slow down the perf_event_open syscall.\n"
+				   "Consider putting a hw event as a leader.\n\n");
+			warn = false;
+		}
+	}
+}
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index e1a450322bc5..a5b0a1d03a00 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -387,4 +387,5 @@ int evlist__ctlfd_ack(struct evlist *evlist);
 #define EVLIST_DISABLED_MSG "Events disabled\n"
 
 struct evsel *evlist__find_evsel(struct evlist *evlist, int idx);
+void evlist__warn_mixed_group(struct evlist *evlist);
 #endif /* __PERF_EVLIST_H */
-- 
2.29.0.rc1.297.gfa9743e501-goog