[PATCH v2] perf stat: Start tool events when enable_on_exec is used

[TAKANO Mitsuhiro <takano32@gmail.com>]

When perf stat runs a workload, target__enable_on_exec() is true and the
tool events are left to the kernel to start on exec.  Tool events are
however emulated in perf (duration_time, user_time, system_time,
num_cpus) and have nothing in the kernel to start them, so they stay
disabled and are read as zero.

For duration_time this makes CPUs_utilized divide by zero, and plain
"perf stat" prints "nan CPUs":

  $ perf stat -e task-clock,duration_time -- sleep 0.1
               0      duration_time

Start the tool events explicitly in that case.  They are started through
evsel__tool_pmu_enable() rather than evsel__enable(), because the latter
also enables the members of a mixed PMU group, whose fds are not
available in this process, and dereferences a NULL fd:

  $ perf stat -e '{user_time,cycles}' -- sleep 0.1
  Segmentation fault

The kernel starts the kernel-side members of such a group on exec, so
only the tool event itself is enabled here.

Fixes: 0b18bced5444 ("perf tool_pmu: Make tool PMU events respect enable/disable")
Assisted-by: LLM

---
v1 -> v2: Do not call evsel__enable(), which also enables the members of a
mixed PMU group and dereferences a NULL fd; call evsel__tool_pmu_enable()
instead.  Reported by Sashiko review.

Signed-off-by: TAKANO Mitsuhiro <takano32@gmail.com>
---
 tools/perf/builtin-stat.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index c50cf230f16e..ca4483f34add 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -510,6 +510,18 @@ static int enable_counters(void)
 	if (!target__enable_on_exec(&target)) {
 		if (!all_counters_use_bpf)
 			evlist__enable(evsel_list);
+	} else {
+		/*
+		 * Tool events are emulated in perf, so enable_on_exec above
+		 * does not start them.  Do not go through evsel__enable(),
+		 * which would also enable the members of a mixed PMU group
+		 * the kernel already handles.
+		 */
+		evlist__for_each_entry(evsel_list, evsel) {
+			if (evsel__is_tool(evsel) &&
+			    !evsel__tool_pmu_enable(evsel))
+				evsel->disabled = false;
+		}
 	}
 	return 0;
 }

base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
-- 
2.53.0