[PATCH v3] 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, and those cannot be used
here: get_group_fd() passes the leader's descriptor to
sys_perf_event_open(), which fails with EINVAL for a userspace PMU
leader, so the members end up unsupported with core.fd released.  Only
the tool event itself is started; the kernel starts the members of a
group it can open.

Skip the events that failed to open, the same way the evlist and the
counter read paths skip them, so a tool event that never opened is not
started and then read through a NULL descriptor.

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.

v2 -> v3: Also require evsel->supported, as reading a tool event that never
opened dereferences a NULL core.fd, and correct the explanation of why the
members of a mixed PMU group have no descriptor: get_group_fd() forwards the
leader's descriptor to sys_perf_event_open(), which fails with EINVAL for a
userspace PMU leader.  Reported by Sashiko review.

Signed-off-by: TAKANO Mitsuhiro <takano32@gmail.com>
---
 tools/perf/builtin-stat.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index c50cf230f16e..334b875d6a2f 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -510,6 +510,19 @@ 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.  Start them here rather than through
+		 * evsel__enable(), which also walks the members of a mixed
+		 * PMU group; those are not opened here and their core.fd can
+		 * be NULL.
+		 */
+		evlist__for_each_entry(evsel_list, evsel) {
+			if (evsel__is_tool(evsel) && evsel->supported &&
+			    !evsel__tool_pmu_enable(evsel))
+				evsel->disabled = false;
+		}
 	}
 	return 0;
 }

base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
-- 
2.53.0