[PATCH v2] perf stat: Count kernel events in a group led by a tool event

[Hang Zhengyang via B4 Relay <devnull+hang.zhengyang1010.gmail.com@kernel.org>]

From: Hang Zhengyang <hang.zhengyang1010@gmail.com>

A tool event like user_time has no perf event in the kernel, but
get_group_fd() still passes its fd, the /proc/stat fd, as the group_fd
of the members.  The kernel rejects each member with EBADF and nothing
is counted:

  $ perf stat -a -e '{user_time,cycles,instructions}' -- sleep 0.05
                   0      user_time
     <not supported>      cycles
     <not supported>      instructions

Open the members with group_fd = -1, as the enable/disable code already
assumes.  They are then on their own, so read them without
PERF_FORMAT_GROUP and open them disabled like a leader, so that -D,
--control and enable_on_exec still apply.  Keep reporting a broken
group when the leader failed to open.

  $ perf stat -a -e '{user_time,cycles,instructions}' -- sleep 0.05
                   0      user_time
            38257394      cycles
            21228043      instructions

The kernel events in such a group are no longer scheduled together.

Fixes: 6828d6929b76 ("perf evsel: Refactor tool events")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Hang Zhengyang <hang.zhengyang1010@gmail.com>
---
This goes on top of TAKANO's segfault fix, as asked in that thread.
A tool event as a member, as in '{cycles,user_time}', is still not
counted; I'll send that separately.

Ian, the other way is to take tool events out of such groups in perf
stat with a warning, the way mismatched cpus are handled, which also
covers the member case.  I went with the group_fd = -1 that
0b18bced5444 describes; happy to switch if you'd rather split them.
---
Changes in v2:
- Open the members disabled like a leader, so -D, --control and
  enable_on_exec apply to them (sashiko)
- Report a broken group when the leader failed to open, as before, so
  perf record is unchanged (sashiko)
- Limit to tool event leaders: a hwmon leader can't enable its members
- Link to v1: https://lore.kernel.org/r/20261111-tool-pmu-leader-v1-1-6173b5569f59@gmail.com
---
 tools/perf/builtin-stat.c | 10 ++++++----
 tools/perf/util/evsel.c   |  8 ++++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 37752d6bfef4..07d01028bdea 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -719,9 +719,10 @@ static int create_perf_stat_counter(struct evsel *evsel,
 	/*
 	 * The event is part of non trivial group, let's enable
 	 * the group read (for leader) and ID retrieval for all
-	 * members.
+	 * members.  Members of a tool event leader are opened and
+	 * read on their own.
 	 */
-	if (leader->core.nr_members > 1)
+	if (leader->core.nr_members > 1 && !evsel__is_tool(leader))
 		attr->read_format |= PERF_FORMAT_ID|PERF_FORMAT_GROUP;
 
 	attr->inherit = !config->no_inherit && list_empty(&evsel->bpf_counter_list);
@@ -748,9 +749,10 @@ static int create_perf_stat_counter(struct evsel *evsel,
 	/*
 	 * Disabling all counters initially, they will be enabled
 	 * either manually by us or by kernel via enable_on_exec
-	 * set later.
+	 * set later.  Members of a tool event leader are opened on
+	 * their own and enabled with it, so they need it too.
 	 */
-	if (evsel__is_group_leader(evsel)) {
+	if (evsel__is_group_leader(evsel) || evsel__is_tool(leader)) {
 		attr->disabled = 1;
 
 		if (target__enable_on_exec(&target))
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 560741d9599d..e3ed645cc289 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2378,6 +2378,14 @@ static int get_group_fd(struct evsel *evsel, int cpu_map_idx, int thread)
 	if (!evsel->supported || evsel__is_group_leader(evsel))
 		return -1;
 
+	/*
+	 * A tool event leader has no perf event to group with, so open
+	 * the member on its own, or report the group as broken (-2) if
+	 * the leader failed to open.
+	 */
+	if (evsel__is_tool(leader))
+		return leader->supported ? -1 : -2;
+
 	/*
 	 * Leader must be already processed/open,
 	 * if not it's a bug.

---
base-commit: cc905a18a647a8e3ad42cd1596baa0e08fba26eb
change-id: 20261111-tool-pmu-leader-d25fe8153599
prerequisite-message-id: <20261023085629.1694941-1-takano32@gmail.com>
prerequisite-patch-id: b92ce8b0c6248a4af182670458912b760b98a410

Best regards,
-- 
Hang Zhengyang <hang.zhengyang1010@gmail.com>