[PATCH v8 1/2] perf: Reveal PMU type in fdinfo

[Chun-Tse Shao <ctshao@google.com>]

It gives useful info on knowing which PMUs are reserved by this process.
Also add config which would be useful.
Testing cycles:

  $ ./perf stat -e cycles &
  $ cat /proc/`pidof perf`/fdinfo/3
  pos:    0
  flags:  02000002
  mnt_id: 16
  ino:    3081
  perf_event_attr.type:   0
  perf_event_attr.config: 0x0
  perf_event_attr.config1:        0x0
  perf_event_attr.config2:        0x0
  perf_event_attr.config3:        0x0
  perf_event_attr.config4:        0x0

Testing L1-dcache-load-misses:

  $ ./perf stat -e L1-dcache-load-misses &
  $ cat /proc/`pidof perf`/fdinfo/3
  pos:    0
  flags:  02000002
  mnt_id: 16
  ino:    1072
  perf_event_attr.type:   3
  perf_event_attr.config: 0x10000
  perf_event_attr.config1:        0x0
  perf_event_attr.config2:        0x0
  perf_event_attr.config3:        0x0
  perf_event_attr.config4:        0x0

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Assisted-by: Gemini:gemini-3.1-pro-preview
---
v8:
  - Fix a race condition on 32-bit architectures by acquiring parent or
    child child_mutex inside perf_show_fdinfo() to prevent concurrent
    mutations via perf_event_modify_breakpoint() from causing torn reads
    of breakpoint fields (which alias config1 and config2).
  - Expose missing config4 field in the fdinfo output.
---
 kernel/events/core.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7935d5663944..95d806bba654 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -58,6 +58,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/unwind_deferred.h>
 #include <linux/kvm_types.h>
+#include <linux/seq_file.h>

 #include "internal.h"

@@ -7546,6 +7547,33 @@ static int perf_fasync(int fd, struct file *filp, int on)
 	return 0;
 }

+static void perf_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct perf_event *event = f->private_data;
+	struct perf_event_context *ctx;
+	struct mutex *child_mutex;
+
+	ctx = perf_event_ctx_lock(event);
+	child_mutex = event->parent ? &event->parent->child_mutex : &event->child_mutex;
+	mutex_lock(child_mutex);
+
+	seq_printf(m, "perf_event_attr.type:\t%u\n", event->orig_type);
+	if (event->pmu)
+		seq_printf(m, "pmu_type:\t%u\n", event->pmu->type);
+	seq_printf(m, "perf_event_attr.config:\t0x%llx\n", (unsigned long long)event->attr.config);
+	seq_printf(m, "perf_event_attr.config1:\t0x%llx\n",
+		   (unsigned long long)event->attr.config1);
+	seq_printf(m, "perf_event_attr.config2:\t0x%llx\n",
+		   (unsigned long long)event->attr.config2);
+	seq_printf(m, "perf_event_attr.config3:\t0x%llx\n",
+		   (unsigned long long)event->attr.config3);
+	seq_printf(m, "perf_event_attr.config4:\t0x%llx\n",
+		   (unsigned long long)event->attr.config4);
+
+	mutex_unlock(child_mutex);
+	perf_event_ctx_unlock(event, ctx);
+}
+
 static const struct file_operations perf_fops = {
 	.release		= perf_release,
 	.read			= perf_read,
@@ -7554,6 +7582,7 @@ static const struct file_operations perf_fops = {
 	.compat_ioctl		= perf_compat_ioctl,
 	.mmap			= perf_mmap,
 	.fasync			= perf_fasync,
+	.show_fdinfo		= perf_show_fdinfo,
 };

 /*
--
2.54.0.1013.g208068f2d8-goog