[RFC] perf/core: Add an ioctl to get a number of lost samples

[Namhyung Kim]

Sometimes we want to know an accurate number of samples even if it's
lost.  Currenlty PERF_RECORD_LOST is generated for a ring-buffer which
might be shared with other events.  So it's hard to know per-event
lost count.

Add event->lost_samples field and PERF_EVENT_IOC_LOST_SAMPLES to
retrieve it from userspace.

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 include/linux/perf_event.h      | 2 ++
 include/uapi/linux/perf_event.h | 1 +
 kernel/events/core.c            | 9 +++++++++
 kernel/events/ring_buffer.c     | 5 ++++-
 4 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index f5a6a2f069ed..44d72079c77a 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -756,6 +756,8 @@ struct perf_event {
 	struct pid_namespace		*ns;
 	u64				id;
 
+	atomic_t			lost_samples;
+
 	u64				(*clock)(void);
 	perf_overflow_handler_t		overflow_handler;
 	void				*overflow_handler_context;
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index bf8143505c49..24397799127d 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -505,6 +505,7 @@ struct perf_event_query_bpf {
 #define PERF_EVENT_IOC_PAUSE_OUTPUT		_IOW('$', 9, __u32)
 #define PERF_EVENT_IOC_QUERY_BPF		_IOWR('$', 10, struct perf_event_query_bpf *)
 #define PERF_EVENT_IOC_MODIFY_ATTRIBUTES	_IOW('$', 11, struct perf_event_attr *)
+#define PERF_EVENT_IOC_LOST_SAMPLES		_IOR('$', 12, __u64 *)
 
 enum perf_event_ioc_flags {
 	PERF_IOC_FLAG_GROUP		= 1U << 0,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0e125ae2fa92..a4d6736b6594 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5664,6 +5664,15 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 
 		return perf_event_modify_attr(event,  &new_attr);
 	}
+
+	case PERF_EVENT_IOC_LOST_SAMPLES: {
+		u64 lost = atomic_read(&event->lost_samples);
+
+		if (copy_to_user((void __user *)arg, &lost, sizeof(lost)))
+			return -EFAULT;
+		return 0;
+	}
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 52868716ec35..06d7dacb05da 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -172,8 +172,10 @@ __perf_output_begin(struct perf_output_handle *handle,
 		goto out;
 
 	if (unlikely(rb->paused)) {
-		if (rb->nr_pages)
+		if (rb->nr_pages) {
 			local_inc(&rb->lost);
+			atomic_inc(&event->lost_samples);
+		}
 		goto out;
 	}
 
@@ -254,6 +256,7 @@ __perf_output_begin(struct perf_output_handle *handle,
 
 fail:
 	local_inc(&rb->lost);
+	atomic_inc(&event->lost_samples);
 	perf_output_put_handle(handle);
 out:
 	rcu_read_unlock();
-- 
2.32.0.605.g8dce9f2422-goog