[RFC PATCH 03/11] perf: Add PERF_EVENT_IOC_FLUSH ioctl

[Robert Bragg]

To allow for pmus that may have internal buffering (e.g. the hardware
itself writes out data to its own circular buffer which is only
periodically forwarded to userspace via perf) this ioctl enables
userspace to explicitly ensure it has received all samples before a
point in time.

Signed-off-by: Robert Bragg <robert at sixbynine.org>
---
 include/linux/perf_event.h      | 7 +++++++
 include/uapi/linux/perf_event.h | 1 +
 kernel/events/core.c            | 5 +++++
 3 files changed, 13 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 1af35b4..69a0cb9 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -266,6 +266,13 @@ struct pmu {
 	 * flush branch stack on context-switches (needed in cpu-wide mode)
 	 */
 	void (*flush_branch_stack)	(void);
+
+	/*
+	 * Flush buffered samples (E.g. for pmu hardware that writes samples to
+	 * some intermediate buffer) userspace may need to explicitly ensure
+	 * such samples have been forwarded to perf.
+	 */
+	void (*flush)			(struct perf_event *event); /*optional */
 };
 
 /**
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 9b79abb..a25967b 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -360,6 +360,7 @@ struct perf_event_attr {
 #define PERF_EVENT_IOC_SET_OUTPUT	_IO ('$', 5)
 #define PERF_EVENT_IOC_SET_FILTER	_IOW('$', 6, char *)
 #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
+#define PERF_EVENT_IOC_FLUSH		_IO ('$', 8)
 
 enum perf_event_ioc_flags {
 	PERF_IOC_FLAG_GROUP		= 1U << 0,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7218b01..340deaa 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3981,6 +3981,11 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 	case PERF_EVENT_IOC_SET_FILTER:
 		return perf_event_set_filter(event, (void __user *)arg);
 
+	case PERF_EVENT_IOC_FLUSH:
+		if (event->pmu->flush)
+			event->pmu->flush(event);
+		return 0;
+
 	default:
 		return -ENOTTY;
 	}
-- 
2.3.2