[RFC PATCH v2] perf: Add PERF_EVENT_IOC_FLUSH ioctl

[Robert Bragg <robert@sixbynine.org>]

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.

v2: return int error status

Signed-off-by: Robert Bragg <robert@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 cf1d096..0c591eb 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -305,6 +305,13 @@ struct pmu {
 	 * Free pmu-private AUX data structures
 	 */
 	void (*free_aux)		(void *aux); /* optional */
+
+	/*
+	 * 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.
+	 */
+	int (*flush)			(struct perf_event *event); /*optional */
 };
 
 /**
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 309211b..cbf1b80 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -389,6 +389,7 @@ struct perf_event_attr {
 #define PERF_EVENT_IOC_SET_FILTER	_IOW('$', 6, char *)
 #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
 #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
+#define PERF_EVENT_IOC_FLUSH		_IO ('$', 9)
 
 enum perf_event_ioc_flags {
 	PERF_IOC_FLAG_GROUP		= 1U << 0,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3fe532a..72daee6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4079,6 +4079,11 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 	case PERF_EVENT_IOC_SET_BPF:
 		return perf_event_set_bpf_prog(event, arg);
 
+	case PERF_EVENT_IOC_FLUSH:
+		if (event->pmu->flush)
+			return event->pmu->flush(event);
+		return 0;
+
 	default:
 		return -ENOTTY;
 	}
-- 
2.4.1