[PATCH 3/3] perf/core: Save calculated sample data size

[Namhyung Kim <namhyung@kernel.org>]

To avoid duplicate work in perf_prepare_sample(), save the final header
size in data->saved_size.  It's initialized to 0 and set to an actual
value at the end of perf_prepare_sample().

If it sees a non-zero value that means it's the second time of the call
and it knows the sample data is populated already.  So update the header
size with the data->saved_size and bail out.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 include/linux/perf_event.h |  2 ++
 kernel/events/core.c       | 11 +++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index dd565306f479..ccde631a0cb4 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1103,6 +1103,7 @@ struct perf_sample_data {
 	 */
 	u64				sample_flags;
 	u64				period;
+	u64				saved_size;
 
 	/*
 	 * Fields commonly set by __perf_event_header__init_id(),
@@ -1158,6 +1159,7 @@ static inline void perf_sample_data_init(struct perf_sample_data *data,
 	/* remaining struct members initialized in perf_prepare_sample() */
 	data->sample_flags = PERF_SAMPLE_PERIOD;
 	data->period = period;
+	data->saved_size = 0;
 
 	if (addr) {
 		data->addr = addr;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 70bff8a04583..dac4d76e2877 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7575,6 +7575,15 @@ void perf_prepare_sample(struct perf_event_header *header,
 	header->misc = 0;
 	header->misc |= perf_misc_flags(regs);
 
+	/*
+	 * If it called perf_prepare_sample() already, it set the all data fields
+	 * and recorded the final size to data->saved_size.
+	 */
+	if (data->saved_size) {
+		header->size = data->saved_size;
+		return;
+	}
+
 	/*
 	 * Clear the sample flags that have already been done by the
 	 * PMU driver.
@@ -7796,6 +7805,8 @@ void perf_prepare_sample(struct perf_event_header *header,
 	 * do here next.
 	 */
 	WARN_ON_ONCE(header->size & 7);
+
+	data->saved_size = header->size;
 }
 
 static __always_inline int
-- 
2.39.0.314.g84b9a713c41-goog