[PATCH 02/10] perf tool: fix: Don't discard prev in backward mode

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

Perf record can switch output. The new output should only store the data
after switching. However, in overwrite backward mode, the new output
still have the data from old output.

At the end of mmap_read, the position of processed ring buffer is saved
in md->prev. Next mmap_read should be end in md->prev.
However, the md->prev is discarded. So next mmap_read has to process
whole valid ring buffer, which definitely include the old processed
data.

Set the prev as the end of the range in backward mode.

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 tools/perf/util/evlist.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 33b8837..7d23cf5 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -742,13 +742,25 @@ static int
 rb_find_range(void *data, int mask, u64 head, u64 old,
 	      u64 *start, u64 *end, bool backward)
 {
+	int ret;
+
 	if (!backward) {
 		*start = old;
 		*end = head;
 		return 0;
 	}
 
-	return backward_rb_find_range(data, mask, head, start, end);
+	ret = backward_rb_find_range(data, mask, head, start, end);
+
+	/*
+	 * The start and end from backward_rb_find_range is the range for all
+	 * valid data in ring buffer.
+	 * However, part of the data is processed previously.
+	 * Reset the end to drop the processed data
+	 */
+	*end = old;
+
+	return ret;
 }
 
 /*
-- 
2.5.5