[PATCH 2/2] perf mmap: Preserve the consumed cursor in forward mode

[Paul Moses <p@1g4.org>]

perf_mmap__read_event() updates prev as it consumes forward-buffer
records. perf_mmap__read_done() then overwrites prev with a fresh
producer head, even when unread records remain.

perf_kvm__mmap_read_idx() stops after 25 records per mmap and calls
read_done(). If more records are pending, prev advances past them. The
next drain starts at that new position and skips the unread remainder.

An event arriving after the last empty read_event() and before
read_done() can also be skipped. read_done() does not itself update the
kernel's data_tail.

Only refresh prev for overwrite buffers, which need the current head as
the end of the next backward read. Leave the forward cursor at the
consumed position.

Fixes: 53172f9057e9 ("perf kvm: Switch to new perf_mmap__read_event() interface")
Signed-off-by: Paul Moses <p@1g4.org>
---
 tools/lib/perf/mmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
index ec124eb0ec0abbcc4a395eca7478129a74309965..ab3adf1d8baf933d5386faf743f178b0a7da44c5 100644
--- a/tools/lib/perf/mmap.c
+++ b/tools/lib/perf/mmap.c
@@ -200,7 +200,7 @@ void perf_mmap__read_done(struct perf_mmap *map)
 	/*
 	 * Check if event was unmapped due to a POLLHUP/POLLERR.
 	 */
-	if (!refcount_read(&map->refcnt))
+	if (!refcount_read(&map->refcnt) || !map->overwrite)
 		return;
 
 	map->prev = perf_mmap__read_head(map);
-- 
2.56.GIT