perf mmap: Introduce perf_mmap__read_event()

Except for 'perf record', the other perf tools read events one by one
from the ring buffer using perf_mmap__read_forward(). But it only
supports non-overwrite mode.

Introduce perf_mmap__read_event() to support both non-overwrite and
overwrite mode.

Usage:
perf_mmap__read_init()
while(event = perf_mmap__read_event()) {
        //process the event
        perf_mmap__consume()
}
perf_mmap__read_done()

It cannot use perf_mmap__read_backward(). Because it always reads the
stale buffer which is already processed. Furthermore, the forward and
backward concepts have been removed. The perf_mmap__read_backward() will
be replaced and discarded later.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1516310792-208685-9-git-send-email-kan.liang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/mmap.c

@@ -113,6 +113,45 @@ union perf_event *perf_mmap__read_backward(struct perf_mmap *map)
 	return perf_mmap__read(map, &map->prev, end);
 }
 
+/*
+ * Read event from ring buffer one by one.
+ * Return one event for each call.
+ *
+ * Usage:
+ * perf_mmap__read_init()
+ * while(event = perf_mmap__read_event()) {
+ *	//process the event
+ *	perf_mmap__consume()
+ * }
+ * perf_mmap__read_done()
+ */
+union perf_event *perf_mmap__read_event(struct perf_mmap *map,
+					bool overwrite,
+					u64 *startp, u64 end)
+{
+	union perf_event *event;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!refcount_read(&map->refcnt))
+		return NULL;
+
+	if (startp == NULL)
+		return NULL;
+
+	/* non-overwirte doesn't pause the ringbuffer */
+	if (!overwrite)
+		end = perf_mmap__read_head(map);
+
+	event = perf_mmap__read(map, startp, end);
+
+	if (!overwrite)
+		map->prev = *startp;
+
+	return event;
+}
+
 void perf_mmap__read_catchup(struct perf_mmap *map)
 {
 	u64 head;