perf evlist: Support non overwrite mode in perf_evlist__read_on_cpu

I.e. stash the overwrite mode in struct perf_evlist and act accordingly
in perf_evlist__read_on_cpu, not checking for overwrites and touching
the tail after consuming one event, like perf record does, for instance.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -116,24 +116,25 @@ event_t *perf_evlist__read_on_cpu(struct perf_evlist *evlist, int cpu)
 	unsigned int old = md->prev;
 	unsigned char *data = md->base + page_size;
 	event_t *event = NULL;
-	int diff;
-
-	/*
-	 * If we're further behind than half the buffer, there's a chance
-	 * the writer will bite our tail and mess up the samples under us.
-	 *
-	 * If we somehow ended up ahead of the head, we got messed up.
-	 *
-	 * In either case, truncate and restart at head.
-	 */
-	diff = head - old;
-	if (diff > md->mask / 2 || diff < 0) {
-		fprintf(stderr, "WARNING: failed to keep up with mmap data.\n");
 
+	if (evlist->overwrite) {
 		/*
-		 * head points to a known good entry, start there.
+		 * If we're further behind than half the buffer, there's a chance
+		 * the writer will bite our tail and mess up the samples under us.
+		 *
+		 * If we somehow ended up ahead of the head, we got messed up.
+		 *
+		 * In either case, truncate and restart at head.
 		 */
-		old = head;
+		int diff = head - old;
+		if (diff > md->mask / 2 || diff < 0) {
+			fprintf(stderr, "WARNING: failed to keep up with mmap data.\n");
+
+			/*
+			 * head points to a known good entry, start there.
+			 */
+			old = head;
+		}
 	}
 
 	if (old != head) {
@@ -166,5 +167,9 @@ event_t *perf_evlist__read_on_cpu(struct perf_evlist *evlist, int cpu)
 	}
 
 	md->prev = old;
+
+	if (!evlist->overwrite)
+		perf_mmap__write_tail(md, old);
+
 	return event;
 }

tools/perf/util/evlist.h

@@ -16,6 +16,7 @@ struct perf_evlist {
 	int		 nr_entries;
 	int		 nr_fds;
 	int		 mmap_len;
+	bool		 overwrite;
 	event_t		 event_copy;
 	struct perf_mmap *mmap;
 	struct pollfd	 *pollfd;

tools/perf/util/evsel.c

@@ -327,6 +327,7 @@ int perf_evlist__mmap(struct perf_evlist *evlist, struct cpu_map *cpus,
 	    perf_evlist__alloc_pollfd(evlist, cpus->nr, threads->nr) < 0)
 		return -ENOMEM;
 
+	evlist->overwrite = overwrite;
 	evlist->mmap_len = (pages + 1) * page_size;
 	first_evsel = list_entry(evlist->entries.next, struct perf_evsel, node);