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>

3 files changed, 22 insertions, 15 deletions

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index df0610e9c61b..b498eecbe850 100644
--- a/tools/perf/util/evlist.c
+++ b/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;
 }
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index acbe48eac608..2706ae40c8b3 100644
--- a/tools/perf/util/evlist.h
+++ b/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;
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 76ab553637d6..f98b3e585039 100644
--- a/tools/perf/util/evsel.c
+++ b/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);