perf record: Read from backward ring buffer

Introduce rb_find_range() to find start and end position from a backward
ring buffer.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1463987628-163563-5-git-send-email-wangnan0@huawei.com
Signed-off-by: He Kuang <hekuang@huawei.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

3 files changed, 54 insertions, 0 deletions

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 73ce651c84f6..dc3fcb597e4c 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -83,6 +83,54 @@ static int process_synthesized_event(struct perf_tool *tool,
         return record__write(rec, event, event->header.size);
 }
 
+static int
+backward_rb_find_range(void *buf, int mask, u64 head, u64 *start, u64 *end)
+{
+        struct perf_event_header *pheader;
+        u64 evt_head = head;
+        int size = mask + 1;
+
+        pr_debug2("backward_rb_find_range: buf=%p, head=%"PRIx64"\n", buf, head);
+        pheader = (struct perf_event_header *)(buf + (head & mask));
+        *start = head;
+        while (true) {
+                if (evt_head - head >= (unsigned int)size) {
+                        pr_debug("Finshed reading backward ring buffer: rewind\n");
+                        if (evt_head - head > (unsigned int)size)
+                                evt_head -= pheader->size;
+                        *end = evt_head;
+                        return 0;
+                }
+
+                pheader = (struct perf_event_header *)(buf + (evt_head & mask));
+
+                if (pheader->size == 0) {
+                        pr_debug("Finshed reading backward ring buffer: get start\n");
+                        *end = evt_head;
+                        return 0;
+                }
+
+                evt_head += pheader->size;
+                pr_debug3("move evt_head: %"PRIx64"\n", evt_head);
+        }
+        WARN_ONCE(1, "Shouldn't get here\n");
+        return -1;
+}
+
+static int
+rb_find_range(struct perf_evlist *evlist,
+              void *data, int mask, u64 head, u64 old,
+              u64 *start, u64 *end)
+{
+        if (!evlist->backward) {
+                *start = old;
+                *end = head;
+                return 0;
+        }
+
+        return backward_rb_find_range(data, mask, head, start, end);
+}
+
 static int record__mmap_read(struct record *rec, int idx)
 {
         struct perf_mmap *md = &rec->evlist->mmap[idx];
@@ -94,6 +142,10 @@ static int record__mmap_read(struct record *rec, int idx)
         void *buf;
         int rc = 0;
 
+        if (rb_find_range(rec->evlist, data, md->mask, head,
+                          old, &start, &end))
+                return -1;
+
         if (start == end)
                 return 0;
 
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 904523a2be90..e82ba90cc969 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -44,6 +44,7 @@ void perf_evlist__init(struct perf_evlist *evlist, struct cpu_map *cpus,
         perf_evlist__set_maps(evlist, cpus, threads);
         fdarray__init(&evlist->pollfd, 64);
         evlist->workload.pid = -1;
+        evlist->backward = false;
 }
 
 struct perf_evlist *perf_evlist__new(void)
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 97090b70976d..d740fb877ab6 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -44,6 +44,7 @@ struct perf_evlist {
         bool             overwrite;
         bool             enabled;
         bool             has_user_cpus;
+        bool             backward;
         size_t           mmap_len;
         int              id_pos;
         int              is_pos;