ring_buffer: compressed event header

RB_MAX_SMALL_DATA = 28bytes is too small for most tracers, it wastes
an 'u32' to save the actually length for events which data size > 28.

This fix uses compressed event header and enlarges RB_MAX_SMALL_DATA.

[ Impact: saves about 0%-12.5%(depends on tracer) memory in ring_buffer ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <49F13189.3090000@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 9 insertions, 7 deletions

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index fac8f1ac6f49..1c2f80911fbe 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -11,7 +11,7 @@ struct ring_buffer_iter;
  * Don't refer to this struct directly, use functions below.
  */
 struct ring_buffer_event {
-        u32             type:2, len:3, time_delta:27;
+        u32             type_len:5, time_delta:27;
         u32             array[];
 };
 
@@ -24,7 +24,8 @@ struct ring_buffer_event {
  *                                size is variable depending on how much
  *                                padding is needed
  *                               If time_delta is non zero:
- *                                everything else same as RINGBUF_TYPE_DATA
+ *                                array[0] holds the actual length
+ *                                size = 4 + length (bytes)
  *
  * @RINGBUF_TYPE_TIME_EXTEND:   Extend the time delta
  *                               array[0] = time delta (28 .. 59)
@@ -35,22 +36,23 @@ struct ring_buffer_event {
  *                               array[1..2] = tv_sec
  *                               size = 16 bytes
  *
- * @RINGBUF_TYPE_DATA:          Data record
- *                               If len is zero:
+ * <= @RINGBUF_TYPE_DATA_TYPE_LEN_MAX:
+ *                              Data record
+ *                               If type_len is zero:
  *                                array[0] holds the actual length
  *                                array[1..(length+3)/4] holds data
- *                                size = 4 + 4 + length (bytes)
+ *                                size = 4 + length (bytes)
  *                               else
- *                                length = len << 2
+ *                                length = type_len << 2
  *                                array[0..(length+3)/4-1] holds data
  *                                size = 4 + length (bytes)
  */
 enum ring_buffer_type {
+        RINGBUF_TYPE_DATA_TYPE_LEN_MAX = 28,
         RINGBUF_TYPE_PADDING,
         RINGBUF_TYPE_TIME_EXTEND,
         /* FIXME: RINGBUF_TYPE_TIME_STAMP not implemented */
         RINGBUF_TYPE_TIME_STAMP,
-        RINGBUF_TYPE_DATA,
 };
 
 unsigned ring_buffer_event_length(struct ring_buffer_event *event);