ring-buffer: try to discard unneeded timestamps

There are times that a race may happen that we add a timestamp in a
nested write. This timestamp would just contain a zero delta and serves
no purpose.

Now that we have a way to discard events, this patch will try to discard
the timestamp instead of just wasting the space in the ring buffer.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 94530236869125aa91bbb5fd54723309c73acc76..50926601a28dd3da0239746865d30643f76d0649 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1335,6 +1335,38 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
        return event;
 }
 
+static inline int
+rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
+                 struct ring_buffer_event *event)
+{
+       unsigned long new_index, old_index;
+       struct buffer_page *bpage;
+       unsigned long index;
+       unsigned long addr;
+
+       new_index = rb_event_index(event);
+       old_index = new_index + rb_event_length(event);
+       addr = (unsigned long)event;
+       addr &= PAGE_MASK;
+
+       bpage = cpu_buffer->tail_page;
+
+       if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
+               /*
+                * This is on the tail page. It is possible that
+                * a write could come in and move the tail page
+                * and write to the next page. That is fine
+                * because we just shorten what is on this page.
+                */
+               index = local_cmpxchg(&bpage->write, old_index, new_index);
+               if (index == old_index)
+                       return 1;
+       }
+
+       /* could not discard */
+       return 0;
+}
+
 static int
 rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
                  u64 *ts, u64 *delta)
@@ -1384,10 +1416,13 @@ rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
                /* let the caller know this was the commit */
                ret = 1;
        } else {
-               /* Darn, this is just wasted space */
-               event->time_delta = 0;
-               event->array[0] = 0;
-               ret = 0;
+               /* Try to discard the event */
+               if (!rb_try_to_discard(cpu_buffer, event)) {
+                       /* Darn, this is just wasted space */
+                       event->time_delta = 0;
+                       event->array[0] = 0;
+                       ret = 0;
+               }
        }
 
        *delta = 0;
@@ -1682,10 +1717,6 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer,
                                struct ring_buffer_event *event)
 {
        struct ring_buffer_per_cpu *cpu_buffer;
-       unsigned long new_index, old_index;
-       struct buffer_page *bpage;
-       unsigned long index;
-       unsigned long addr;
        int cpu;
 
        /* The event is discarded regardless */
@@ -1701,24 +1732,8 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer,
        cpu = smp_processor_id();
        cpu_buffer = buffer->buffers[cpu];
 
-       new_index = rb_event_index(event);
-       old_index = new_index + rb_event_length(event);
-       addr = (unsigned long)event;
-       addr &= PAGE_MASK;
-
-       bpage = cpu_buffer->tail_page;
-
-       if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
-               /*
-                * This is on the tail page. It is possible that
-                * a write could come in and move the tail page
-                * and write to the next page. That is fine
-                * because we just shorten what is on this page.
-                */
-               index = local_cmpxchg(&bpage->write, old_index, new_index);
-               if (index == old_index)
-                       goto out;
-       }
+       if (!rb_try_to_discard(cpu_buffer, event))
+               goto out;
 
        /*
         * The commit is still visible by the reader, so we