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>

1 files changed, 41 insertions, 26 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 945302368691..50926601a28d 100644
--- 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