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[for-next][PATCH 5/7] ring-buffer: Remove redundant update of page timestamp

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From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

The first commit of a buffer page updates the timestamp of that page. No
need to have the update to the next page add the timestamp too. It will only
be replaced by the first commit on that page anyway.

Only update to a page if it contains an event.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/ring_buffer.c | 32 ++++++++------------------------
 1 file changed, 8 insertions(+), 24 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index ab102e6259bc..631541a53baf 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1001,17 +1001,13 @@ static int rb_head_page_replace(struct buffer_page *old,
 
 /*
  * rb_tail_page_update - move the tail page forward
- *
- * Returns 1 if moved tail page, 0 if someone else did.
  */
-static int rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
+static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
 			       struct buffer_page *tail_page,
 			       struct buffer_page *next_page)
 {
-	struct buffer_page *old_tail;
 	unsigned long old_entries;
 	unsigned long old_write;
-	int ret = 0;
 
 	/*
 	 * The tail page now needs to be moved forward.
@@ -1061,14 +1057,9 @@ static int rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
 		 */
 		local_set(&next_page->page->commit, 0);
 
-		old_tail = cmpxchg(&cpu_buffer->tail_page,
-				   tail_page, next_page);
-
-		if (old_tail == tail_page)
-			ret = 1;
+		/* Again, either we update tail_page or an interrupt does */
+		(void)cmpxchg(&cpu_buffer->tail_page, tail_page, next_page);
 	}
-
-	return ret;
 }
 
 static int rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer,
@@ -2150,7 +2141,6 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
 	struct ring_buffer *buffer = cpu_buffer->buffer;
 	struct buffer_page *next_page;
 	int ret;
-	u64 ts;
 
 	next_page = tail_page;
 
@@ -2224,15 +2214,7 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
 		}
 	}
 
-	ret = rb_tail_page_update(cpu_buffer, tail_page, next_page);
-	if (ret) {
-		/*
-		 * Nested commits always have zero deltas, so
-		 * just reread the time stamp
-		 */
-		ts = rb_time_stamp(buffer);
-		next_page->page->time_stamp = ts;
-	}
+	rb_tail_page_update(cpu_buffer, tail_page, next_page);
 
  out_again:
 
@@ -2422,8 +2404,10 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
 		local_set(&cpu_buffer->commit_page->page->commit,
 			  rb_page_write(cpu_buffer->commit_page));
 		rb_inc_page(cpu_buffer, &cpu_buffer->commit_page);
-		cpu_buffer->write_stamp =
-			cpu_buffer->commit_page->page->time_stamp;
+		/* Only update the write stamp if the page has an event */
+		if (rb_page_write(cpu_buffer->commit_page))
+			cpu_buffer->write_stamp =
+				cpu_buffer->commit_page->page->time_stamp;
 		/* add barrier to keep gcc from optimizing too much */
 		barrier();
 	}
-- 
2.6.1


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[for-next][PATCH 0/7] tracing: Updates for 4.5 (and some stuff going into 4.4) Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100
  [for-next][PATCH 7/7] tracing: Update cond flag when enabling or disabling a trigger Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100
  [for-next][PATCH 4/7] ring-buffer: Use READ_ONCE() for most tail_page access Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100
  [for-next][PATCH 3/7] ftracetest: Add instance create and delete test Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100
    Re: [for-next][PATCH 3/7] ftracetest: Add instance create and  delete test Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:40 +0100
  [for-next][PATCH 1/7] ring-buffer: Update read stamp with first real commit on page Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100
  [for-next][PATCH 5/7] ring-buffer: Remove redundant update of page timestamp Steven Rostedt <rostedt@goodmis.org> - 2015-11-25 17:20 +0100

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