Commit edd813bf authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
Browse files

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: default avatarSteven Rostedt <rostedt@goodmis.org>
parent a2023556

kernel/trace/ring_buffer.c

+41 −26
Original line number Diff line number Diff line
@@ -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)
@@ -1384,11 +1416,14 @@ rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer, 
		/* let the caller know this was the commit */

		ret = 1;

	} else {

		/* 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)

	if (!rb_try_to_discard(cpu_buffer, event))

		goto out;

	}



	/*

	 * The commit is still visible by the reader, so we

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