[PATCH 2/3] ring-buffer: try to discard unneeded timestamps

Steven Rostedt
2009-06-03T14:17:30Z

[-- Attachment #1: 0002-ring-buffer-try-to-discard-unneeded-timestamps.patch --]
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From: Steven Rostedt <srostedt@redhat.com>

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>
---
kernel/trace/ring_buffer.c | 67 +++++++++++++++++++++++++++-----------------
1 files changed, 41 insertions(+), 26 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9453023..5092660 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
--
1.6.3.1

--