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

Tim Bird
2009-06-03T18:55:48Z

Steven Rostedt wrote:
> 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;
> +}
> +

Is this new routine only for discarding uncommitted events,
or can it be used on committed events?

I assume the former, since I see nothing about adjusting the
commit position.

In the ring_buffer API I see that there's a function for
discarding events (committed ones), but not for free-ing them.
In function duration filtering, it is desirable to free the
last committed event, which for a function exit of short
duration will be it's entry event 99% of the time.
-- Tim

P.S. I'm very sorry about the missing '>' on the Signed-off-by line.
I ran checkpatch and got a passing score, but missed this.

=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Corporation of America
=============================