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

Tim Bird
2009-06-03T20:56:43Z

Steve,

See my last comment for an important fix.

Steven Rostedt wrote:
> 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)

It might be worth using the terminology 'rb_try_to_free' (or something else)
to distinguish the case of backing up the write pointer from the
case of marking an event as discarded (pad) in the log.

> +{
> + 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 sense of this test is wrong. It should be:

if (rb_try_to_discard(cpu_buffer, event))
goto out;

You want to go to 'out' (skipping the increment of entries)
if you successfully freed the event.

In my testing, rb_try_to_discard was almost alway successful.
Since only one statement is jumped over by the goto, the
following might be better:

if (unlikely(!rb_try_to_discard(cpu_buffer, event)))
/*
* The commit is still visible by the reader, so we
* must increment entries.
*/
local_inc(&cpu_buffer->entries);

> /*
> * The commit is still visible by the reader, so we
> -- 1.6.3.1
> --

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