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Message-ID: <alpine.DEB.2.00.0903172108160.10610@gandalf.stny.rr.com>
Date: Tue, 17 Mar 2009 21:21:01 -0400 (EDT)
From: Steven Rostedt <rostedt@...dmis.org>
To: Tom Zanussi <tzanussi@...il.com>
cc: linux-kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>,
Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC][PATCH 2/4] tracing: add rb_event_discard() hack
On Tue, 17 Mar 2009, Tom Zanussi wrote:
> This patch overloads RINGBUF_TYPE_PADDING to provide a way to discard
> events from the ring buffer, for the event-filtering mechanism
> introduced in a subsequent patch.
>
> NOTE: this patch is a true hack and will likely either eventually crash
> your machine or result in an endless loop of printed events. But it
> sometimes works, enough to show that the filtering apparently does what
> it should.
>
> I'm posting it only to solicit comments from any ringbuffer gurus who
> might know know how to do it correctly, or more likely could suggest a
> better approach.
Actually, we can make this work. Just some tweaks. I may take this patch,
and massage it up to something that will work.
>
> Signed-off-by: Tom Zanussi <tzanussi@...il.com>
>
> ---
> include/linux/ring_buffer.h | 9 ++++-
> kernel/trace/ring_buffer.c | 73 ++++++++++++++++++++++++++++++-------------
> 2 files changed, 58 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index b237e20..4292774 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -22,10 +22,13 @@ struct ring_buffer_event {
> /**
> * enum ring_buffer_type - internal ring buffer types
> *
> - * @RINGBUF_TYPE_PADDING: Left over page padding
> + * @RINGBUF_TYPE_PADDING: Left over page padding or discarded event
> * array is ignored
> - * size is variable depending on how much
> + * If time_delta is 0:
> + * size is variable depending on how much
> * padding is needed
> + * If time_delta is 1:
> + * everything else same as RINGBUF_TYPE_DATA
I would make array not ignored, unless all is zero, then everything
would be ignored. That is, the true padding would just be zeroed out.
> *
> * @RINGBUF_TYPE_TIME_EXTEND: Extend the time delta
> * array[0] = time delta (28 .. 59)
> @@ -69,6 +72,8 @@ ring_buffer_event_time_delta(struct ring_buffer_event *event)
> return event->time_delta;
> }
>
> +void rb_event_discard(struct ring_buffer_event *event);
Note, the "rb_" prefix is only used for static functions since we can get
into name space collisions with rbtree. Functions that are global outside
this file have the "ring_buffer_" prefix.
> +
> /*
> * size is in bytes for each per CPU buffer.
> */
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1d3b07d..c59e910 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -213,16 +213,50 @@ enum {
> RB_LEN_TIME_STAMP = 16,
> };
>
> -/* inline for ring buffer fast paths */
> +static inline int rb_null_event(struct ring_buffer_event *event)
> +{
> + return event->type == RINGBUF_TYPE_PADDING && event->time_delta == 0;
> +}
> +
> +static inline int rb_discarded_event(struct ring_buffer_event *event)
> +{
> + return event->type == RINGBUF_TYPE_PADDING && event->time_delta == 1;
> +}
> +
> +static void rb_event_set_padding(struct ring_buffer_event *event)
> +{
> + event->type = RINGBUF_TYPE_PADDING;
> + event->time_delta = 0;
> +}
the above seems reasonable, and should work fine.
> +
> +void rb_event_discard(struct ring_buffer_event *event)
> +{
> + event->type = RINGBUF_TYPE_PADDING;
> + event->time_delta = 1;
> +}
> +
> static unsigned
> -rb_event_length(struct ring_buffer_event *event)
> +rb_event_data_length(struct ring_buffer_event *event)
> {
> unsigned length;
>
> + if (event->len)
> + length = event->len * RB_ALIGNMENT;
> + else
> + length = event->array[0];
> + return length + RB_EVNT_HDR_SIZE;
> +}
> +
> +/* inline for ring buffer fast paths */
> +static unsigned
> +rb_event_length(struct ring_buffer_event *event)
> +{
> switch (event->type) {
> case RINGBUF_TYPE_PADDING:
> - /* undefined */
> - return -1;
> + if (rb_null_event(event))
> + /* undefined */
> + return -1;
> + return rb_event_data_length(event);
>
> case RINGBUF_TYPE_TIME_EXTEND:
> return RB_LEN_TIME_EXTEND;
> @@ -231,11 +265,7 @@ rb_event_length(struct ring_buffer_event *event)
> return RB_LEN_TIME_STAMP;
>
> case RINGBUF_TYPE_DATA:
> - if (event->len)
> - length = event->len * RB_ALIGNMENT;
> - else
> - length = event->array[0];
> - return length + RB_EVNT_HDR_SIZE;
> + return rb_event_data_length(event);
This is all reasonable.
> default:
> BUG();
> }
> @@ -828,11 +858,6 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
> }
> EXPORT_SYMBOL_GPL(ring_buffer_resize);
>
> -static inline int rb_null_event(struct ring_buffer_event *event)
> -{
> - return event->type == RINGBUF_TYPE_PADDING;
> -}
> -
> static inline void *
> __rb_data_page_index(struct buffer_data_page *bpage, unsigned index)
> {
> @@ -1203,7 +1228,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> /* Mark the rest of the page with padding */
> event = __rb_page_index(tail_page, tail);
> kmemcheck_annotate_bitfield(event->bitfield);
> - event->type = RINGBUF_TYPE_PADDING;
> + rb_event_set_padding(event);
> }
>
> if (tail <= BUF_PAGE_SIZE)
> @@ -1954,7 +1979,7 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
>
> event = rb_reader_event(cpu_buffer);
>
> - if (event->type == RINGBUF_TYPE_DATA)
> + if (event->type == RINGBUF_TYPE_DATA || rb_discarded_event(event))
> cpu_buffer->entries--;
Actually, we should decrement the entries counter at time of discard.
>
> rb_update_read_stamp(cpu_buffer, event);
> @@ -2026,8 +2051,8 @@ rb_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
> * can have. Nesting 10 deep of interrupts is clearly
> * an anomaly.
> */
> - if (RB_WARN_ON(cpu_buffer, ++nr_loops > 10))
> - return NULL;
> +// if (RB_WARN_ON(cpu_buffer, ++nr_loops > 10))
> +// return NULL;
Were you hitting this? If so, we need to investigate.
>
> reader = rb_get_reader_page(cpu_buffer);
> if (!reader)
> @@ -2037,7 +2062,8 @@ rb_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
>
> switch (event->type) {
> case RINGBUF_TYPE_PADDING:
> - RB_WARN_ON(cpu_buffer, 1);
> + if (rb_null_event(event))
> + RB_WARN_ON(cpu_buffer, 1);
> rb_advance_reader(cpu_buffer);
> return NULL;
Actually, we want to simply skip over the discarded event. This will cause
the above loops to hit more often. We should make the warning count be
BUFFER_PAGE_SIZE, if we hit it that many times, we know we are in trouble.
In other words, we do not want to return NULL, but we want to goto again.
>
> @@ -2089,8 +2115,8 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
> * can have. Nesting 10 deep of interrupts is clearly
> * an anomaly.
> */
> - if (RB_WARN_ON(cpu_buffer, ++nr_loops > 10))
> - return NULL;
> +// if (RB_WARN_ON(cpu_buffer, ++nr_loops > 10))
> +// return NULL;
>
> if (rb_per_cpu_empty(cpu_buffer))
> return NULL;
> @@ -2099,7 +2125,10 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
>
> switch (event->type) {
> case RINGBUF_TYPE_PADDING:
> - rb_inc_iter(iter);
> + if (rb_null_event(event))
> + rb_inc_iter(iter);
> + else
> + rb_advance_iter(iter);
> goto again;
This should work.
>
> case RINGBUF_TYPE_TIME_EXTEND:
> --
> 1.5.6.3
I'll take a look at your patch and see if I can get it to work.
Thanks,
-- Steve
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