Re: [PATCH 2/4] tracing: add ring_buffer_event_discard() to ring buffer

[Frederic Weisbecker <fweisbec@gmail.com>]

On Sun, Mar 22, 2009 at 03:30:49AM -0500, 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.
> 
> I did the initial version but thanks to Steven Rostedt for adding the
> parts that actually made it work. ;-)
> 
> ---
>  include/linux/ring_buffer.h |   11 +++-
>  kernel/trace/ring_buffer.c  |  117 +++++++++++++++++++++++++++++++++++-------
>  2 files changed, 105 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index e20b886..ae5b210 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
> - *				 array is ignored
> - *				 size is variable depending on how much
> + * @RINGBUF_TYPE_PADDING:	Left over page padding or discarded event
> + *				 If time_delta is 0:
> + *				  array is ignored
> + *				  size is variable depending on how much
>   *				  padding is needed
> + *				 If time_delta is non zero:
> + *				  everything else same as RINGBUF_TYPE_DATA
>   *
>   * @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 ring_buffer_event_discard(struct ring_buffer_event *event);
> +
>  /*
>   * size is in bytes for each per CPU buffer.
>   */
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7e05e2c..5cd2342 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -190,16 +190,65 @@ 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;
> +}
> +
> +static void rb_event_set_padding(struct ring_buffer_event *event)
> +{
> +	event->type = RINGBUF_TYPE_PADDING;
> +	event->time_delta = 0;
> +}
> +
> +/**
> + * ring_buffer_event_discard - discard an event in the ring buffer
> + * @buffer: the ring buffer
> + * @event: the event to discard
> + *
> + * Sometimes a event that is in the ring buffer needs to be ignored.
> + * This function lets the user discard an event in the ring buffer
> + * and then that event will not be read later.
> + *
> + * Note, it is up to the user to be careful with this, and protect
> + * against races. If the user discards an event that has been consumed
> + * it is possible that it could corrupt the ring buffer.
> + */
> +void ring_buffer_event_discard(struct ring_buffer_event *event)
> +{
> +	event->type = RINGBUF_TYPE_PADDING;
> +	/* time delta must be non zero */
> +	if (!event->time_delta)
> +		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 */


I don't see an inline here.


> +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;
> @@ -208,11 +257,7 @@ 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);
>  	default:
>  		BUG();
>  	}
> @@ -846,11 +891,6 @@ 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)
>  {
> @@ -1221,7 +1261,7 @@ __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)
> @@ -1972,7 +2012,7 @@ 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--;
>  
>  	rb_update_read_stamp(cpu_buffer, event);
> @@ -2055,9 +2095,18 @@ 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);
> +		/*
> +		 * Because the writer could be discarding every
> +		 * event it creates (which would probably be bad)
> +		 * if we were to go back to "again" then we may never
> +		 * catch up, and will trigger the warn on, or lock
> +		 * the box. Return the padding, and we will release
> +		 * the current locks, and try again.
> +		 */
>  		rb_advance_reader(cpu_buffer);
> -		return NULL;
> +		return event;
>  
>  	case RINGBUF_TYPE_TIME_EXTEND:
>  		/* Internal data, OK to advance */
> @@ -2118,8 +2167,12 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
>  
>  	switch (event->type) {
>  	case RINGBUF_TYPE_PADDING:
> -		rb_inc_iter(iter);
> -		goto again;
> +		if (rb_null_event(event)) {
> +			rb_inc_iter(iter);
> +			goto again;
> +		}
> +		rb_advance_iter(iter);
> +		return event;
>  
>  	case RINGBUF_TYPE_TIME_EXTEND:
>  		/* Internal data, OK to advance */
> @@ -2166,10 +2219,16 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
>  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  		return NULL;
>  
> + again:
>  	spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  	event = rb_buffer_peek(buffer, cpu, ts);
>  	spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
> +	if (event && event->type == RINGBUF_TYPE_PADDING) {
> +		cpu_relax();
> +		goto again;
> +	}
> +
>  	return event;
>  }
>  
> @@ -2188,10 +2247,16 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
>  	struct ring_buffer_event *event;
>  	unsigned long flags;
>  
> + again:
>  	spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  	event = rb_iter_peek(iter, ts);
>  	spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
> +	if (event && event->type == RINGBUF_TYPE_PADDING) {
> +		cpu_relax();
> +		goto again;
> +	}
> +
>  	return event;
>  }
>  
> @@ -2210,6 +2275,7 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
>  	struct ring_buffer_event *event = NULL;
>  	unsigned long flags;
>  
> + again:
>  	/* might be called in atomic */
>  	preempt_disable();
>  
> @@ -2231,6 +2297,11 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
>   out:
>  	preempt_enable();
>  
> +	if (event && event->type == RINGBUF_TYPE_PADDING) {
> +		cpu_relax();
> +		goto again;
> +	}
> +
>  	return event;
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_consume);
> @@ -2309,6 +2380,7 @@ ring_buffer_read(struct ring_buffer_iter *iter, u64 *ts)
>  	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
>  	unsigned long flags;
>  
> + again:
>  	spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  	event = rb_iter_peek(iter, ts);
>  	if (!event)
> @@ -2318,6 +2390,11 @@ ring_buffer_read(struct ring_buffer_iter *iter, u64 *ts)
>   out:
>  	spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
> +	if (event && event->type == RINGBUF_TYPE_PADDING) {
> +		cpu_relax();
> +		goto again;
> +	}
> +
>  	return event;
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_read);
> -- 


Looks good!

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>