[lttng-dev] [PATCH babeltrace] Fix: lttng-live discarded event count after inactivity

[Jonathan Rajotte-Julien]

On Tue, Jan 09, 2018 at 05:12:54PM -0500, Julien Desfossez wrote:
> When a stream is inactive, the consumer produces fake indexes which are
> beacons to let the viewer know a stream has not produced any data up to
> a certain timestamp. These beacon are actually real packet indexes with
> all the fields set to 0 except for the timestamp_end. Currently we keep
> these beacons just like we keep real indexes. The problem is that when
> we switch packet, we compare the events_discarded field in the index we
> just received with the same field in the previous index. In the case
> where a stream has been inactive, we have received inactivity beacons,
> and set the discarded_event field to 0, so the difference with the next
> real index might be wrong.
> 
> In fact, since the inactivity beacons are only used to push the
> timestamp end of a stream, we don't need to keep them and we actually
> need to keep most of the data from the real previous index. So we now
> copy the entire prev_index into the cur_index when we receive an
> inactivity beacon. We could refactor the code to avoid performing the
> pointer swap of cur and prev indexes, but this implies a redesign of
> much of the packet switching code which would affect other code paths.
> 

Tested-by: Jonathan Rajotte-Julien <jonathan.rajotte-julien at efficios.com>

> Signed-off-by: Julien Desfossez <jdesfossez at efficios.com>
> ---
>  formats/lttng-live/lttng-live-comm.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/formats/lttng-live/lttng-live-comm.c b/formats/lttng-live/lttng-live-comm.c
> index 77bf34a..3ffa257 100644
> --- a/formats/lttng-live/lttng-live-comm.c
> +++ b/formats/lttng-live/lttng-live-comm.c
> @@ -1275,6 +1275,7 @@ retry:
>  		pos->offset = 0;
>  	}
>  
> +	/* Beacon packet index */
>  	if (cur_index->content_size == 0) {
>  		if (file_stream->parent.stream_class) {
>  			file_stream->parent.cycles_timestamp =
> @@ -1282,8 +1283,23 @@ retry:
>  			file_stream->parent.real_timestamp = ctf_get_real_timestamp(
>  					&file_stream->parent,
>  					cur_index->ts_cycles.timestamp_end);
> +
> +			/*
> +			 * Duplicate the data from the previous index, because
> +			 * the one we just received is only a beacon with no
> +			 * relevant information except the timestamp_end. We
> +			 * don't need to keep this timestamp_end because we already
> +			 * updated the file_stream timestamps, so we only need
> +			 * to keep the last real index data as prev_index. That
> +			 * way, we keep the original prev timestamps and
> +			 * discarded events counter. This is the same behaviour
> +			 * as if we were reading a local trace, we would not
> +			 * have fake indexes between real indexes.
> +			 */
> +			memcpy(cur_index, prev_index, sizeof(struct packet_index));
>  		}
>  	} else {
> +	/* Real packet index */
>  		if (file_stream->parent.stream_class) {
>  			/* Convert the timestamps and append to the real_index. */
>  			cur_index->ts_real.timestamp_begin = ctf_get_real_timestamp(
> -- 
> 2.7.4
> 

-- 
Jonathan Rajotte-Julien
EfficiOS