Fix: invalid discarded events on start/stop without event production

Observed issue
==============

On consecutive start/stop command sequence the reported discarded event
count is N * CPU, where N is the number of start/stop pair executed.

Note that no event generation occurred between each start/stop pair.

    lttng start
    lttng stop
    Tracing stopped for session auto-20200616-094338
    lttng start
    lttng stop
    Waiting for data availability
    Warning: 4 events were discarded, please refer to the documentation on channel configuration.
    Tracing stopped for session auto-20200616-094338
    lttng start
    lttng stop
    Waiting for data availability
    Warning: 8 events were discarded, please refer to the documentation on channel configuration.
    Tracing stopped for session auto-20200616-094338

The issue was bisected down to:
  commit 6f9449c22eef59294cf1e1dc3610a5cbf14baec0 (HEAD)
  Author: Jérémie Galarneau <jeremie.galarneau@efficios.com>
  Date:   Sun May 10 18:00:26 2020 -0400

  consumerd: refactor: split read_subbuf into sub-operations
  [...]

Cause
=====

The discarded event local variable, in `consumer_stream_update_stats()`
is initialized with the subbuffer sequence count instead of the
subbuffer discarded event count.

Solution
========

Use the subbuffer discarded event count to initialized the variable.

Known drawbacks
=========

None

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5ff213d0464cdb591b550f6e610bf15085b18888

diff --git a/src/common/consumer/consumer-stream.c b/src/common/consumer/consumer-stream.c
index b262b54d56868c27d2704b7cadeb52880c3ec40e..0c6dc8f518bbaf54af0c04fb352d52400e7feac1 100644 (file)
--- a/src/common/consumer/consumer-stream.c
+++ b/src/common/consumer/consumer-stream.c
@@ -69,8 +69,7 @@ static int consumer_stream_update_stats(struct lttng_consumer_stream *stream,
 {
        int ret = 0;
        uint64_t sequence_number;
-       const uint64_t discarded_events =
-                       LTTNG_OPTIONAL_GET(subbuf->info.data.sequence_number);
+       const uint64_t discarded_events = subbuf->info.data.events_discarded;
 
        if (!subbuf->info.data.sequence_number.is_set) {
                /* Command not supported by the tracer. */