[lttng-dev] [GIT PULL v3] Accounting packet lost and discarded events at runtime

[Julien Desfossez]

This patchset is composed of commits in lttng-tools, lttng-ust and
lttng-modules, it allows the sessiond, to report to the user the
discarded events (when a channel is in discard mode) or the lost packets
(when a channel is in overwrite mode).

Please pull from:
https://github.com/jdesfossez/lttng-tools-dev/tree/seqnum
a312e89 Store the instance id and packet_seq_num in indexes
e40456e kernctl commands to extract the stream instance id
53d9b1d Account the lost packets in snapshot mode
17d0d29 Display discarded and lost events at destroy and stop
291246b Extract the lost packets and discarded events counters
0965b2f Metadata for instance_id and packet_seq_num
a0badfd Reference pointers from ust_app_chan to parent structs
082f79d Explicitly stop the session on lttng destroy

https://github.com/jdesfossez/lttng-modules-dev/tree/seqnum
fa326dd Add stream instance id to the packet header
cf556ea Add a packet sequence number

https://github.com/jdesfossez/lttng-ust-dev/tree/seqnum
66eb100 Export the stream instance ID
72d75d7 Generate and export the sequence number
885e9ba Add stream instance id to the packet header


Since we are in the last sprint of the 2.7 release, I will try to keep
these branches up-to-date by rebasing regularly until the maintainers
have some time to review and merge into master, so the commit IDs might
change, but the commit messages won't. I can resubmit as emails instead
of a pull request if it is easier for the review.

Change from v2:
- actually pushed the updates intended for v1 in UST and tools...
- fixed the comments from Mathieu on the v2

Change from v1:
- the packet sequence number is not dependant on the subbuf_size and now
always a uint64_t, so it got rid of the need of the
packet_seq_num_padding field

Original cover letter:
The reporting is detailled on a per-channel basis in the "lttng list"
command (and associated API calls) at runtime. On lttng stop/destroy, a
warning is now displayed to the user if events/packets were
discarded/lost during the session.

The information about the packet lost is extracted from a packet
sequence number now written in each CTF packet and CTF index, allowing
the relayd and the viewers to know when parsing the trace/index that
entire packets were lost between two data packets.

Finally, a stream_instance_id is now written in each packet, this
feature allows a viewer to link multiple tracefiles belonging to the
same channel and stream. This is particularly useful when dealing with
tracefile rotation because the viewers will now be able to compute the
accurate number of discarded events.

Julien

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