gstreamer/docs/design/draft-tracing.txt

Tracing
=======

This subsystem will provide a mechanism to get structured tracing info from
GStreamer applications. This can be used for post-run analysis as well as for
live introspection.

We are going to introduce a GstTracer object. There will be only a single
instance per process or none if tracing is off (not enabled via envvar or
compiled out).

Certain GStreamer core function (such as gst_pad_push or gst_element_add_pad)
will call into the tracer. The tracer will dispatch into loaded tracing plugins.
Developers will be able to select a list of plugins by setting an environment
variable, such as GST_TRACE="meminfo,dbus". One can also pass parameters to
plugins:  GST_TRACE="log(events;buffers),stats(all)".
When then plugins are loaded, we'll add them to certain hooks according to that
they are interested in.

Another env var GST_TRACE_CHANNEL can be used to send the tracing to a file or
a socket (Do the same for GST_DEBUG_CHANNEL). The syntax could be
GST_XXX_CHANNEL=file:///path/to/file or GST_XXX_CHANNEL=tcp://<ip>:<port>.
If no channel is set, the tracing goes to stderr like the debug logging.

TODO(ensonic): we might want to have GST_{DEBUG|TRACE)_FORMAT envars as well.
These could be raw, ansi-color, binary, suitable for babeltrace (see lttng), ...

With these we can deprecate GST_DEBUG_FILE and GST_DEBUG_NO_COLOR.

Hook api
--------
e.g. gst_pad_push() would become:

#ifndef GST_DISABLE_GST_DEBUG
static inline GstFlowReturn __gst_pad_push (GstPad * pad, GstBuffer * buffer);
#endif

GstFlowReturn
gst_pad_push (GstPad * pad, GstBuffer * buffer)
#ifndef GST_DISABLE_GST_DEBUG
{
  if (__tracer_enabled && __tracer_hook_is_used)
    gst_tracer_push_buffer_pre (pad, buffer);
  GstFlowReturn res = __gst_pad_push (GstPad * pad, GstBuffer * buffer);
  if (__tracer_enabled && __tracer_hook_is_used)
    gst_tracer_push_buffer_post (pad, res);
  return res;
}

static inline GstFlowReturn
__gst_pad_push (GstPad * pad, GstBuffer * buffer)
#endif
{
  g_return_val_if_fail (GST_IS_PAD (pad), GST_FLOW_ERROR);
  g_return_val_if_fail (GST_PAD_IS_SRC (pad), GST_FLOW_ERROR);
  g_return_val_if_fail (GST_IS_BUFFER (buffer), GST_FLOW_ERROR);

  return gst_pad_push_data (pad,
      GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_PUSH, buffer);
}

TODO(ensonic): gcc has some magic for wrapping functions
- http://gcc.gnu.org/onlinedocs/gcc/Constructing-Calls.html
- http://www.clifford.at/cfun/gccfeat/#gccfeat05.c

TODO(ensonic): we should eval if we can use something like jump_label in the kernel
- http://lwn.net/Articles/412072/ + http://lwn.net/Articles/435215/
- http://lxr.free-electrons.com/source/kernel/jump_label.c
- http://lxr.free-electrons.com/source/include/linux/jump_label.h
- http://lxr.free-electrons.com/source/arch/x86/kernel/jump_label.c
TODO(ensonic): liblttng-ust provides such a mechanism for user-space
- but this is mostly about logging traces
- it is linux specific :/

In addition to api hooks we should also provide timer hooks. Interval timers are
useful to get e.g. resource usage snapshots. Also absolute timers might make
sense. All this could be implemented with a clock thread.

Hooks
-----
- gst_bin_add
- gst_bin_remove
- gst_element_add_pad
- gst_element_post_message
- gst_element_query
- gst_element_remove_pad
- gst_pad_link
- gst_pad_pull_range
* gst_pad_push
* gst_pad_push_list
- gst_pad_push_event
- gst_pad_unlink

Plugin api
----------

TracerPlugins are plugin features. They have a simple api:

'instance creation'
Plugins can attach handlers to one or more hooks. They use a HookMask to tell
which events they are interested in. The params are the extra detail from the
environment var.

void invoke(GstTracerHookId id, GstStructure *s);
Hooks marshall the parameters given to a trace hook into a GstStructure and also
add some extra into such as a timestamp. The hooks will receive this structure.
Hooks will be called from misc threads. The trace plugins should only consume 
(=read) the provided data. Most trace plugins will log data to a trace channel.

'instance destruction'
Plugins can output results and release data. This would ideally be done at the
end of the applications, but gst_deinit() is not mandatory. gst_tracelib was
using a gcc_destructor

Plugins ideas
=============

We can have some under gstreamer/plugins/tracers/

meminfo
-------
- register to an interval-timer hook.
- call mallinfo() and log memory usage

rusage
------
- register to an interval-timer hook.
- call getrusage() and log resource usage

dbus
----
- provide a dbus iface to announce applications that are traced
- tracing UIs can use the dbus iface to find the channels where logging and
  tracing is getting logged to
- one would start the tracing UI first and when the application is started with
  tracing activated, the dbus plugin will announce the new application,
  upon which the tracing UI can start reading from the log channels, this avoid
  missing some data

topology
--------
- register to pipeline topology hooks
- tracing UIs can show a live pipeline graph

stats
-----
- register to buffer, event, message and query flow
- tracing apps can do e.g. statistics

UI
==

gst-debug-viewer
----------------
gst-debug-viewer could be given the tracelog in addition to the debug log.
Alternatively it would show a dialog that shows all local apps (if the dbus
plugin is loaded) and read the log streams from the sockets/files that are
configured for the app.

gst-tracer
----------
Counterpart of gst-tracelib-ui


Problems / Open items
=====================
- when hooking into a timer, should we just have some predefined intervals?
- how to trigger the shutdown processing?
  gst_deinit() (apps can call it from atexit())
- how to share the quark table from gst/gstquark?
  use a separate table in gst/gsttracers or add a getter to gst/gstquark
- how to chain tracers (use the rank?)
  e.g. how can the stats tracer get data from rusage/mallinfo tracers
  or should all tracers just log and we would use a gst-stats tool to collect
  the data from the log and process it then (that would also solve the deinit()
  issue)
- when connecting to a running app, we can't easily get the 'current' state if
  logging is using a socket, as past events are not stored


Try it
======
GST_DEBUG="GST_REG*:4,GST_TRACER:4,log:7" GST_TRACE=log gst-launch-1.0 fakesrc num-buffers=10 ! fakesink
- traces for buffer flow in TRACE level and default category