tor/doc/HACKING/Tracing.md

Tracing

This document describes how the event tracing subsystem works in tor so developers can add events to the code base but also hook them to an event tracing framework.

Basics

Event tracing is separated in two concepts, trace events and a tracer. The tracing subsystem can be found in src/trace. The events.h header file is the main file that maps the different tracers to trace events.

Events

A trace event is basically a function from which we can pass any data that we want to collect. In addition, we specify a context for the event such as a subsystem and an event name.

A trace event in tor has the following standard format:

tor_trace(subsystem, event\_name, args...)

The subsystem parameter is the name of the subsytem the trace event is in. For example that could be "scheduler" or "vote" or "hs". The idea is to add some context to the event so when we collect them we know where it's coming from. The event_name is the name of the event which helps a lot with adding some semantic to the event. Finally, args is any number of arguments we want to collect.

Here is an example of a possible tracepoint in main():

tor_trace(main, init_phase, argc)

The above is a tracepoint in the main subsystem with init_phase as the event name and the int argc is passed to the event as well.

How argc is collected or used has nothing to do with the instrumentation (adding trace events to the code). It is the work of the tracer so this is why the trace events and collection framework (tracer) are decoupled. You can have trace events without a tracer.

Tracer

In src/trace/events.h, we map the tor_trace() function to the right tracer. A tracer support is only enabled at compile time. For instance, the file src/trace/debug.h contains the mapping of the generic tracing function tor_trace() to the log_debug() function. More specialized function can be mapped depending on the tracepoint.

Build System

This section describes how it is integrated into the build system of tor.

By default, every tracing events are disabled in tor that is tor_trace() is a NOP.

To enable a tracer, there is a configure option on the form of:

--enable-tracing-<tracer>

We have an option that will send every trace events to a log_debug() (as mentionned above) which will print you the subsystem and name of the event but not the arguments for technical reasons. This is useful if you want to quickly see if your trace event is being hit or well written. To do so, use this configure option:

--enable-tracing-debug

Instrument Tor

This is pretty easy. Let's say you want to add a trace event in src/feature/rend/rendcache.c, you only have to add this include statement:

#include "trace/events.h"

Once done, you can add as many as you want tor_trace() that you need. Please use the right subsystem (here it would be hs) and a unique name that tells what the event is for. For example:

tor_trace(hs, store_desc_as_client, desc, desc_id);

If you look in src/trace/events.h, you'll see that if tracing is enabled it will be mapped to a function called:

tor_trace_hs_store_desc_as_client(desc, desc_id)

And the point of all this is for that function to be defined in a new file that you might want to add named src/trace/hs.{c|h} which would defined how to collect the data for the tor_trace_hs_store_desc_as_client() function like for instance sending it to a log_debug() or do more complex operations or use a userspace tracer like LTTng (https://lttng.org).