4.8 KiB
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 (i.e. tracer).
Basics
Tracing is separated in two different concepts. The tracing API and the tracing probes.
The API is in src/lib/trace/ which defines how to call tracepoints in the
tor code. Every C files should include `src/lib/trace/events.h" if they want
to call a tracepoint.
The probes are what actually record the tracepoint data. Because they often
need to access specific subsystem objects, the probes are within each
subsystem. They are defined in the trace-probes-<subsystem>.c files.
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 the 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 adds better semantic to the
event.
The args can be 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 one argument.
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.
Instrumentation
In src/lib/trace/events.h, we map the high level tor\_trace() macro to one
or many enabled instrumentation.
Currently, we have 3 types of possible instrumentation:
- Debug
This will map every tracepoint to log\_debug(). However, none of the
arguments will be passed on because we don't know their type nor the string
format of the debug log. The output is standardized like this:
[debug] __FUNC__: Tracepoint <event_name> from subsystem <subsystem> hit.
- USDT
User Statically-Defined Tracing (USDT) is a kind of probe which can be handled by a variety of tracers such as SystemTap, DTrace, perf, eBPF and ftrace.
For each tracer, one will need to define the ABI in order for the tracer to
be able to extract the data from the tracepoint objects. For instance, the
tracer needs to know how to print the circuit state of a circuit\_t
object.
- LTTng-UST
LTTng Userspace is a tracer that has it own type of instrumentation. The probe definitions are created within the C code and is strongly typed.
For more information, see https://lttng.org/docs.
Build System
This section describes how the instrumentation is integrated into the build system of tor.
By default, every tracing events are disabled in tor that is tor\_trace() is
a NOP thus has no execution cost time.
To enable a specific instrumentation, there are configure options:
-
Debug:
--enable-tracing-instrumentation-debug -
USDT:
--enable-tracing-instrumentation-usdt -
LTTng:
--enable-tracing-instrumentation-lttng
They can all be used together or independently. If one of them is set,
HAVE\_TRACING define is set. And for each instrumentation, a
USE\_TRACING\_INSTRUMENTATION\_<type> is set.
Adding a Tracepoint
This is pretty easy. Let's say you want to add a trace event in
src/feature/rend/rendcache.c, you first need to include this file:
#include "lib/trace/events.h"
Then, the tor\_trace() macro can be used with the specific format detailled
before in a previous section. As an example:
tor_trace(hs, store_desc_as_client, desc, desc_id);
For Debug instrumentation, you have nothing else to do.
For USDT, instrumentation, you will need to define the probes in a way the
specific tracer can understand. For instance, SystemTap requires you to define
a tapset for each tracepoints.
For LTTng, you will need to define the probes in the
trace-probes-<subsystem>.{c|h} file. See the trace-probes-circuit.{c|h}
file as an example and https://lttng.org/docs/v2.11/#doc-instrumenting.
Performance
A word about performance when a tracepoint is enabled. One of the goal of a
tracepoint (USDT, LTTng-UST, ...) is that they can be enabled or disabled. By
default, they are disabled which means the tracer will not record the data but
it has to do a check thus the cost is basically the one of a branch.
If enabled, then the performance depends on the tracer. In the case of LTTng-UST, the event costs around 110nsec.