6 Pigweed's tracing module provides facilities for applications to trace
7 information about the execution of their application. The module is split into
10 1. The facade (this module) which is only a macro interface layer
11 2. The backend, provided elsewhere, implements the low level tracing.
16 This module is currently in development, and is therefore still undergoing
21 1. A Python library to generate trace files which can be viewed. (pwbug/205)
22 2. Add more examples with sample output. (pwbug/207)
23 3. Implement a trace backend module. (pwbug/260)
28 Traces provide a useful view which shows the flow of events through a system,
29 and can greatly assist in understanding complex software problems. These traces
30 can either show what tasks are running at any given time, or use added code
31 (similar to logging), to help annotate specific interesting flows.
33 Fundamentally, tracing is similar to logging in that it provides the developer
34 with a view into what the system was doing at a particular time. The fundamental
35 difference between logs and tracing is that logs present information as ad-hoc
36 human-readable strings and are better suited to providing the current state of
37 the system. Instead, traces capture sequences of events with precise timestamps,
38 and are therefore useful at understanding the flow of events in the system over
43 Most of the facade is compatible with C and C++, the only exception to this is
44 the Scope and Function tracing macros which are convenience wrappers only
56 #define PW_TRACE_MODULE_NAME "Input"
57 #include "pw_trace/trace.h"
66 auto event = WaitNewInputEvent()
67 TRACE_SCOPE("Handle Event"); // measure until loop finished
68 if (event == kNewButton){
70 PW_TRACE_END("button"); // Trace event was started in ButtonIsr
72 PW_TRACE_INSTANT("Unknown event");
78 PW_TRACE_START("button");
79 SendNewInputEvent(kNewButton);
87 The ``pw_trace`` public API provides three basic trace events:
89 - ``PW_TRACE_INSTANT`` - This is used to trace an instant event, which has no
91 - ``PW_TRACE_START`` & ``PW_TRACE_END`` - Trace 'start' and 'end' events are
92 paired together to measure the duration of an event.
94 These trace event macros all have the same arguments:
96 - *label* - Each of these trace events must have a label, which is a string
97 describing the event. In addition to the required label, each of these traces
98 can optionally provide a group label and trace id.
99 - *group_label* - The *optional* group label is used if many traces are all
100 measuring the same thing and should be grouped together. This information will
101 be used when visualizing the trace to ensure the information appears together.
102 - *trace_id* - The *optional* trace id is similar to the group_id, but instead
103 groups events using a runtime value. This can be used if multiple trace flow
104 might happen at the same time and need to be grouped together.
105 For example, this could be used to trace data packets flowing through the
106 system; when a new sample might arrive before the previous packet is finished
107 processing. This would result in two start events occurring before the end
108 event. By providing a trace id with a different value for each packet, these
109 can be separated when decoding.
113 All of these arguments must be the same for a *start* and *end* pair.
115 This results in 9 different trace calls:
117 .. cpp:function:: PW_TRACE_INSTANT(label)
118 .. cpp:function:: PW_TRACE_INSTANT(label, group_label)
119 .. cpp:function:: PW_TRACE_INSTANT(label, group_label, trace_id)
120 .. cpp:function:: PW_TRACE_START(label)
121 .. cpp:function:: PW_TRACE_START(label, group_label)
122 .. cpp:function:: PW_TRACE_START(label, group_label, trace_id)
123 .. cpp:function:: PW_TRACE_END(label)
124 .. cpp:function:: PW_TRACE_END(label, group_label)
125 .. cpp:function:: PW_TRACE_END(label, group_label, trace_id)
129 In addition to these arguments, traces can be grouped into modules similar to
130 logging. This is done by redefining the ``PW_TRACE_MODULE_NAME``.
134 Each trace event also has a flags field which can be used to provide additional
135 compile time trace information. Each trace macro has a matching macro which
136 allows specifying the flag:
138 .. cpp:function:: PW_TRACE_INSTANT_FLAG(flag, label)
139 .. cpp:function:: PW_TRACE_INSTANT_FLAG(flag, label, group_label)
140 .. cpp:function:: PW_TRACE_INSTANT_FLAG(flag, label, group_label, trace_id)
141 .. cpp:function:: PW_TRACE_START_FLAG(flag, label)
142 .. cpp:function:: PW_TRACE_START_FLAG(flag, label, group_label)
143 .. cpp:function:: PW_TRACE_START_FLAG(flag, label, group_label, trace_id)
144 .. cpp:function:: PW_TRACE_END_FLAG(flag, label)
145 .. cpp:function:: PW_TRACE_END_FLAG(flag, label, group_label)
146 .. cpp:function:: PW_TRACE_END_FLAG(flag, label, group_label, trace_id)
148 When not specified the flag uses the value of the macro ``PW_TRACE_FLAGS``.
152 Each macro also has a variant which allows appending additional data:
154 .. cpp:function:: PW_TRACE_INSTANT_DATA(label, data_format_string, data, size)
155 .. cpp:function:: PW_TRACE_INSTANT_DATA(\
156 label, group_label, data_format_string, data, size)
157 .. cpp:function:: PW_TRACE_INSTANT_DATA(\
158 label, group_label, trace_id, data_format_string, data, size)
159 .. cpp:function:: PW_TRACE_START_DATA(label, data_format_string, data, size)
160 .. cpp:function:: PW_TRACE_START_DATA(\
161 label, group_label, data_format_string, data, size)
162 .. cpp:function:: PW_TRACE_START_DATA(\
163 label, group_label, trace_id, data_format_string, data, size)
164 .. cpp:function:: PW_TRACE_END_DATA(label, data_format_string, data, size)
165 .. cpp:function:: PW_TRACE_END_DATA(\
166 label, group_label, data_format_string, data, size)
167 .. cpp:function:: PW_TRACE_END_DATA(\
168 label, group_label, trace_id, data_format_string, data, size)
170 These macros require 3 additional arguments:
172 - *data_format_string* - A string which is used by the decoder to identify the
173 data. This could for example either be printf style tokens, python struct
174 packed fmt string or a custom label recognized by the decoder.
175 - *data* - A pointer to a buffer of arbitrary caller-provided data (void*).
176 - *size* - The size of the data (size_t).
178 Currently the included python tool supports a few different options for
179 *data_format_string*:
181 - *@pw_arg_label* - Uses the string in the data as the trace event label.
182 - *@pw_arg_group* - Uses the string in the data as the trace event group.
183 - *@pw_arg_counter* - Uses the data as a little endian integer value, and
184 visualizes it as a counter value in the trace (on a graph).
185 - *@pw_py_struct_fmt:* - Interprets the string after the ":" as a python struct
186 format string, and uses that format string to unpack the data elements. This
187 can be used to either provide a single value type, or provide multiple
188 different values with a variety of types. Options for format string types can
189 be found here: https://docs.python.org/3/library/struct.html#format-characters
193 It is ok for only one event of a start/end pair to contain data, as long the
194 *label*, *group_label*, and *trace_id*, are all the same.
198 Scope API measures durations until the object loses scope. This can for
199 example, provide a convenient method of tracing functions or loops.
201 .. cpp:function:: PW_TRACE_SCOPE(label)
202 .. cpp:function:: PW_TRACE_SCOPE(label, group_label)
204 Function API measures durations until the function returns. This is the only
205 macro which does not require a *label*, and instead uses the function name as the
206 label. It still can optionally be provided a *group_id*.
208 .. cpp:function:: PW_TRACE_FUNCTION()
209 .. cpp:function:: PW_TRACE_FUNCTION(group_label)
214 Each of the trace event macros get sent to one of two macros which are
215 implemented by the backend:
217 .. cpp:function:: PW_TRACE(event_type, flags, label, group_label, trace_id)
218 .. cpp:function:: PW_TRACE_DATA(event_type, flags, label, group_label, \
219 trace_id, data_format_string, data, size)
221 The ``event_type`` value will be whatever the backend defined for that specific
222 trace type using the macros defined below.
224 The backend can optionally not define ``PW_TRACE_DATA`` to have those traces
229 Although there are only 3 basic trace types, each has 3 variants:
233 - Label, group, and trace_id
235 This combination creates 9 different trace event types:
237 - *PW_TRACE_TYPE_INSTANT*: Instant trace, with only a label.
238 - *PW_TRACE_TYPE_DURATION_START*: Start trace, with only a label.
239 - *PW_TRACE_TYPE_DURATION_END*: End trace, with only a label.
240 - *PW_TRACE_TYPE_INSTANT_GROUP*: Instant trace, with a label and a group.
241 - *PW_TRACE_TYPE_DURATION_GROUP_START*: Start trace, with a label and a group.
242 - *PW_TRACE_TYPE_DURATION_GROUP_END*: End trace, with a label and a group.
243 - *PW_TRACE_TYPE_ASYNC_INSTANT*: Instant trace, with label, group, and trace_id
244 - *PW_TRACE_TYPE_ASYNC_START*: Start trace, with label, group, and trace_id.
245 - *PW_TRACE_TYPE_ASYNC_END*: End trace, with label, group, and trace_id.
247 The backend must define these macros to have them enabled. If any are left
248 undefined, any traces of that type are removed.
252 The backend can use these macros to change what the default value is if not
255 - *PW_TRACE_FLAGS_DEFAULT*: Default value if no flags are provided.
256 - *PW_TRACE_TRACE_ID_DEFAULT*: Default value if no trace_id provided.
257 - *PW_TRACE_GROUP_LABEL_DEFAULT*: Default value if no group_label provided.
262 A sample application is provided in the examples folder. This code attempts to
263 provide examples of the multiple ways tracing can be used. Furthermore,
264 trace backends can include the sample app in their own examples to show how to
267 The sample app contains 3 "fake" tasks, which are each in their own
270 - *Input*: Simulating a driver, which gets data periodically, and sends to
272 - *Processing*: Has a work queue, which handles processing the jobs.
273 - *Kernel*: A simple work loop which demonstrates a possible integration of
274 tracing with a RTOS/Kernel.
276 Jobs are intentionally made to have multiple stages of processing (simulated by
277 being re-added to the work-queue). This results in multiple jobs being handled
278 at the same time, the trace_id is used to separate these traces.
280 ----------------------
281 Python Trace Generator
282 ----------------------
283 The Python tool is still in early developments, but currently it supports
284 generating a list of json lines from a list of trace events.
286 To view the trace, these lines can be saved to a file and loaded into
289 Future work will look to add:
291 - Config options to customize output.
292 - A method of providing custom data formatters.