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LLVM 5.0
author kono
date Fri, 27 Oct 2017 17:07:41 +0900
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1 ======================================
2 XRay Flight Data Recorder Trace Format
3 ======================================
4
5 :Version: 1 as of 2017-07-20
6
7 .. contents::
8 :local:
9
10
11 Introduction
12 ============
13
14 When gathering XRay traces in Flight Data Recorder mode, each thread of an
15 application will claim buffers to fill with trace data, which at some point
16 is finalized and flushed.
17
18 A goal of the profiler is to minimize overhead, so the flushed data directly
19 corresponds to the buffer.
20
21 This document describes the format of a trace file.
22
23
24 General
25 =======
26
27 Each trace file corresponds to a sequence of events in a particular thread.
28
29 The file has a header followed by a sequence of discriminated record types.
30
31 The endianness of byte fields matches the endianess of the platform which
32 produced the trace file.
33
34
35 Header Section
36 ==============
37
38 A trace file begins with a 32 byte header.
39
40 +-------------------+-----------------+----------------------------------------+
41 | Field | Size (bytes) | Description |
42 +===================+=================+========================================+
43 | version | ``2`` | Anticipates versioned readers. This |
44 | | | document describes the format when |
45 | | | version == 1 |
46 +-------------------+-----------------+----------------------------------------+
47 | type | ``2`` | An enumeration encoding the type of |
48 | | | trace. Flight Data Recorder mode |
49 | | | traces have type == 1 |
50 +-------------------+-----------------+----------------------------------------+
51 | bitfield | ``4`` | Holds parameters that are not aligned |
52 | | | to bytes. Further described below. |
53 +-------------------+-----------------+----------------------------------------+
54 | cycle_frequency | ``8`` | The frequency in hertz of the CPU |
55 | | | oscillator used to measure duration of |
56 | | | events in ticks. |
57 +-------------------+-----------------+----------------------------------------+
58 | buffer_size | ``8`` | The size in bytes of the data portion |
59 | | | of the trace following the header. |
60 +-------------------+-----------------+----------------------------------------+
61 | reserved | ``8`` | Reserved for future use. |
62 +-------------------+-----------------+----------------------------------------+
63
64 The bitfield parameter of the file header is composed of the following fields.
65
66 +-------------------+----------------+-----------------------------------------+
67 | Field | Size (bits) | Description |
68 +===================+================+=========================================+
69 | constant_tsc | ``1`` | Whether the platform's timestamp |
70 | | | counter used to record ticks between |
71 | | | events ticks at a constant frequency |
72 | | | despite CPU frequency changes. |
73 | | | 0 == non-constant. 1 == constant. |
74 +-------------------+----------------+-----------------------------------------+
75 | nonstop_tsc | ``1`` | Whether the tsc continues to count |
76 | | | despite whether the CPU is in a low |
77 | | | power state. 0 == stop. 1 == non-stop. |
78 +-------------------+----------------+-----------------------------------------+
79 | reserved | ``30`` | Not meaningful. |
80 +-------------------+----------------+-----------------------------------------+
81
82
83 Data Section
84 ============
85
86 Following the header in a trace is a data section with size matching the
87 buffer_size field in the header.
88
89 The data section is a stream of elements of different types.
90
91 There are a few categories of data in the sequence.
92
93 - ``Function Records``: Function Records contain the timing of entry into and
94 exit from function execution. Function Records have 8 bytes each.
95
96 - ``Metadata Records``: Metadata records serve many purposes. Mostly, they
97 capture information that may be too costly to record for each function, but
98 that is required to contextualize the fine-grained timings. They also are used
99 as markers for user-defined Event Data payloads. Metadata records have 16
100 bytes each.
101
102 - ``Event Data``: Free form data may be associated with events that are traced
103 by the binary and encode data defined by a handler function. Event data is
104 always preceded with a marker record which indicates how large it is.
105
106 - ``Function Arguments``: The arguments to some functions are included in the
107 trace. These are either pointer addresses or primitives that are read and
108 logged independently of their types in a high level language. To the tracer,
109 they are all simply numbers. Function Records that have attached arguments
110 will indicate their presence on the function entry record. We only support
111 logging contiguous function argument sequences starting with argument zero,
112 which will be the "this" pointer for member function invocations. For example,
113 we don't support logging the first and third argument.
114
115 A reader of the memory format must maintain a state machine. The format makes no
116 attempt to pad for alignment, and it is not seekable.
117
118
119 Function Records
120 ----------------
121
122 Function Records have an 8 byte layout. This layout encodes information to
123 reconstruct a call stack of instrumented function and their durations.
124
125 +---------------+--------------+-----------------------------------------------+
126 | Field | Size (bits) | Description |
127 +===============+==============+===============================================+
128 | discriminant | ``1`` | Indicates whether a reader should read a |
129 | | | Function or Metadata record. Set to ``0`` for |
130 | | | Function records. |
131 +---------------+--------------+-----------------------------------------------+
132 | action | ``3`` | Specifies whether the function is being |
133 | | | entered, exited, or is a non-standard entry |
134 | | | or exit produced by optimizations. |
135 +---------------+--------------+-----------------------------------------------+
136 | function_id | ``28`` | A numeric ID for the function. Resolved to a |
137 | | | name via the xray instrumentation map. The |
138 | | | instrumentation map is built by xray at |
139 | | | compile time into an object file and pairs |
140 | | | the function ids to addresses. It is used for |
141 | | | patching and as a lookup into the binary's |
142 | | | symbols to obtain names. |
143 +---------------+--------------+-----------------------------------------------+
144 | tsc_delta | ``32`` | The number of ticks of the timestamp counter |
145 | | | since a previous record recorded a delta or |
146 | | | other TSC resetting event. |
147 +---------------+--------------+-----------------------------------------------+
148
149 On little-endian machines, the bitfields are ordered from least significant bit
150 bit to most significant bit. A reader can read an 8 bit value and apply the mask
151 ``0x01`` for the discriminant. Similarly, they can read 32 bits and unsigned
152 shift right by ``0x04`` to obtain the function_id field.
153
154 On big-endian machine, the bitfields are written in order from most significant
155 bit to least significant bit. A reader would read an 8 bit value and unsigned
156 shift right by 7 bits for the discriminant. The function_id field could be
157 obtained by reading a 32 bit value and applying the mask ``0x0FFFFFFF``.
158
159 Function action types are as follows.
160
161 +---------------+--------------+-----------------------------------------------+
162 | Type | Number | Description |
163 +===============+==============+===============================================+
164 | Entry | ``0`` | Typical function entry. |
165 +---------------+--------------+-----------------------------------------------+
166 | Exit | ``1`` | Typical function exit. |
167 +---------------+--------------+-----------------------------------------------+
168 | Tail_Exit | ``2`` | An exit from a function due to tail call |
169 | | | optimization. |
170 +---------------+--------------+-----------------------------------------------+
171 | Entry_Args | ``3`` | A function entry that records arguments. |
172 +---------------+--------------+-----------------------------------------------+
173
174 Entry_Args records do not contain the arguments themselves. Instead, metadata
175 records for each of the logged args follow the function record in the stream.
176
177
178 Metadata Records
179 ----------------
180
181 Interspersed throughout the buffer are 16 byte Metadata records. For typically
182 instrumented binaries, they will be sparser than Function records, and they
183 provide a fuller picture of the binary execution state.
184
185 Metadata record layout is partially record dependent, but they share a common
186 structure.
187
188 The same bit field rules described for function records apply to the first byte
189 of MetadataRecords. Within this byte, little endian machines use lsb to msb
190 ordering and big endian machines use msb to lsb ordering.
191
192 +---------------+--------------+-----------------------------------------------+
193 | Field | Size | Description |
194 +===============+==============+===============================================+
195 | discriminant | ``1 bit`` | Indicates whether a reader should read a |
196 | | | Function or Metadata record. Set to ``1`` for |
197 | | | Metadata records. |
198 +---------------+--------------+-----------------------------------------------+
199 | record_kind | ``7 bits`` | The type of Metadata record. |
200 +---------------+--------------+-----------------------------------------------+
201 | data | ``15 bytes`` | A data field used differently for each record |
202 | | | type. |
203 +---------------+--------------+-----------------------------------------------+
204
205 Here is a table of the enumerated record kinds.
206
207 +--------+---------------------------+
208 | Number | Type |
209 +========+===========================+
210 | 0 | NewBuffer |
211 +--------+---------------------------+
212 | 1 | EndOfBuffer |
213 +--------+---------------------------+
214 | 2 | NewCPUId |
215 +--------+---------------------------+
216 | 3 | TSCWrap |
217 +--------+---------------------------+
218 | 4 | WallTimeMarker |
219 +--------+---------------------------+
220 | 5 | CustomEventMarker |
221 +--------+---------------------------+
222 | 6 | CallArgument |
223 +--------+---------------------------+
224
225
226 NewBuffer Records
227 -----------------
228
229 Each buffer begins with a NewBuffer record immediately after the header.
230 It records the thread ID of the thread that the trace belongs to.
231
232 Its data segment is as follows.
233
234 +---------------+--------------+-----------------------------------------------+
235 | Field | Size (bytes) | Description |
236 +===============+==============+===============================================+
237 | thread_Id | ``2`` | Thread ID for buffer. |
238 +---------------+--------------+-----------------------------------------------+
239 | reserved | ``13`` | Unused. |
240 +---------------+--------------+-----------------------------------------------+
241
242
243 WallClockTime Records
244 ---------------------
245
246 Following the NewBuffer record, each buffer records an absolute time as a frame
247 of reference for the durations recorded by timestamp counter deltas.
248
249 Its data segment is as follows.
250
251 +---------------+--------------+-----------------------------------------------+
252 | Field | Size (bytes) | Description |
253 +===============+==============+===============================================+
254 | seconds | ``8`` | Seconds on absolute timescale. The starting |
255 | | | point is unspecified and depends on the |
256 | | | implementation and platform configured by the |
257 | | | tracer. |
258 +---------------+--------------+-----------------------------------------------+
259 | microseconds | ``4`` | The microsecond component of the time. |
260 +---------------+--------------+-----------------------------------------------+
261 | reserved | ``3`` | Unused. |
262 +---------------+--------------+-----------------------------------------------+
263
264
265 NewCpuId Records
266 ----------------
267
268 Each function entry invokes a routine to determine what CPU is executing.
269 Typically, this is done with readtscp, which reads the timestamp counter at the
270 same time.
271
272 If the tracing detects that the execution has switched CPUs or if this is the
273 first instrumented entry point, the tracer will output a NewCpuId record.
274
275 Its data segment is as follows.
276
277 +---------------+--------------+-----------------------------------------------+
278 | Field | Size (bytes) | Description |
279 +===============+==============+===============================================+
280 | cpu_id | ``2`` | CPU Id. |
281 +---------------+--------------+-----------------------------------------------+
282 | absolute_tsc | ``8`` | The absolute value of the timestamp counter. |
283 +---------------+--------------+-----------------------------------------------+
284 | reserved | ``5`` | Unused. |
285 +---------------+--------------+-----------------------------------------------+
286
287
288 TSCWrap Records
289 ---------------
290
291 Since each function record uses a 32 bit value to represent the number of ticks
292 of the timestamp counter since the last reference, it is possible for this value
293 to overflow, particularly for sparsely instrumented binaries.
294
295 When this delta would not fit into a 32 bit representation, a reference absolute
296 timestamp counter record is written in the form of a TSCWrap record.
297
298 Its data segment is as follows.
299
300 +---------------+--------------+-----------------------------------------------+
301 | Field | Size (bytes) | Description |
302 +===============+==============+===============================================+
303 | absolute_tsc | ``8`` | Timestamp counter value. |
304 +---------------+--------------+-----------------------------------------------+
305 | reserved | ``7`` | Unused. |
306 +---------------+--------------+-----------------------------------------------+
307
308
309 CallArgument Records
310 --------------------
311
312 Immediately following an Entry_Args type function record, there may be one or
313 more CallArgument records that contain the traced function's parameter values.
314
315 The order of the CallArgument Record sequency corresponds one to one with the
316 order of the function parameters.
317
318 CallArgument data segment:
319
320 +---------------+--------------+-----------------------------------------------+
321 | Field | Size (bytes) | Description |
322 +===============+==============+===============================================+
323 | argument | ``8`` | Numeric argument (may be pointer address). |
324 +---------------+--------------+-----------------------------------------------+
325 | reserved | ``7`` | Unused. |
326 +---------------+--------------+-----------------------------------------------+
327
328
329 CustomEventMarker Records
330 -------------------------
331
332 XRay provides the feature of logging custom events. This may be leveraged to
333 record tracing info for RPCs or similarly trace data that is application
334 specific.
335
336 Custom Events themselves are an unstructured (application defined) segment of
337 memory with arbitrary size within the buffer. They are preceded by
338 CustomEventMarkers to indicate their presence and size.
339
340 CustomEventMarker data segment:
341
342 +---------------+--------------+-----------------------------------------------+
343 | Field | Size (bytes) | Description |
344 +===============+==============+===============================================+
345 | event_size | ``4`` | Size of preceded event. |
346 +---------------+--------------+-----------------------------------------------+
347 | absolute_tsc | ``8`` | A timestamp counter of the event. |
348 +---------------+--------------+-----------------------------------------------+
349 | reserved | ``3`` | Unused. |
350 +---------------+--------------+-----------------------------------------------+
351
352
353 EndOfBuffer Records
354 -------------------
355
356 An EndOfBuffer record type indicates that there is no more trace data in this
357 buffer. The reader is expected to seek past the remaining buffer_size expressed
358 before the start of buffer and look for either another header or EOF.
359
360
361 Format Grammar and Invariants
362 =============================
363
364 Not all sequences of Metadata records and Function records are valid data. A
365 sequence should be parsed as a state machine. The expectations for a valid
366 format can be expressed as a context free grammar.
367
368 This is an attempt to explain the format with statements in EBNF format.
369
370 - Format := Header ThreadBuffer* EOF
371
372 - ThreadBuffer := NewBuffer WallClockTime NewCPUId BodySequence* End
373
374 - BodySequence := NewCPUId | TSCWrap | Function | CustomEvent
375
376 - Function := (Function_Entry_Args CallArgument*) | Function_Other_Type
377
378 - CustomEvent := CustomEventMarker CustomEventUnstructuredMemory
379
380 - End := EndOfBuffer RemainingBufferSizeToSkip
381
382
383 Function Record Order
384 ---------------------
385
386 There are a few clarifications that may help understand what is expected of
387 Function records.
388
389 - Functions with an Exit are expected to have a corresponding Entry or
390 Entry_Args function record precede them in the trace.
391
392 - Tail_Exit Function records record the Function ID of the function whose return
393 address the program counter will take. In other words, the final function that
394 would be popped off of the call stack if tail call optimization was not used.
395
396 - Not all functions marked for instrumentation are necessarily in the trace. The
397 tracer uses heuristics to preserve the trace for non-trivial functions.
398
399 - Not every entry must have a traced Exit or Tail Exit. The buffer may run out
400 of space or the program may request for the tracer to finalize toreturn the
401 buffer before an instrumented function exits.