mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-12-05 02:07:56 +00:00
ee14ad5043
llvm-svn: 333354
402 lines
19 KiB
ReStructuredText
402 lines
19 KiB
ReStructuredText
======================================
|
|
XRay Flight Data Recorder Trace Format
|
|
======================================
|
|
|
|
:Version: 1 as of 2017-07-20
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
When gathering XRay traces in Flight Data Recorder mode, each thread of an
|
|
application will claim buffers to fill with trace data, which at some point
|
|
is finalized and flushed.
|
|
|
|
A goal of the profiler is to minimize overhead, the flushed data directly
|
|
corresponds to the buffer.
|
|
|
|
This document describes the format of a trace file.
|
|
|
|
|
|
General
|
|
=======
|
|
|
|
Each trace file corresponds to a sequence of events in a particular thread.
|
|
|
|
The file has a header followed by a sequence of discriminated record types.
|
|
|
|
The endianness of byte fields matches the endianess of the platform which
|
|
produced the trace file.
|
|
|
|
|
|
Header Section
|
|
==============
|
|
|
|
A trace file begins with a 32 byte header.
|
|
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===================+=================+========================================+
|
|
| version | ``2`` | Anticipates versioned readers. This |
|
|
| | | document describes the format when |
|
|
| | | version == 1 |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| type | ``2`` | An enumeration encoding the type of |
|
|
| | | trace. Flight Data Recorder mode |
|
|
| | | traces have type == 1 |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| bitfield | ``4`` | Holds parameters that are not aligned |
|
|
| | | to bytes. Further described below. |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| cycle_frequency | ``8`` | The frequency in hertz of the CPU |
|
|
| | | oscillator used to measure duration of |
|
|
| | | events in ticks. |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| buffer_size | ``8`` | The size in bytes of the data portion |
|
|
| | | of the trace following the header. |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
| reserved | ``8`` | Reserved for future use. |
|
|
+-------------------+-----------------+----------------------------------------+
|
|
|
|
The bitfield parameter of the file header is composed of the following fields.
|
|
|
|
+-------------------+----------------+-----------------------------------------+
|
|
| Field | Size (bits) | Description |
|
|
+===================+================+=========================================+
|
|
| constant_tsc | ``1`` | Whether the platform's timestamp |
|
|
| | | counter used to record ticks between |
|
|
| | | events ticks at a constant frequency |
|
|
| | | despite CPU frequency changes. |
|
|
| | | 0 == non-constant. 1 == constant. |
|
|
+-------------------+----------------+-----------------------------------------+
|
|
| nonstop_tsc | ``1`` | Whether the tsc continues to count |
|
|
| | | despite whether the CPU is in a low |
|
|
| | | power state. 0 == stop. 1 == non-stop. |
|
|
+-------------------+----------------+-----------------------------------------+
|
|
| reserved | ``30`` | Not meaningful. |
|
|
+-------------------+----------------+-----------------------------------------+
|
|
|
|
|
|
Data Section
|
|
============
|
|
|
|
Following the header in a trace is a data section with size matching the
|
|
buffer_size field in the header.
|
|
|
|
The data section is a stream of elements of different types.
|
|
|
|
There are a few categories of data in the sequence.
|
|
|
|
- ``Function Records``: Function Records contain the timing of entry into and
|
|
exit from function execution. Function Records have 8 bytes each.
|
|
|
|
- ``Metadata Records``: Metadata records serve many purposes. Mostly, they
|
|
capture information that may be too costly to record for each function, but
|
|
that is required to contextualize the fine-grained timings. They also are used
|
|
as markers for user-defined Event Data payloads. Metadata records have 16
|
|
bytes each.
|
|
|
|
- ``Event Data``: Free form data may be associated with events that are traced
|
|
by the binary and encode data defined by a handler function. Event data is
|
|
always preceded with a marker record which indicates how large it is.
|
|
|
|
- ``Function Arguments``: The arguments to some functions are included in the
|
|
trace. These are either pointer addresses or primitives that are read and
|
|
logged independently of their types in a high level language. To the tracer,
|
|
they are all numbers. Function Records that have attached arguments will
|
|
indicate their presence on the function entry record. We only support logging
|
|
contiguous function argument sequences starting with argument zero, which will
|
|
be the "this" pointer for member function invocations. For example, we don't
|
|
support logging the first and third argument.
|
|
|
|
A reader of the memory format must maintain a state machine. The format makes no
|
|
attempt to pad for alignment, and it is not seekable.
|
|
|
|
|
|
Function Records
|
|
----------------
|
|
|
|
Function Records have an 8 byte layout. This layout encodes information to
|
|
reconstruct a call stack of instrumented function and their durations.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bits) | Description |
|
|
+===============+==============+===============================================+
|
|
| discriminant | ``1`` | Indicates whether a reader should read a |
|
|
| | | Function or Metadata record. Set to ``0`` for |
|
|
| | | Function records. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| action | ``3`` | Specifies whether the function is being |
|
|
| | | entered, exited, or is a non-standard entry |
|
|
| | | or exit produced by optimizations. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| function_id | ``28`` | A numeric ID for the function. Resolved to a |
|
|
| | | name via the xray instrumentation map. The |
|
|
| | | instrumentation map is built by xray at |
|
|
| | | compile time into an object file and pairs |
|
|
| | | the function ids to addresses. It is used for |
|
|
| | | patching and as a lookup into the binary's |
|
|
| | | symbols to obtain names. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| tsc_delta | ``32`` | The number of ticks of the timestamp counter |
|
|
| | | since a previous record recorded a delta or |
|
|
| | | other TSC resetting event. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
On little-endian machines, the bitfields are ordered from least significant bit
|
|
bit to most significant bit. A reader can read an 8 bit value and apply the mask
|
|
``0x01`` for the discriminant. Similarly, they can read 32 bits and unsigned
|
|
shift right by ``0x04`` to obtain the function_id field.
|
|
|
|
On big-endian machine, the bitfields are written in order from most significant
|
|
bit to least significant bit. A reader would read an 8 bit value and unsigned
|
|
shift right by 7 bits for the discriminant. The function_id field could be
|
|
obtained by reading a 32 bit value and applying the mask ``0x0FFFFFFF``.
|
|
|
|
Function action types are as follows.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Type | Number | Description |
|
|
+===============+==============+===============================================+
|
|
| Entry | ``0`` | Typical function entry. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Exit | ``1`` | Typical function exit. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Tail_Exit | ``2`` | An exit from a function due to tail call |
|
|
| | | optimization. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Entry_Args | ``3`` | A function entry that records arguments. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
Entry_Args records do not contain the arguments themselves. Instead, metadata
|
|
records for each of the logged args follow the function record in the stream.
|
|
|
|
|
|
Metadata Records
|
|
----------------
|
|
|
|
Interspersed throughout the buffer are 16 byte Metadata records. For typically
|
|
instrumented binaries, they will be sparser than Function records, and they
|
|
provide a fuller picture of the binary execution state.
|
|
|
|
Metadata record layout is partially record dependent, but they share a common
|
|
structure.
|
|
|
|
The same bit field rules described for function records apply to the first byte
|
|
of MetadataRecords. Within this byte, little endian machines use lsb to msb
|
|
ordering and big endian machines use msb to lsb ordering.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size | Description |
|
|
+===============+==============+===============================================+
|
|
| discriminant | ``1 bit`` | Indicates whether a reader should read a |
|
|
| | | Function or Metadata record. Set to ``1`` for |
|
|
| | | Metadata records. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| record_kind | ``7 bits`` | The type of Metadata record. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| data | ``15 bytes`` | A data field used differently for each record |
|
|
| | | type. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
Here is a table of the enumerated record kinds.
|
|
|
|
+--------+---------------------------+
|
|
| Number | Type |
|
|
+========+===========================+
|
|
| 0 | NewBuffer |
|
|
+--------+---------------------------+
|
|
| 1 | EndOfBuffer |
|
|
+--------+---------------------------+
|
|
| 2 | NewCPUId |
|
|
+--------+---------------------------+
|
|
| 3 | TSCWrap |
|
|
+--------+---------------------------+
|
|
| 4 | WallTimeMarker |
|
|
+--------+---------------------------+
|
|
| 5 | CustomEventMarker |
|
|
+--------+---------------------------+
|
|
| 6 | CallArgument |
|
|
+--------+---------------------------+
|
|
|
|
|
|
NewBuffer Records
|
|
-----------------
|
|
|
|
Each buffer begins with a NewBuffer record immediately after the header.
|
|
It records the thread ID of the thread that the trace belongs to.
|
|
|
|
Its data segment is as follows.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| thread_Id | ``2`` | Thread ID for buffer. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``13`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
WallClockTime Records
|
|
---------------------
|
|
|
|
Following the NewBuffer record, each buffer records an absolute time as a frame
|
|
of reference for the durations recorded by timestamp counter deltas.
|
|
|
|
Its data segment is as follows.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| seconds | ``8`` | Seconds on absolute timescale. The starting |
|
|
| | | point is unspecified and depends on the |
|
|
| | | implementation and platform configured by the |
|
|
| | | tracer. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| microseconds | ``4`` | The microsecond component of the time. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``3`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
NewCpuId Records
|
|
----------------
|
|
|
|
Each function entry invokes a routine to determine what CPU is executing.
|
|
Typically, this is done with readtscp, which reads the timestamp counter at the
|
|
same time.
|
|
|
|
If the tracing detects that the execution has switched CPUs or if this is the
|
|
first instrumented entry point, the tracer will output a NewCpuId record.
|
|
|
|
Its data segment is as follows.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| cpu_id | ``2`` | CPU Id. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| absolute_tsc | ``8`` | The absolute value of the timestamp counter. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``5`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
TSCWrap Records
|
|
---------------
|
|
|
|
Since each function record uses a 32 bit value to represent the number of ticks
|
|
of the timestamp counter since the last reference, it is possible for this value
|
|
to overflow, particularly for sparsely instrumented binaries.
|
|
|
|
When this delta would not fit into a 32 bit representation, a reference absolute
|
|
timestamp counter record is written in the form of a TSCWrap record.
|
|
|
|
Its data segment is as follows.
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| absolute_tsc | ``8`` | Timestamp counter value. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``7`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
CallArgument Records
|
|
--------------------
|
|
|
|
Immediately following an Entry_Args type function record, there may be one or
|
|
more CallArgument records that contain the traced function's parameter values.
|
|
|
|
The order of the CallArgument Record sequency corresponds one to one with the
|
|
order of the function parameters.
|
|
|
|
CallArgument data segment:
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| argument | ``8`` | Numeric argument (may be pointer address). |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``7`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
CustomEventMarker Records
|
|
-------------------------
|
|
|
|
XRay provides the feature of logging custom events. This may be leveraged to
|
|
record tracing info for RPCs or similarly trace data that is application
|
|
specific.
|
|
|
|
Custom Events themselves are an unstructured (application defined) segment of
|
|
memory with arbitrary size within the buffer. They are preceded by
|
|
CustomEventMarkers to indicate their presence and size.
|
|
|
|
CustomEventMarker data segment:
|
|
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| Field | Size (bytes) | Description |
|
|
+===============+==============+===============================================+
|
|
| event_size | ``4`` | Size of preceded event. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| absolute_tsc | ``8`` | A timestamp counter of the event. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
| reserved | ``3`` | Unused. |
|
|
+---------------+--------------+-----------------------------------------------+
|
|
|
|
|
|
EndOfBuffer Records
|
|
-------------------
|
|
|
|
An EndOfBuffer record type indicates that there is no more trace data in this
|
|
buffer. The reader is expected to seek past the remaining buffer_size expressed
|
|
before the start of buffer and look for either another header or EOF.
|
|
|
|
|
|
Format Grammar and Invariants
|
|
=============================
|
|
|
|
Not all sequences of Metadata records and Function records are valid data. A
|
|
sequence should be parsed as a state machine. The expectations for a valid
|
|
format can be expressed as a context free grammar.
|
|
|
|
This is an attempt to explain the format with statements in EBNF format.
|
|
|
|
- Format := Header ThreadBuffer* EOF
|
|
|
|
- ThreadBuffer := NewBuffer WallClockTime NewCPUId BodySequence* End
|
|
|
|
- BodySequence := NewCPUId | TSCWrap | Function | CustomEvent
|
|
|
|
- Function := (Function_Entry_Args CallArgument*) | Function_Other_Type
|
|
|
|
- CustomEvent := CustomEventMarker CustomEventUnstructuredMemory
|
|
|
|
- End := EndOfBuffer RemainingBufferSizeToSkip
|
|
|
|
|
|
Function Record Order
|
|
---------------------
|
|
|
|
There are a few clarifications that may help understand what is expected of
|
|
Function records.
|
|
|
|
- Functions with an Exit are expected to have a corresponding Entry or
|
|
Entry_Args function record precede them in the trace.
|
|
|
|
- Tail_Exit Function records record the Function ID of the function whose return
|
|
address the program counter will take. In other words, the final function that
|
|
would be popped off of the call stack if tail call optimization was not used.
|
|
|
|
- Not all functions marked for instrumentation are necessarily in the trace. The
|
|
tracer uses heuristics to preserve the trace for non-trivial functions.
|
|
|
|
- Not every entry must have a traced Exit or Tail Exit. The buffer may run out
|
|
of space or the program may request for the tracer to finalize toreturn the
|
|
buffer before an instrumented function exits.
|