trace-cmd.dat.v7 - Man Page

trace-cmd version 7 file format

Synopsis

trace-cmd.dat ignore

Description

The trace-cmd(1) utility produces a "trace.dat" file. The file may also be named anything depending if the user specifies a different output name, but it must have a certain binary format. The file is used by trace-cmd to save kernel traces into it and be able to extract the trace from it at a later point (see trace-cmd-report(1)).

Initial Format

The first three bytes contain the magic value:
0x17 0x08  0x44
The next 7 bytes contain the characters:
"tracing"
The next set of characters contain a null '\0' terminated string
that contains the version of the file:
"7\0"
The next 1 byte contains the flags for the file endianess:
0 = little endian
1 = big endian
The next byte contains the number of bytes per "long" value:
4 - 32-bit long values
8 - 64-bit long values
Note: This is the long size of the target's user space. Not the
kernel space size.
[ Now all numbers are written in file defined endianess. ]
The next 4 bytes are a 32-bit word that defines what the traced
host machine page size was.
The compression algorithm header is written next:
   "name\0version\0"
where "name" and "version" are strings, name and version of the
compression algorithm used to compress the trace file. If the name
is "none", the data in the file is not compressed.
The next 8 bytes are 64-bit integer, the offset within the file where
the first OPTIONS section is located.
The rest of the file consists of different sections. The only mandatory
is the first OPTIONS section, all others are optional. The location and
the order of the sections is not strict. Each section starts with a header:

Format of the Section Header

<2 bytes> unsigned short integer, ID of the section.
<2 bytes> unsigned short integer, section flags:
  1 = the section is compressed.
<4 bytes> ID of a string, description of the section.
<4 bytes> unsigned integer, size of the section in the file.
If the section is compressed, the above is the compressed size.
The section must be uncompressed on reading. The described format of
the sections refers to the uncompressed data.

Compression Format of the File Sections

Some of the sections in the file may be compressed with the compression algorithm,
specified in the compression algorithm header. Compressed sections have a compression
header, written after the section header and right before the compressed data:
  <4 bytes> unsigned int, size of compressed data in this section.
  <4 bytes> unsigned int, size of uncompressed data.
  <data> binary compressed data, with the specified size.

Compression Format of the Trace Data

There are two special sections, BUFFER FLYRECORD and BUFFER LATENCY, containing
trace data. These sections may be compressed with the compression algorithm, specified
in the compression header. Usually the size of these sections is huge, that's why its
compression format is different from the other sections. The trace data is compressed
in chunks The size of one chunk is specified in the file creation time. The format
of compressed trace data is:
   <4 bytes> unsigned int, count of chunks.
   Follows the compressed chunks of given count. For each chunk:
      <4 bytes> unsigned int, size of compressed data in this chunk.
      <4 bytes> unsigned int, size of uncompressed data, aligned with the trace page size.
      <data> binary compressed data, with the specified size.
These chunks must be uncompressed on reading. The described format of
trace data refers to the uncompressed data.

Options Section

Section ID: 0
This is the the only mandatory section in the file. There can be multiple
options sections, the first one is located at the offset specified right
after the compression algorithm header. The section consists of multiple
trace options, each option has the following format:
  <2 bytes> unsigned short integer, ID of the option.
  <4 bytes> unsigned integer, size of the option's data.
  <binary data> bytes of the size specified above, data of the option.
Options, supported by the trace file version 7:
DONE: id 0, size 8
  This option indicates the end of the options section, it is written
  always as last option. The DONE option data is:
     <8 bytes> long long unsigned integer, offset in the trace file where
     the next options section is located. If this offset is 0, then there
     are no more options sections.
DATE: id 1, size vary
  The DATE option data is a null terminated ASCII string, which represents
  the time difference between trace events timestamps and the Generic Time
  of Day of the system.
CPUSTAT: id 2, size vary
  The CPUSTAT option data is a null terminated ASCII string, the content of the
  "per_cpu/cpu<id>/stats" file from the trace directory. There is a CPUSTAT option
  for each CPU.
BUFFER: id 3, size vary
  The BUFFER option describes the flyrecord trace data saved in the file, collected
  from one trace instance. There is BUFFER option for each trace instance. The format
  of the BUFFER data is:
    <8 bytes> long long unsigned integer, offset in the trace file where the
    BUFFER FLYRECORD section is located, containing flyrecord trace data.
    <string> a null terminated ASCII string, name of the trace instance. Empty string ""
    is saved as name of the top instance.
    <string> a null terminated ASCII string, trace clock used for events timestamps in
    this trace instance.
    <4 bytes> unsigned integer, size of the trace buffer page.
    <4 bytes> unsigned integer, count of the CPUs with trace data.
    For each CPU of the above count:
       <4 bytes> unsigned integer, ID of the CPU.
       <8 bytes> long long unsigned integer, offset in the trace file where the trace data
       for this CPU is located.
       <8 bytes> long long unsigned integer, size of the trace data for this CPU.
TRACECLOCK: id 4, size vary
  The TRACECLOCK option data is a null terminated ASCII string, the content of the
  "trace_clock" file from the trace directory.
UNAME: id 5, size vary
  The UNAME option data is a null terminated ASCII string, identifying the system where
  the trace data is collected. The string is retrieved by the uname() system call.
HOOK: id 6, size vary
  The HOOK option data is a null terminated ASCII string, describing event hooks: custom
  event matching to connect any two events together.
OFFSET: id 7, size vary
  The OFFSET option data is a null terminated ASCII string, representing a fixed time that
  is added to each event timestamp on reading.
CPUCOUNT: id 8, size 4
  The CPUCOUNT option data is:
    <4 bytes> unsigned integer, number of CPUs in the system.
VERSION: id 9, size vary
  The VERSION option data is a null terminated ASCII string, representing the version of
  the trace-cmd application, used to collect these trace logs.
PROCMAPS: id 10, size vary
  The PROCMAPS option data is a null terminated ASCII string, representing the memory map
  of each traced filtered process. The format of the string is, for each filtered process:
    <procss ID> <libraries count> <process command> \n
      <memory start address> <memory end address> <full path of the mapped library file> \n
      ...
       separate line for each library, used by this process
      ...
    ...
TRACEID: id 11, size 8
  The TRACEID option data is a unique identifier of this tracing session:
    <8 bytes> long long unsigned integer, trace session identifier.
TIME_SHIFT: id 12, size vary
  The TIME_SHIFT option stores time synchronization information, collected during host and guest
  tracing session. Usually it is saved in the guest trace file. This information is used to
  synchronize guest with host events timestamps, when displaying all files from this tracing
  session. The format of the TIME_SHIFT option data is:
    <8 bytes> long long unsigned integer, trace identifier of the peer (usually the host).
    <4 bytes> unsigned integer, flags specific to the time synchronization protocol, used in this
    trace session.
    <4 bytes> unsigned integer, number of traced CPUs. For each CPU, timestamps corrections
    are recorded:
       <4 bytes> unsigned integer, count of the recorded timestamps corrections for this CPU.
       <array of unsigned long long integers of the above count>, times when the corrections are calculated
       <array of unsigned long long integers of the above count>, corrections offsets
       <array of unsigned long long integers of the above count>, corrections scaling ratio
GUEST: id 13, size vary
  The GUEST option stores information about traced guests in this tracing session. Usually it is
  saved in the host trace file. There is a separate GUEST option for each traced guest.
  The information is used when displaying all files from this tracing session. The format of
  the GUEST option data is:
     <string> a null terminated ASCII string, name of the guest.
     <8 bytes> long long unsigned integer, trace identifier of the guest for this session.
     <4 bytes> unsigned integer, number of guest's CPUs. For each CPU:
        <4 bytes> unsigned integer, ID of the CPU.
        <4 bytes> unsigned integer, PID of the host task, emulating this guest CPU.
TSC2NSEC: id 14, size 16
  The TSC2NSEC option stores information, used to convert TSC events timestamps to nanoseconds.
  The format of the TSC2NSEC option data is:
     <4 bytes> unsigned integer, time multiplier.
     <4 bytes> unsigned integer, time shift.
     <8 bytes> unsigned long long integer, time offset.
HEADER_INFO: id 16, size 8
  The HEADER_INFO option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the HEADER INFO
    section is located
FTRACE_EVENTS: id 17, size 8
  The FTRACE_EVENTS option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the
    FTRACE EVENT FORMATS section is located.
EVENT_FORMATS: id 18, size 8
  The EVENT_FORMATS option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the EVENT FORMATS
    section is located.
KALLSYMS: id 19, size 8
  The KALLSYMS option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the KALLSYMS
    section is located.
PRINTK: id 20, size 8
  The PRINTK option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the TRACE_PRINTK
    section is located.
CMDLINES: id 21, size 8
  The CMDLINES option data is:
    <8 bytes> long long unsigned integer, offset into the trace file where the
    SAVED COMMAND LINES section is located.
BUFFER_TEXT: id 22, size
  The BUFFER_LAT option describes the latency trace data saved in the file. The format
  of the BUFFER_LAT data is:
    <8 bytes> long long unsigned integer, offset in the trace file where the
    BUFFER LATENCY section is located, containing latency trace data.
    <string> a null terminated ASCII string, name of the trace instance. Empty string ""
    is saved as name of the top instance.
    <string> a null terminated ASCII string, trace clock used for events timestamps in
    this trace instance.

Header Info Section

Section ID: 16
The first 12 bytes of the section, after the section header, contain the string:
"header_page\0"
The next 8 bytes are a 64-bit word containing the size of the
page header information stored next.
The next set of data is of the size read from the previous 8 bytes,
and contains the data retrieved from debugfs/tracing/events/header_page.
Note: The size of the second field \fBcommit\fR contains the target
kernel long size. For example:
field: local_t commit;        offset:8;       \fBsize:8;\fR   signed:1;
shows the kernel has a 64-bit long.
The next 13 bytes contain the string:
"header_event\0"
The next 8 bytes are a 64-bit word containing the size of the
event header information stored next.
The next set of data is of the size read from the previous 8 bytes
and contains the data retrieved from debugfs/tracing/events/header_event.
This data allows the trace-cmd tool to know if the ring buffer format
of the kernel made any changes.

Ftrace Event Formats Section

Section ID: 17
Directly after the section header comes the information about
the Ftrace specific events. These are the events used by the Ftrace plugins
and are not enabled by the event tracing.
The next 4 bytes contain a 32-bit word of the number of Ftrace event
format files that are stored in the file.
For the number of times defined by the previous 4 bytes is the
following:
8 bytes for the size of the Ftrace event format file.
The Ftrace event format file copied from the target machine:
debugfs/tracing/events/ftrace/<event>/format

Event Formats Section

Section ID: 18
Directly after the section header comes the information about
the event layout.
The next 4 bytes are a 32-bit word containing the number of
event systems that are stored in the file. These are the
directories in debugfs/tracing/events excluding the \fBftrace\fR
directory.
For the number of times defined by the previous 4 bytes is the
following:
A null-terminated string containing the system name.
4 bytes containing a 32-bit word containing the number
of events within the system.
For the number of times defined in the previous 4 bytes is the
following:
8 bytes for the size of the event format file.
The event format file copied from the target machine:
debugfs/tracing/events/<system>/<event>/format

Kallsyms Section

Section ID: 19
Directly after the section header comes the information of the mapping
of function addresses to the function names.
The next 4 bytes are a 32-bit word containing the size of the
data holding the function mappings.
The next set of data is of the size defined by the previous 4 bytes
and contains the information from the target machine's file:
/proc/kallsyms

Trace_printk Section

Section ID: 20
If a developer used trace_printk() within the kernel, it may
store the format string outside the ring buffer.
This information can be found in:
debugfs/tracing/printk_formats
The next 4 bytes are a 32-bit word containing the size of the
data holding the printk formats.
The next set of data is of the size defined by the previous 4 bytes
and contains the information from debugfs/tracing/printk_formats.

Saved Command Lines Section

Section ID: 21
Directly after the section header comes the information mapping
a PID to a process name.
The next 8 bytes contain a 64-bit word that holds the size of the
data mapping the PID to a process name.
The next set of data is of the size defined by the previous 8 bytes
and contains the information from debugfs/tracing/saved_cmdlines.

Buffer Flyrecord Section

This section contains flyrecord tracing data, collected in one trace instance.
The data is saved per CPU. Each BUFFER FLYRECORD section has a corresponding BUFFER
option, containing information about saved CPU's trace data. Padding is placed between
the section header and the CPU data, placing the CPU data at a page aligned (target page)
position in the file.
This data is copied directly from the Ftrace ring buffer and is of the
same format as the ring buffer specified by the event header files
loaded in the header format file.
The trace-cmd tool will try to \fBmmap(2)\fR the data page by page with the
target's page size if possible. If it fails to mmap, it will just read the
data instead.

BUFFER TEXT SECTION

.ft C
  This section contains latency tracing data, ASCII text taken from the
  target's debugfs/tracing/trace file.

STRINGS SECTION
.ft
All strings from trace file metadata are stored in string section in the file. The section
contains a list of NULL terminated ASCII strings. An ID of the string is used in the file
meta data, which is the offset of the actual string into the string section. Strings can be stored
into multiple string sections in the file.

See Also

trace-cmd(1), trace-cmd-record(1), trace-cmd-report(1), trace-cmd-start(1), trace-cmd-stop(1), trace-cmd-extract(1), trace-cmd-reset(1), trace-cmd-split(1), trace-cmd-list(1), trace-cmd-listen(1), trace-cmd.dat(5)

Author

Written by Steven Rostedt, <rostedt@goodmis.org[1]>

Resources

https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/

Copying

Copyright (C) 2010 Red Hat, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).

Notes

1.

rostedt@goodmis.org
mailto:rostedt@goodmis.org

Info

07/23/2022 libtracefs Manual