Provided by: lttng-tools_2.11.2-1build1_amd64 
NAME
lttng-enable-event - Create or enable LTTng event rules
SYNOPSIS
Create or enable Linux kernel event rules:
lttng [GENERAL OPTIONS] enable-event --kernel
[--probe=SOURCE | --function=SOURCE | --syscall |
--userspace-probe=SOURCE]
[--filter=EXPR] [--session=SESSION]
[--channel=CHANNEL] EVENT[,EVENT]...
Create or enable an "all" Linux kernel event rule:
lttng [GENERAL OPTIONS] enable-event --kernel --all [--syscall]
[--filter=EXPR] [--session=SESSION] [--channel=CHANNEL]
Create or enable application/library event rules:
lttng [GENERAL OPTIONS] enable-event
(--userspace | --jul | --log4j | --python)
[--filter=EXPR] [--exclude=EVENT[,EVENT]...]
[--loglevel=LOGLEVEL | --loglevel-only=LOGLEVEL]
[--session=SESSION] [--channel=CHANNEL] (--all | EVENT[,EVENT]...)
DESCRIPTION
The lttng enable-event command can create a new event rule, or enable one or more existing
and disabled ones.
An event rule created by lttng enable-event is a set of conditions that must be satisfied
in order for an actual event to be emitted by an LTTng tracer when the execution of an
application or a library or the Linux kernel reaches an event source (tracepoint, system
call, dynamic probe). Event sources can be listed with the lttng-list(1) command.
The lttng-disable-event(1) command can be used to disable existing event rules.
Event rules are always assigned to a channel when they are created. If the --channel
option is omitted, a default channel named channel0 is used (and created automatically if
it does not exist for the specified domain in the selected tracing session).
If the --session option is omitted, the chosen channel is picked from the current tracing
session.
Events can be enabled while tracing is active (use lttng-start(1) to make a tracing
session active).
Event source types
Five types of event sources are available in the Linux kernel tracing domain (--kernel
option):
Tracepoint (--tracepoint option; default)
A Linux kernel tracepoint, that is, a static instrumentation point placed in the
kernel source code. Standard tracepoints are designed and placed in the source code by
developers and record useful payload fields.
Dynamic kernel probe (--probe option)
A Linux kernel kprobe, that is, an instrumentation point placed dynamically in the
compiled kernel code. Dynamic probe events do not record any payload field.
Dynamic user space probe (--userspace-probe option)
A Linux kernel uprobe, that is, an instrumentation point placed dynamically in the
compiled user space application/library through the kernel. Dynamic user space probe
events do not record any payload field.
See the Dynamic user space probes section for more information.
Function probe (--function option)
A Linux kernel kretprobe, that is, two instrumentation points placed dynamically where
a function is entered and where it returns in the compiled kernel code. Function probe
events do not record any payload field.
System call (--syscall option)
A Linux kernel system call. Two instrumentation points are statically placed where a
system call function is entered and where it returns in the compiled kernel code.
System call event sources record useful payload fields.
The application tracing domains (--userspace, --jul, --log4j, or --python options) only
support tracepoints. In the cases of the JUL, Apache log4j, and Python domains, the event
names correspond to logger names.
Understanding event rule conditions
When creating an event rule with lttng enable-event, conditions are specified using
options. The logical conjunction (logical AND) of all those conditions must be true when
an event source is reached by an application or by the Linux kernel in order for an actual
event to be emitted by an LTTng tracer.
Any condition that is not explicitly specified on creation is considered a don’t care.
For example, consider the following commands:
$ lttng enable-event --userspace hello:world
$ lttng enable-event --userspace hello:world --loglevel=TRACE_INFO
Here, two event rules are created. The first one has a single condition: the tracepoint
name must match hello:world. The second one has two conditions:
• The tracepoint name must match hello:world, and
• The tracepoint’s defined log level must be at least as severe as the TRACE_INFO level.
In this case, the second event rule is pointless because the first one is more general: it
does not care about the tracepoint’s log level. If an event source matching both event
rules is reached by the application’s execution, only one event is emitted.
The available conditions for the Linux kernel domain are:
• Tracepoint/system call name (EVENT argument with --tracepoint or --syscall options) or
dynamic probe/function name/address (--probe, --userspace-probe, and --function
option’s argument) which must match event source’s equivalent.
You can use * characters at any place in the tracepoint or system call name as
wildcards to match zero or more characters. To use a literal * character, use \*.
• Filter expression (--filter option) executed against the dynamic values of event
fields at execution time that must evaluate to true. See the Filter expression section
below for more information.
The available conditions for the application domains are:
• Tracepoint name (EVENT with --tracepoint option) which must match event source’s
equivalent.
You can use * characters at any place in the tracepoint name as wildcards to match
zero or more characters. To use a literal * character, use \*. When you create an
event rule with a tracepoint name containing a wildcard, you can exclude specific
tracepoint names from the match with the --exclude option.
• Filter expression (--filter option) executed against the dynamic values of event
fields at execution time that must evaluate to true. See the Filter expression section
below for more information.
• Event’s log level that must be at least as severe as a given log level (--loglevel
option) or match exactly a given log level (--loglevel-only option).
When using lttng enable-event with a set of conditions that does not currently exist for
the chosen tracing session, domain, and channel, a new event rule is created. Otherwise,
the existing event rule is enabled if it is currently disabled (see lttng-disable-
event(1)).
The --all option can be used alongside the --tracepoint or --syscall options. When this
option is used, no EVENT argument must be specified. This option defines a single event
rule matching all the possible events of a given tracing domain for the chosen channel and
tracing session. It is the equivalent of an EVENT argument named * (wildcard).
Filter expression
A filter expression can be specified with the --filter option when creating a new event
rule. If the filter expression evaluates to true when executed against the dynamic values
of an event’s fields when tracing, the filtering condition passes.
Note
Make sure to single-quote the filter expression when running the command from a shell,
as filter expressions typically include characters having a special meaning for most
shells.
The filter expression syntax is similar to C language conditional expressions (expressions
that can be evaluated by an if statement), albeit with a few differences:
• C integer and floating point number constants are supported, as well as literal
strings between double quotes ("). You can use * characters at any place in a literal
string as wildcards to match zero or more characters. To use a literal * character,
use \*.
Examples: 32, -0x17, 0755, 12.34, "a \"literal string\"", "src/*/*.h".
• The dynamic value of an event field is read by using its name as a C identifier.
The dot and square bracket notations are available, like in the C language, to access
nested structure and array/sequence fields. Only a constant, positive integer number
can be used within square brackets. If the index is out of bounds, the whole filter
expression evaluates to false (the event is discarded).
An enumeration field’s value is an integer.
When the expression’s field does not exist, the whole filter expression evaluates to
false.
Examples: my_field, target_cpu, seq[7], msg.user[1].data[2][17].
• The dynamic value of a statically-known context field is read by prefixing its name
with $ctx.. Statically-known context fields are context fields added to channels
without the $app. prefix using the lttng-add-context(1) command.
When the expression’s statically-known context field does not exist, the whole filter
expression evaluates to false.
Examples: $ctx.prio, $ctx.preemptible, $ctx.perf:cpu:stalled-cycles-frontend.
• The dynamic value of an application-specific context field is read by prefixing its
name with $app. (follows the format used to add such a context field with the lttng-
add-context(1) command).
When the expression’s application-specific context field does not exist, the whole
filter expression evaluates to false.
Example: $app.server:cur_user.
The following precedence table shows the operators which are supported in a filter
expression. In this table, the highest precedence is 1. Parentheses are supported to
bypass the default order.
Important
Unlike the C language, the lttng enable-event filter expression syntax’s bitwise AND
and OR operators (& and |) take precedence over relational operators (<, <=, >, >=,
==, and !=). This means the filter expression 2 & 2 == 2 is true while the equivalent
C expression is false.
┌───────────┬──────────┬─────────────────────┬───────────────┐
│Precedence │ Operator │ Description │ Associativity │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│1 │ - │ Unary minus │ Right-to-left │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│1 │ + │ Unary plus │ Right-to-left │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│1 │ ! │ Logical NOT │ Right-to-left │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│1 │ ~ │ Bitwise NOT │ Right-to-left │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│2 │ << │ Bitwise left shift │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│2 │ >> │ Bitwise right shift │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│3 │ & │ Bitwise AND │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│4 │ ^ │ Bitwise XOR │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│5 │ | │ Bitwise OR │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│6 │ < │ Less than │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│6 │ <= │ Less than or equal │ Left-to-right │
│ │ │ to │ │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│6 │ > │ Greater than │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│6 │ >= │ Greater than or │ Left-to-right │
│ │ │ equal to │ │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│7 │ == │ Equal to │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│7 │ != │ Not equal to │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│8 │ && │ Logical AND │ Left-to-right │
├───────────┼──────────┼─────────────────────┼───────────────┤
│ │ │ │ │
│9 │ || │ Logical OR │ Left-to-right │
└───────────┴──────────┴─────────────────────┴───────────────┘
The arithmetic operators are NOT supported.
All integer constants and fields are first casted to signed 64-bit integers. The
representation of negative integers is two’s complement. This means that, for example, the
signed 8-bit integer field 0xff (-1) becomes 0xffffffffffffffff (still -1) once casted.
Before a bitwise operator is applied, all its operands are casted to unsigned 64-bit
integers, and the result is casted back to a signed 64-bit integer. For the bitwise NOT
operator, it is the equivalent of this C expression:
(int64_t) ~((uint64_t) val)
For the binary bitwise operators, it is the equivalent of those C expressions:
(int64_t) ((uint64_t) lhs >> (uint64_t) rhs)
(int64_t) ((uint64_t) lhs << (uint64_t) rhs)
(int64_t) ((uint64_t) lhs & (uint64_t) rhs)
(int64_t) ((uint64_t) lhs ^ (uint64_t) rhs)
(int64_t) ((uint64_t) lhs | (uint64_t) rhs)
If the right-hand side of a bitwise shift operator (<< and >>) is not in the [0, 63]
range, the whole filter expression evaluates to false.
Note
Although it is possible to filter the process ID of an event when the pid context has
been added to its channel using, for example, $ctx.pid == 2832, it is recommended to
use the PID tracker instead, which is much more efficient (see lttng-track(1)).
Filter expression examples:
msg_id == 23 && size >= 2048
$ctx.procname == "lttng*" && (!flag || poel < 34)
$app.my_provider:my_context == 17.34e9 || some_enum >= 14
$ctx.cpu_id == 2 && filename != "*.log"
eax_reg & 0xff7 == 0x240 && x[4] >> 12 <= 0x1234
Log levels
Tracepoints and log statements in applications have an attached log level. Application
event rules can contain a log level condition.
With the --loglevel option, the event source’s log level must be at least as severe as the
option’s argument. With the --loglevel-only option, the event source’s log level must
match the option’s argument.
The available log levels are:
User space domain (--userspace option)
Shortcuts such as system are allowed.
• TRACE_EMERG (0)
• TRACE_ALERT (1)
• TRACE_CRIT (2)
• TRACE_ERR (3)
• TRACE_WARNING (4)
• TRACE_NOTICE (5)
• TRACE_INFO (6)
• TRACE_DEBUG_SYSTEM (7)
• TRACE_DEBUG_PROGRAM (8)
• TRACE_DEBUG_PROCESS (9)
• TRACE_DEBUG_MODULE (10)
• TRACE_DEBUG_UNIT (11)
• TRACE_DEBUG_FUNCTION (12)
• TRACE_DEBUG_LINE (13)
• TRACE_DEBUG (14)
java.util.logging domain (--jul option)
Shortcuts such as severe are allowed.
• JUL_OFF (INT32_MAX)
• JUL_SEVERE (1000)
• JUL_WARNING (900)
• JUL_INFO (800)
• JUL_CONFIG (700)
• JUL_FINE (500)
• JUL_FINER (400)
• JUL_FINEST (300)
• JUL_ALL (INT32_MIN)
Apache log4j domain (--log4j option)
Shortcuts such as severe are allowed.
• LOG4J_OFF (INT32_MAX)
• LOG4J_FATAL (50000)
• LOG4J_ERROR (40000)
• LOG4J_WARN (30000)
• LOG4J_INFO (20000)
• LOG4J_DEBUG (10000)
• LOG4J_TRACE (5000)
• LOG4J_ALL (INT32_MIN)
Python domain (--python option)
Shortcuts such as critical are allowed.
• PYTHON_CRITICAL (50)
• PYTHON_ERROR (40)
• PYTHON_WARNING (30)
• PYTHON_INFO (20)
• PYTHON_DEBUG (10)
• PYTHON_NOTSET (0)
Dynamic user space probes
With the --userspace-probe option, you can instrument function entries of any user space
binary (application or library) using either an available symbol name or a SystemTap
User-level Statically Defined Tracing (USDT, a DTrace-style marker) probe’s provider and
probe names. As of this version, only USDT probes that are NOT surrounded by a reference
counter (semaphore) are supported.
The --userspace-probe option must be specified with the --kernel option because it uses
Linux’s uprobe feature to dynamically instrument a user space application or library.
As of this version, dynamic probe events do not record any payload field.
OPTIONS
General options are described in lttng(1).
Domain
One of:
-j, --jul
Create or enable event rules in the java.util.logging (JUL) domain.
-k, --kernel
Create or enable event rules in the Linux kernel domain.
-l, --log4j
Create or enable event rules in the Apache log4j domain.
-p, --python
Create or enable event rules in the Python domain.
-u, --userspace
Create or enable event rules in the user space domain.
Target
-c CHANNEL, --channel=CHANNEL
Create or enable event rules in the channel named CHANNEL instead of the default
channel name channel0.
-s SESSION, --session=SESSION
Create or enable event rules in the tracing session named SESSION instead of the
current tracing session.
Event source type
One of:
--function=SOURCE
Dynamic kernel return probe (kretprobe). Only available with the --kernel domain
option. SOURCE is one of:
• Function address (0x prefix supported)
• Function symbol name
• Function symbol name and offset (SYMBOL+OFFSET format)
--probe=SOURCE
Dynamic kernel probe (kprobe). Only available with the --kernel domain option. SOURCE
is one of:
• Address (0x prefix supported)
• Symbol name
• Symbol name and offset (SYMBOL+OFFSET format)
--userspace-probe=SOURCE
Dynamic user space probe (uprobe). Only available with the --kernel domain option. See
the Dynamic user space probes section.
SOURCE is one of:
[elf:]PATH:SYMBOL
Dynamically instrument an available symbol within a user space application or
library.
PATH
Application or library path.
This can be:
• An absolute path.
• A relative path.
• An application’s name as found in the directories listed in the PATH
environment variable.
SYMBOL
Symbol name of the function of which to instrument the entry.
This can be any defined code symbol listed by the nm(1) command (including
with its --dynamic option which lists dynamic symbols).
As of this version, not specifying elf: is equivalent to specifying it.
Examples:
• --userspace-probe=/usr/lib/libc.so.6:malloc
• --userspace-probe=./myapp:createUser
• --userspace-probe=httpd:ap_run_open_htaccess
sdt:PATH:PROVIDER:NAME
Dynamically instrument a USDT probe within a user space application or library.
PATH
Application or library path.
This can be:
• An absolute path.
• A relative path.
• An application’s name as found in the directories listed in the PATH
environment variable.
PROVIDER:NAME
USDT provider and probe names.
For example, with the following USDT probe:
DTRACE_PROBE2("server", "accept_request",
request_id, ip_addr);
The provider/probe name pair is server:accept_request.
Example:
• --userspace-probe=sdt:./build/server:server:accept_request
--syscall
Linux kernel system call. Only available with the --kernel domain option.
--tracepoint
Linux kernel or application tracepoint (default).
Log level
One of:
--loglevel=LOGLEVEL
Add log level condition to the event rule: the event source’s defined log level must
be at least as severe as LOGLEVEL. See the Log levels section above for the available
log levels. Only available with application domains.
--loglevel-only=LOGLEVEL
Add log level condition to the event rule: the event source’s defined log level must
match LOGLEVEL. See the Log levels section above for the available log levels. Only
available with application domains.
Filtering and exclusion
-x EVENT[,EVENT]..., --exclude=EVENT[,EVENT]...
Exclude events named EVENT from the event rule. This option can be used when the
command’s EVENT argument contains at least one wildcard star (*) to exclude specific
names. EVENT can also contain wildcard stars. To use a literal , character, use \,.
Only available with the --userspace domain.
-f EXPR, --filter=EXPR
Add filter expression condition to the event rule. Expression EXPR must evaluate to
true when executed against the dynamic values of event fields. See the Filter
expression section above for more information.
Shortcuts
-a, --all
Equivalent to an EVENT argument named * (wildcard) when also using the --tracepoint
(default) or --syscall option.
Program information
-h, --help
Show command help.
This option, like lttng-help(1), attempts to launch /usr/bin/man to view the command’s
man page. The path to the man pager can be overridden by the LTTNG_MAN_BIN_PATH
environment variable.
--list-options
List available command options.
ENVIRONMENT VARIABLES
LTTNG_ABORT_ON_ERROR
Set to 1 to abort the process after the first error is encountered.
LTTNG_HOME
Overrides the $HOME environment variable. Useful when the user running the commands
has a non-writable home directory.
LTTNG_MAN_BIN_PATH
Absolute path to the man pager to use for viewing help information about LTTng
commands (using lttng-help(1) or lttng COMMAND --help).
LTTNG_SESSION_CONFIG_XSD_PATH
Path in which the session.xsd session configuration XML schema may be found.
LTTNG_SESSIOND_PATH
Full session daemon binary path.
The --sessiond-path option has precedence over this environment variable.
Note that the lttng-create(1) command can spawn an LTTng session daemon automatically if
none is running. See lttng-sessiond(8) for the environment variables influencing the
execution of the session daemon.
FILES
$LTTNG_HOME/.lttngrc
User LTTng runtime configuration.
This is where the per-user current tracing session is stored between executions of
lttng(1). The current tracing session can be set with lttng-set-session(1). See lttng-
create(1) for more information about tracing sessions.
$LTTNG_HOME/lttng-traces
Default output directory of LTTng traces. This can be overridden with the --output
option of the lttng-create(1) command.
$LTTNG_HOME/.lttng
User LTTng runtime and configuration directory.
$LTTNG_HOME/.lttng/sessions
Default location of saved user tracing sessions (see lttng-save(1) and lttng-load(1)).
/etc/lttng/sessions
System-wide location of saved tracing sessions (see lttng-save(1) and lttng-load(1)).
Note
$LTTNG_HOME defaults to $HOME when not explicitly set.
EXIT STATUS
0
Success
1
Command error
2
Undefined command
3
Fatal error
4
Command warning (something went wrong during the command)
BUGS
If you encounter any issue or usability problem, please report it on the LTTng bug tracker
<https://bugs.lttng.org/projects/lttng-tools>.
RESOURCES
• LTTng project website <https://lttng.org>
• LTTng documentation <https://lttng.org/docs>
• Git repositories <http://git.lttng.org>
• GitHub organization <http://github.com/lttng>
• Continuous integration <http://ci.lttng.org/>
• Mailing list <http://lists.lttng.org> for support and development: lttng-
dev@lists.lttng.org
• IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net
COPYRIGHTS
This program is part of the LTTng-tools project.
LTTng-tools is distributed under the GNU General Public License version 2
<http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>. See the LICENSE
<https://github.com/lttng/lttng-tools/blob/master/LICENSE> file for details.
THANKS
Special thanks to Michel Dagenais and the DORSAL laboratory
<http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal for the LTTng journey.
Also thanks to the Ericsson teams working on tracing which helped us greatly with detailed
bug reports and unusual test cases.
SEE ALSO
lttng-disable-event(1), lttng(1)