Event Tracing

Author:Theodore Ts’o
Updated:Li Zefan and Tom Zanussi

1. Introduction

Tracepoints (see Documentation/trace/tracepoints.txt) can be used without creating custom kernel modules to register probe functions using the event tracing infrastructure.

Not all tracepoints can be traced using the event tracing system; the kernel developer must provide code snippets which define how the tracing information is saved into the tracing buffer, and how the tracing information should be printed.

2. Using Event Tracing

2.1 Via the ‘set_event’ interface

The events which are available for tracing can be found in the file /sys/kernel/debug/tracing/available_events.

To enable a particular event, such as ‘sched_wakeup’, simply echo it to /sys/kernel/debug/tracing/set_event. For example:

# echo sched_wakeup >> /sys/kernel/debug/tracing/set_event


‘>>’ is necessary, otherwise it will firstly disable all the events.

To disable an event, echo the event name to the set_event file prefixed with an exclamation point:

# echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event

To disable all events, echo an empty line to the set_event file:

# echo > /sys/kernel/debug/tracing/set_event

To enable all events, echo *:* or *: to the set_event file:

# echo *:* > /sys/kernel/debug/tracing/set_event

The events are organized into subsystems, such as ext4, irq, sched, etc., and a full event name looks like this: <subsystem>:<event>. The subsystem name is optional, but it is displayed in the available_events file. All of the events in a subsystem can be specified via the syntax <subsystem>:*; for example, to enable all irq events, you can use the command:

# echo 'irq:*' > /sys/kernel/debug/tracing/set_event

2.2 Via the ‘enable’ toggle

The events available are also listed in /sys/kernel/debug/tracing/events/ hierarchy of directories.

To enable event ‘sched_wakeup’:

# echo 1 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable

To disable it:

# echo 0 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable

To enable all events in sched subsystem:

# echo 1 > /sys/kernel/debug/tracing/events/sched/enable

To enable all events:

# echo 1 > /sys/kernel/debug/tracing/events/enable

When reading one of these enable files, there are four results:

  • 0 - all events this file affects are disabled
  • 1 - all events this file affects are enabled
  • X - there is a mixture of events enabled and disabled
  • ? - this file does not affect any event

2.3 Boot option

In order to facilitate early boot debugging, use boot option:


event-list is a comma separated list of events. See section 2.1 for event format.

3. Defining an event-enabled tracepoint

See The example provided in samples/trace_events

4. Event formats

Each trace event has a ‘format’ file associated with it that contains a description of each field in a logged event. This information can be used to parse the binary trace stream, and is also the place to find the field names that can be used in event filters (see section 5).

It also displays the format string that will be used to print the event in text mode, along with the event name and ID used for profiling.

Every event has a set of common fields associated with it; these are the fields prefixed with common_. The other fields vary between events and correspond to the fields defined in the TRACE_EVENT definition for that event.

Each field in the format has the form:

field:field-type field-name; offset:N; size:N;

where offset is the offset of the field in the trace record and size is the size of the data item, in bytes.

For example, here’s the information displayed for the ‘sched_wakeup’ event:

# cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/format

name: sched_wakeup
ID: 60
        field:unsigned short common_type;       offset:0;       size:2;
        field:unsigned char common_flags;       offset:2;       size:1;
        field:unsigned char common_preempt_count;       offset:3;       size:1;
        field:int common_pid;   offset:4;       size:4;
        field:int common_tgid;  offset:8;       size:4;

        field:char comm[TASK_COMM_LEN]; offset:12;      size:16;
        field:pid_t pid;        offset:28;      size:4;
        field:int prio; offset:32;      size:4;
        field:int success;      offset:36;      size:4;
        field:int cpu;  offset:40;      size:4;

print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid,
           REC->prio, REC->success, REC->cpu

This event contains 10 fields, the first 5 common and the remaining 5 event-specific. All the fields for this event are numeric, except for ‘comm’ which is a string, a distinction important for event filtering.

5. Event filtering

Trace events can be filtered in the kernel by associating boolean ‘filter expressions’ with them. As soon as an event is logged into the trace buffer, its fields are checked against the filter expression associated with that event type. An event with field values that ‘match’ the filter will appear in the trace output, and an event whose values don’t match will be discarded. An event with no filter associated with it matches everything, and is the default when no filter has been set for an event.

5.1 Expression syntax

A filter expression consists of one or more ‘predicates’ that can be combined using the logical operators ‘&&’ and ‘||’. A predicate is simply a clause that compares the value of a field contained within a logged event with a constant value and returns either 0 or 1 depending on whether the field value matched (1) or didn’t match (0):

field-name relational-operator value

Parentheses can be used to provide arbitrary logical groupings and double-quotes can be used to prevent the shell from interpreting operators as shell metacharacters.

The field-names available for use in filters can be found in the ‘format’ files for trace events (see section 4).

The relational-operators depend on the type of the field being tested:

The operators available for numeric fields are:

==, !=, <, <=, >, >=, &

And for string fields they are:

==, !=, ~

The glob (~) accepts a wild card character (*,?) and character classes ([). For example:

prev_comm ~ "*sh"
prev_comm ~ "sh*"
prev_comm ~ "*sh*"
prev_comm ~ "ba*sh"

5.2 Setting filters

A filter for an individual event is set by writing a filter expression to the ‘filter’ file for the given event.

For example:

# cd /sys/kernel/debug/tracing/events/sched/sched_wakeup
# echo "common_preempt_count > 4" > filter

A slightly more involved example:

# cd /sys/kernel/debug/tracing/events/signal/signal_generate
# echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter

If there is an error in the expression, you’ll get an ‘Invalid argument’ error when setting it, and the erroneous string along with an error message can be seen by looking at the filter e.g.:

# cd /sys/kernel/debug/tracing/events/signal/signal_generate
# echo "((sig >= 10 && sig < 15) || dsig == 17) && comm != bash" > filter
-bash: echo: write error: Invalid argument
# cat filter
((sig >= 10 && sig < 15) || dsig == 17) && comm != bash
parse_error: Field not found

Currently the caret (‘^’) for an error always appears at the beginning of the filter string; the error message should still be useful though even without more accurate position info.

5.3 Clearing filters

To clear the filter for an event, write a ‘0’ to the event’s filter file.

To clear the filters for all events in a subsystem, write a ‘0’ to the subsystem’s filter file.

5.3 Subsystem filters

For convenience, filters for every event in a subsystem can be set or cleared as a group by writing a filter expression into the filter file at the root of the subsystem. Note however, that if a filter for any event within the subsystem lacks a field specified in the subsystem filter, or if the filter can’t be applied for any other reason, the filter for that event will retain its previous setting. This can result in an unintended mixture of filters which could lead to confusing (to the user who might think different filters are in effect) trace output. Only filters that reference just the common fields can be guaranteed to propagate successfully to all events.

Here are a few subsystem filter examples that also illustrate the above points:

Clear the filters on all events in the sched subsystem:

# cd /sys/kernel/debug/tracing/events/sched
# echo 0 > filter
# cat sched_switch/filter
# cat sched_wakeup/filter

Set a filter using only common fields for all events in the sched subsystem (all events end up with the same filter):

# cd /sys/kernel/debug/tracing/events/sched
# echo common_pid == 0 > filter
# cat sched_switch/filter
common_pid == 0
# cat sched_wakeup/filter
common_pid == 0

Attempt to set a filter using a non-common field for all events in the sched subsystem (all events but those that have a prev_pid field retain their old filters):

# cd /sys/kernel/debug/tracing/events/sched
# echo prev_pid == 0 > filter
# cat sched_switch/filter
prev_pid == 0
# cat sched_wakeup/filter
common_pid == 0

5.4 PID filtering

The set_event_pid file in the same directory as the top events directory exists, will filter all events from tracing any task that does not have the PID listed in the set_event_pid file.

# cd /sys/kernel/debug/tracing
# echo $$ > set_event_pid
# echo 1 > events/enable

Will only trace events for the current task.

To add more PIDs without losing the PIDs already included, use ‘>>’.

# echo 123 244 1 >> set_event_pid

6. Event triggers

Trace events can be made to conditionally invoke trigger ‘commands’ which can take various forms and are described in detail below; examples would be enabling or disabling other trace events or invoking a stack trace whenever the trace event is hit. Whenever a trace event with attached triggers is invoked, the set of trigger commands associated with that event is invoked. Any given trigger can additionally have an event filter of the same form as described in section 5 (Event filtering) associated with it - the command will only be invoked if the event being invoked passes the associated filter. If no filter is associated with the trigger, it always passes.

Triggers are added to and removed from a particular event by writing trigger expressions to the ‘trigger’ file for the given event.

A given event can have any number of triggers associated with it, subject to any restrictions that individual commands may have in that regard.

Event triggers are implemented on top of “soft” mode, which means that whenever a trace event has one or more triggers associated with it, the event is activated even if it isn’t actually enabled, but is disabled in a “soft” mode. That is, the tracepoint will be called, but just will not be traced, unless of course it’s actually enabled. This scheme allows triggers to be invoked even for events that aren’t enabled, and also allows the current event filter implementation to be used for conditionally invoking triggers.

The syntax for event triggers is roughly based on the syntax for set_ftrace_filter ‘ftrace filter commands’ (see the ‘Filter commands’ section of Documentation/trace/ftrace.txt), but there are major differences and the implementation isn’t currently tied to it in any way, so beware about making generalizations between the two.

6.1 Expression syntax

Triggers are added by echoing the command to the ‘trigger’ file:

# echo 'command[:count] [if filter]' > trigger

Triggers are removed by echoing the same command but starting with ‘!’ to the ‘trigger’ file:

# echo '!command[:count] [if filter]' > trigger

The [if filter] part isn’t used in matching commands when removing, so leaving that off in a ‘!’ command will accomplish the same thing as having it in.

The filter syntax is the same as that described in the ‘Event filtering’ section above.

For ease of use, writing to the trigger file using ‘>’ currently just adds or removes a single trigger and there’s no explicit ‘>>’ support (‘>’ actually behaves like ‘>>’) or truncation support to remove all triggers (you have to use ‘!’ for each one added.)

6.2 Supported trigger commands

The following commands are supported:

  • enable_event/disable_event

    These commands can enable or disable another trace event whenever the triggering event is hit. When these commands are registered, the other trace event is activated, but disabled in a “soft” mode. That is, the tracepoint will be called, but just will not be traced. The event tracepoint stays in this mode as long as there’s a trigger in effect that can trigger it.

    For example, the following trigger causes kmalloc events to be traced when a read system call is entered, and the :1 at the end specifies that this enablement happens only once:

    # echo 'enable_event:kmem:kmalloc:1' > \

    The following trigger causes kmalloc events to stop being traced when a read system call exits. This disablement happens on every read system call exit:

    # echo 'disable_event:kmem:kmalloc' > \

    The format is:


    To remove the above commands:

    # echo '!enable_event:kmem:kmalloc:1' > \
    # echo '!disable_event:kmem:kmalloc' > \

    Note that there can be any number of enable/disable_event triggers per triggering event, but there can only be one trigger per triggered event. e.g. sys_enter_read can have triggers enabling both kmem:kmalloc and sched:sched_switch, but can’t have two kmem:kmalloc versions such as kmem:kmalloc and kmem:kmalloc:1 or ‘kmem:kmalloc if bytes_req == 256’ and ‘kmem:kmalloc if bytes_alloc == 256’ (they could be combined into a single filter on kmem:kmalloc though).

  • stacktrace

    This command dumps a stacktrace in the trace buffer whenever the triggering event occurs.

    For example, the following trigger dumps a stacktrace every time the kmalloc tracepoint is hit:

    # echo 'stacktrace' > \

    The following trigger dumps a stacktrace the first 5 times a kmalloc request happens with a size >= 64K:

    # echo 'stacktrace:5 if bytes_req >= 65536' > \

    The format is:


    To remove the above commands:

    # echo '!stacktrace' > \
    # echo '!stacktrace:5 if bytes_req >= 65536' > \

    The latter can also be removed more simply by the following (without the filter):

    # echo '!stacktrace:5' > \

    Note that there can be only one stacktrace trigger per triggering event.

  • snapshot

    This command causes a snapshot to be triggered whenever the triggering event occurs.

    The following command creates a snapshot every time a block request queue is unplugged with a depth > 1. If you were tracing a set of events or functions at the time, the snapshot trace buffer would capture those events when the trigger event occurred:

    # echo 'snapshot if nr_rq > 1' > \

    To only snapshot once:

    # echo 'snapshot:1 if nr_rq > 1' > \

    To remove the above commands:

    # echo '!snapshot if nr_rq > 1' > \
    # echo '!snapshot:1 if nr_rq > 1' > \

    Note that there can be only one snapshot trigger per triggering event.

  • traceon/traceoff

    These commands turn tracing on and off when the specified events are hit. The parameter determines how many times the tracing system is turned on and off. If unspecified, there is no limit.

    The following command turns tracing off the first time a block request queue is unplugged with a depth > 1. If you were tracing a set of events or functions at the time, you could then examine the trace buffer to see the sequence of events that led up to the trigger event:

    # echo 'traceoff:1 if nr_rq > 1' > \

    To always disable tracing when nr_rq > 1:

    # echo 'traceoff if nr_rq > 1' > \

    To remove the above commands:

    # echo '!traceoff:1 if nr_rq > 1' > \
    # echo '!traceoff if nr_rq > 1' > \

    Note that there can be only one traceon or traceoff trigger per triggering event.

  • hist

    This command aggregates event hits into a hash table keyed on one or more trace event format fields (or stacktrace) and a set of running totals derived from one or more trace event format fields and/or event counts (hitcount).

    See Documentation/trace/histogram.txt for details and examples.