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=============
Event Tracing
=============
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:Author: Theodore Ts'o
:Updated: Li Zefan and Tom Zanussi
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1. Introduction
===============
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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
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tracing information should be printed.
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2. Using Event Tracing
======================

2.1 Via the 'set_event' interface
---------------------------------
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The events which are available for tracing can be found in the file
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/sys/kernel/debug/tracing/available_events.
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To enable a particular event, such as 'sched_wakeup', simply echo it
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to /sys/kernel/debug/tracing/set_event. For example::
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	# echo sched_wakeup >> /sys/kernel/debug/tracing/set_event
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.. Note:: '>>' is necessary, otherwise it will firstly disable all the events.
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To disable an event, echo the event name to the set_event file prefixed
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with an exclamation point::
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	# echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event
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To disable all events, echo an empty line to the set_event file::
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	# echo > /sys/kernel/debug/tracing/set_event
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To enable all events, echo ``*:*`` or ``*:`` to the set_event file::
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	# echo *:* > /sys/kernel/debug/tracing/set_event
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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
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``<subsystem>:*``; for example, to enable all irq events, you can use the
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command::
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	# echo 'irq:*' > /sys/kernel/debug/tracing/set_event
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2.2 Via the 'enable' toggle
---------------------------

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

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To enable event 'sched_wakeup'::
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	# echo 1 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable
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To disable it::
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	# echo 0 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable
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To enable all events in sched subsystem::
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	# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
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To enable all events::
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	# echo 1 > /sys/kernel/debug/tracing/events/enable
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When reading one of these enable files, there are four results:

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 - 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
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2.3 Boot option
---------------

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In order to facilitate early boot debugging, use boot option::
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	trace_event=[event-list]

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event-list is a comma separated list of events. See section 2.1 for event
format.
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3. Defining an event-enabled tracepoint
=======================================

See The example provided in samples/trace_events

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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.

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

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Each field in the format has the form::
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     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'
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event::
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	# cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/format
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	name: sched_wakeup
	ID: 60
	format:
		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;
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		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;
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	print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid,
		   REC->prio, REC->success, REC->cpu
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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
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on whether the field value matched (1) or didn't match (0)::
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	  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:

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==, !=, <, <=, >, >=, &
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And for string fields they are:

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==, !=, ~
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The glob (~) accepts a wild card character (\*,?) and character classes
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([). For example::
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  prev_comm ~ "*sh"
  prev_comm ~ "sh*"
  prev_comm ~ "*sh*"
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  prev_comm ~ "ba*sh"
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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.

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For example::
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	# cd /sys/kernel/debug/tracing/events/sched/sched_wakeup
	# echo "common_preempt_count > 4" > filter
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A slightly more involved example::
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	# cd /sys/kernel/debug/tracing/events/signal/signal_generate
	# echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter
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If there is an error in the expression, you'll get an 'Invalid
argument' error when setting it, and the erroneous string along with
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an error message can be seen by looking at the filter e.g.::
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	# 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
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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
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at the root of the subsystem.  Note however, that if a filter for any
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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:

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Clear the filters on all events in the sched subsystem::
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	# cd /sys/kernel/debug/tracing/events/sched
	# echo 0 > filter
	# cat sched_switch/filter
	none
	# cat sched_wakeup/filter
	none
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Set a filter using only common fields for all events in the sched
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subsystem (all events end up with the same filter)::
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	# 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
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Attempt to set a filter using a non-common field for all events in the
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sched subsystem (all events but those that have a prev_pid field retain
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their old filters)::
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	# 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
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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.
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::
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	# cd /sys/kernel/debug/tracing
	# echo $$ > set_event_pid
	# echo 1 > events/enable
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Will only trace events for the current task.

To add more PIDs without losing the PIDs already included, use '>>'.
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::
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	# echo 123 244 1 >> set_event_pid
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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
---------------------

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Triggers are added by echoing the command to the 'trigger' file::
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  # echo 'command[:count] [if filter]' > trigger

Triggers are removed by echoing the same command but starting with '!'
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to the 'trigger' file::
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  # 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
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  specifies that this enablement happens only once::
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	  # echo 'enable_event:kmem:kmalloc:1' > \
	      /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
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  The following trigger causes kmalloc events to stop being traced
  when a read system call exits.  This disablement happens on every
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  read system call exit::
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	  # echo 'disable_event:kmem:kmalloc' > \
	      /sys/kernel/debug/tracing/events/syscalls/sys_exit_read/trigger
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  The format is::
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      enable_event:<system>:<event>[:count]
      disable_event:<system>:<event>[:count]

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  To remove the above commands::
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	  # echo '!enable_event:kmem:kmalloc:1' > \
	      /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
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	  # echo '!disable_event:kmem:kmalloc' > \
	      /sys/kernel/debug/tracing/events/syscalls/sys_exit_read/trigger
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  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
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  kmalloc tracepoint is hit::
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	  # echo 'stacktrace' > \
		/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
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  The following trigger dumps a stacktrace the first 5 times a kmalloc
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  request happens with a size >= 64K::
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	  # echo 'stacktrace:5 if bytes_req >= 65536' > \
		/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
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  The format is::
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      stacktrace[:count]

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  To remove the above commands::
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	  # echo '!stacktrace' > \
		/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
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	  # echo '!stacktrace:5 if bytes_req >= 65536' > \
		/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
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  The latter can also be removed more simply by the following (without
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  the filter)::
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	  # echo '!stacktrace:5' > \
		/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
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  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
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  capture those events when the trigger event occurred::
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	  # echo 'snapshot if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  To only snapshot once::
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	  # echo 'snapshot:1 if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  To remove the above commands::
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	  # echo '!snapshot if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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	  # echo '!snapshot:1 if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  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
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  trigger event::
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	  # echo 'traceoff:1 if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  To always disable tracing when nr_rq  > 1::
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	  # echo 'traceoff if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  To remove the above commands::
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	  # echo '!traceoff:1 if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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	  # echo '!traceoff if nr_rq > 1' > \
		/sys/kernel/debug/tracing/events/block/block_unplug/trigger
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  Note that there can be only one traceon or traceoff trigger per
  triggering event.
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- 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).

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  The format of a hist trigger is as follows::
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        hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
          [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
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          [:clear][:name=histname1] [if <filter>]
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  When a matching event is hit, an entry is added to a hash table
  using the key(s) and value(s) named.  Keys and values correspond to
  fields in the event's format description.  Values must correspond to
  numeric fields - on an event hit, the value(s) will be added to a
  sum kept for that field.  The special string 'hitcount' can be used
  in place of an explicit value field - this is simply a count of
  event hits.  If 'values' isn't specified, an implicit 'hitcount'
  value will be automatically created and used as the only value.
  Keys can be any field, or the special string 'stacktrace', which
  will use the event's kernel stacktrace as the key.  The keywords
  'keys' or 'key' can be used to specify keys, and the keywords
  'values', 'vals', or 'val' can be used to specify values.  Compound
  keys consisting of up to two fields can be specified by the 'keys'
  keyword.  Hashing a compound key produces a unique entry in the
  table for each unique combination of component keys, and can be
  useful for providing more fine-grained summaries of event data.
  Additionally, sort keys consisting of up to two fields can be
  specified by the 'sort' keyword.  If more than one field is
  specified, the result will be a 'sort within a sort': the first key
  is taken to be the primary sort key and the second the secondary
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  key.  If a hist trigger is given a name using the 'name' parameter,
  its histogram data will be shared with other triggers of the same
  name, and trigger hits will update this common data.  Only triggers
  with 'compatible' fields can be combined in this way; triggers are
  'compatible' if the fields named in the trigger share the same
  number and type of fields and those fields also have the same names.
  Note that any two events always share the compatible 'hitcount' and
  'stacktrace' fields and can therefore be combined using those
  fields, however pointless that may be.
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  'hist' triggers add a 'hist' file to each event's subdirectory.
  Reading the 'hist' file for the event will dump the hash table in
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  its entirety to stdout.  If there are multiple hist triggers
  attached to an event, there will be a table for each trigger in the
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  output.  The table displayed for a named trigger will be the same as
  any other instance having the same name. Each printed hash table
  entry is a simple list of the keys and values comprising the entry;
  keys are printed first and are delineated by curly braces, and are
  followed by the set of value fields for the entry.  By default,
  numeric fields are displayed as base-10 integers.  This can be
  modified by appending any of the following modifiers to the field
  name:
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        - .hex        display a number as a hex value
	- .sym        display an address as a symbol
	- .sym-offset display an address as a symbol and offset
	- .syscall    display a syscall id as a system call name
	- .execname   display a common_pid as a program name
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  Note that in general the semantics of a given field aren't
  interpreted when applying a modifier to it, but there are some
  restrictions to be aware of in this regard:

    - only the 'hex' modifier can be used for values (because values
      are essentially sums, and the other modifiers don't make sense
      in that context).
    - the 'execname' modifier can only be used on a 'common_pid'.  The
      reason for this is that the execname is simply the 'comm' value
      saved for the 'current' process when an event was triggered,
      which is the same as the common_pid value saved by the event
      tracing code.  Trying to apply that comm value to other pid
      values wouldn't be correct, and typically events that care save
      pid-specific comm fields in the event itself.

  A typical usage scenario would be the following to enable a hist
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  trigger, read its current contents, and then turn it off::
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	  # echo 'hist:keys=skbaddr.hex:vals=len' > \
	    /sys/kernel/debug/tracing/events/net/netif_rx/trigger
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	  # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
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	  # echo '!hist:keys=skbaddr.hex:vals=len' > \
	    /sys/kernel/debug/tracing/events/net/netif_rx/trigger
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  The trigger file itself can be read to show the details of the
  currently attached hist trigger.  This information is also displayed
  at the top of the 'hist' file when read.

  By default, the size of the hash table is 2048 entries.  The 'size'
  parameter can be used to specify more or fewer than that.  The units
  are in terms of hashtable entries - if a run uses more entries than
  specified, the results will show the number of 'drops', the number
  of hits that were ignored.  The size should be a power of 2 between
  128 and 131072 (any non- power-of-2 number specified will be rounded
  up).

  The 'sort' parameter can be used to specify a value field to sort
  on.  The default if unspecified is 'hitcount' and the default sort
  order is 'ascending'.  To sort in the opposite direction, append
  .descending' to the sort key.

  The 'pause' parameter can be used to pause an existing hist trigger
  or to start a hist trigger but not log any events until told to do
  so.  'continue' or 'cont' can be used to start or restart a paused
  hist trigger.

  The 'clear' parameter will clear the contents of a running hist
  trigger and leave its current paused/active state.

  Note that the 'pause', 'cont', and 'clear' parameters should be
  applied using 'append' shell operator ('>>') if applied to an
  existing trigger, rather than via the '>' operator, which will cause
  the trigger to be removed through truncation.

- enable_hist/disable_hist

  The enable_hist and disable_hist triggers can be used to have one
  event conditionally start and stop another event's already-attached
  hist trigger.  Any number of enable_hist and disable_hist triggers
  can be attached to a given event, allowing that event to kick off
  and stop aggregations on a host of other events.

642
  The format is very similar to the enable/disable_event triggers::
643 644 645 646 647 648 649 650 651 652 653 654

      enable_hist:<system>:<event>[:count]
      disable_hist:<system>:<event>[:count]

  Instead of enabling or disabling the tracing of the target event
  into the trace buffer as the enable/disable_event triggers do, the
  enable/disable_hist triggers enable or disable the aggregation of
  the target event into a hash table.

  A typical usage scenario for the enable_hist/disable_hist triggers
  would be to first set up a paused hist trigger on some event,
  followed by an enable_hist/disable_hist pair that turns the hist
655
  aggregation on and off when conditions of interest are hit::
656

657 658
	  # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
	    /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
659

660 661
	  # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
	    /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
662

663 664
	  # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
	    /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
665 666 667 668 669 670 671 672 673 674 675 676 677 678 679

  The above sets up an initially paused hist trigger which is unpaused
  and starts aggregating events when a given program is executed, and
  which stops aggregating when the process exits and the hist trigger
  is paused again.

  The examples below provide a more concrete illustration of the
  concepts and typical usage patterns discussed above.


6.2 'hist' trigger examples
---------------------------

  The first set of examples creates aggregations using the kmalloc
  event.  The fields that can be used for the hist trigger are listed
680
  in the kmalloc event's format file::
681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
    name: kmalloc
    ID: 374
    format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
	field:int common_pid;					offset:4;	size:4;	signed:1;

	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
	field:const void * ptr;					offset:16;	size:8;	signed:0;
	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;

  We'll start by creating a hist trigger that generates a simple table
  that lists the total number of bytes requested for each function in
699
  the kernel that made one or more calls to kmalloc::
700 701 702 703 704 705 706 707 708 709 710 711 712 713

    # echo 'hist:key=call_site:val=bytes_req' > \
            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  This tells the tracing system to create a 'hist' trigger using the
  call_site field of the kmalloc event as the key for the table, which
  just means that each unique call_site address will have an entry
  created for it in the table.  The 'val=bytes_req' parameter tells
  the hist trigger that for each unique entry (call_site) in the
  table, it should keep a running total of the number of bytes
  requested by that call_site.

  We'll let it run for awhile and then dump the contents of the 'hist'
  file in the kmalloc event's subdirectory (for readability, a number
714
  of entries have been omitted)::
715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
    .
    .
    .
    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520

    Totals:
        Hits: 4610
        Entries: 45
        Dropped: 0

  The output displays a line for each entry, beginning with the key
  specified in the trigger, followed by the value(s) also specified in
  the trigger.  At the beginning of the output is a line that displays
  the trigger info, which can also be displayed by reading the
754
  'trigger' file::
755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

  At the end of the output are a few lines that display the overall
  totals for the run.  The 'Hits' field shows the total number of
  times the event trigger was hit, the 'Entries' field shows the total
  number of used entries in the hash table, and the 'Dropped' field
  shows the number of hits that were dropped because the number of
  used entries for the run exceeded the maximum number of entries
  allowed for the table (normally 0, but if not a hint that you may
  want to increase the size of the table using the 'size' parameter).

  Notice in the above output that there's an extra field, 'hitcount',
  which wasn't specified in the trigger.  Also notice that in the
  trigger info output, there's a parameter, 'sort=hitcount', which
  wasn't specified in the trigger either.  The reason for that is that
  every trigger implicitly keeps a count of the total number of hits
  attributed to a given entry, called the 'hitcount'.  That hitcount
  information is explicitly displayed in the output, and in the
  absence of a user-specified sort parameter, is used as the default
  sort field.

  The value 'hitcount' can be used in place of an explicit value in
  the 'values' parameter if you don't really need to have any
  particular field summed and are mainly interested in hit
  frequencies.

  To turn the hist trigger off, simply call up the trigger in the
784
  command history and re-execute it with a '!' prepended::
785 786 787 788 789 790 791

    # echo '!hist:key=call_site:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  Finally, notice that the call_site as displayed in the output above
  isn't really very useful.  It's an address, but normally addresses
  are displayed in hex.  To have a numeric field displayed as a hex
792
  value, simply append '.hex' to the field name in the trigger::
793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836

    # echo 'hist:key=call_site.hex:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
    .
    .
    .
    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600

    Totals:
        Hits: 4775
        Entries: 46
        Dropped: 0

  Even that's only marginally more useful - while hex values do look
  more like addresses, what users are typically more interested in
  when looking at text addresses are the corresponding symbols
  instead.  To have an address displayed as symbolic value instead,
  simply append '.sym' or '.sym-offset' to the field name in the
837
  trigger::
838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886

    # echo 'hist:key=call_site.sym:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
    .
    .
    .
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265

    Totals:
        Hits: 109928
        Entries: 71
        Dropped: 0

  Because the default sort key above is 'hitcount', the above shows a
  the list of call_sites by increasing hitcount, so that at the bottom
  we see the functions that made the most kmalloc calls during the
  run.  If instead we we wanted to see the top kmalloc callers in
  terms of the number of bytes requested rather than the number of
  calls, and we wanted the top caller to appear at the top, we can use
887
  the 'sort' parameter, along with the 'descending' modifier::
888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927

    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
    .
    .
    .
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 32133
        Entries: 81
        Dropped: 0

  To display the offset and size information in addition to the symbol
928
  name, just use 'sym-offset' instead::
929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966

    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
    .
    .
    .
    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 26098
        Entries: 64
        Dropped: 0

  We can also add multiple fields to the 'values' parameter.  For
  example, we might want to see the total number of bytes allocated
  alongside bytes requested, and display the result sorted by bytes
967
  allocated in a descending order::
968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009

    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
    .
    .
    .
    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8

    Totals:
        Hits: 66598
        Entries: 65
        Dropped: 0

  Finally, to finish off our kmalloc example, instead of simply having
  the hist trigger display symbolic call_sites, we can have the hist
  trigger additionally display the complete set of kernel stack traces
  that led to each call_site.  To do that, we simply use the special
1010
  value 'stacktrace' for the key parameter::
1011 1012 1013 1014 1015 1016 1017 1018 1019 1020

    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  The above trigger will use the kernel stack trace in effect when an
  event is triggered as the key for the hash table.  This allows the
  enumeration of every kernel callpath that led up to a particular
  event, along with a running total of any of the event fields for
  that event.  Here we tally bytes requested and bytes allocated for
  every callpath in the system that led up to a kmalloc (in this case
1021
  every callpath to a kmalloc for a kernel compile)::
1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]

    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
         cpuidle_enter+0x17/0x20
         cpu_startup_entry+0x315/0x3e0
         rest_init+0x7c/0x80
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         aa_alloc_task_context+0x27/0x40
         apparmor_cred_prepare+0x1f/0x50
         security_prepare_creds+0x16/0x20
         prepare_creds+0xdf/0x1a0
         SyS_capset+0xb5/0x200
         system_call_fastpath+0x12/0x6a
    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
    .
    .
    .
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
         drm_ioctl+0x349/0x670 [drm]
         do_vfs_ioctl+0x2f0/0x4f0
         SyS_ioctl+0x81/0xa0
         system_call_fastpath+0x12/0x6a
    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         load_elf_phdrs+0x76/0xa0
         load_elf_binary+0x102/0x1650
         search_binary_handler+0x97/0x1d0
         do_execveat_common.isra.34+0x551/0x6e0
         SyS_execve+0x3a/0x50
         return_from_execve+0x0/0x23
    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         apparmor_file_alloc_security+0x27/0x40
         security_file_alloc+0x16/0x20
         get_empty_filp+0x93/0x1c0
         path_openat+0x31/0x5f0
         do_filp_open+0x3a/0x90
         do_sys_open+0x128/0x220
         SyS_open+0x1e/0x20
         system_call_fastpath+0x12/0x6a
    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         seq_buf_alloc+0x1b/0x50
         seq_read+0x2cc/0x370
         proc_reg_read+0x3d/0x80
         __vfs_read+0x28/0xe0
         vfs_read+0x86/0x140
         SyS_read+0x46/0xb0
         system_call_fastpath+0x12/0x6a
    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768

    Totals:
        Hits: 6085872
        Entries: 253
        Dropped: 0

  If you key a hist trigger on common_pid, in order for example to
  gather and display sorted totals for each process, you can use the
  special .execname modifier to display the executable names for the
  processes in the table rather than raw pids.  The example below
1119
  keeps a per-process sum of total bytes read::
1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159

    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]

    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
    .
    .
    .
    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
    { common_pid: init            [         1] } hitcount:          2  count:          2

    Totals:
        Hits: 2116
        Entries: 51
        Dropped: 0

  Similarly, if you key a hist trigger on syscall id, for example to
  gather and display a list of systemwide syscall hits, you can use
  the special .syscall modifier to display the syscall names rather
  than raw ids.  The example below keeps a running total of syscall
1160
  counts for the system during the run::
1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201

    # echo 'hist:key=id.syscall:val=hitcount' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]

    { id: sys_fsync                     [ 74] } hitcount:          1
    { id: sys_newuname                  [ 63] } hitcount:          1
    { id: sys_prctl                     [157] } hitcount:          1
    { id: sys_statfs                    [137] } hitcount:          1
    { id: sys_symlink                   [ 88] } hitcount:          1
    { id: sys_sendmmsg                  [307] } hitcount:          1
    { id: sys_semctl                    [ 66] } hitcount:          1
    { id: sys_readlink                  [ 89] } hitcount:          3
    { id: sys_bind                      [ 49] } hitcount:          3
    { id: sys_getsockname               [ 51] } hitcount:          3
    { id: sys_unlink                    [ 87] } hitcount:          3
    { id: sys_rename                    [ 82] } hitcount:          4
    { id: unknown_syscall               [ 58] } hitcount:          4
    { id: sys_connect                   [ 42] } hitcount:          4
    { id: sys_getpid                    [ 39] } hitcount:          4
    .
    .
    .
    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
    { id: sys_futex                     [202] } hitcount:       1534
    { id: sys_write                     [  1] } hitcount:       2689
    { id: sys_setitimer                 [ 38] } hitcount:       2797
    { id: sys_read                      [  0] } hitcount:       3202
    { id: sys_select                    [ 23] } hitcount:       3773
    { id: sys_writev                    [ 20] } hitcount:       4531
    { id: sys_poll                      [  7] } hitcount:       8314
    { id: sys_recvmsg                   [ 47] } hitcount:      13738
    { id: sys_ioctl                     [ 16] } hitcount:      21843

    Totals:
        Hits: 67612
        Entries: 72
        Dropped: 0

1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350
  The syscall counts above provide a rough overall picture of system
  call activity on the system; we can see for example that the most
  popular system call on this system was the 'sys_ioctl' system call.

  We can use 'compound' keys to refine that number and provide some
  further insight as to which processes exactly contribute to the
  overall ioctl count.

  The command below keeps a hitcount for every unique combination of
  system call id and pid - the end result is essentially a table
  that keeps a per-pid sum of system call hits.  The results are
  sorted using the system call id as the primary key, and the
  hitcount sum as the secondary key::

      # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
             /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

      # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
      # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]

      { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
      { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
      { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
      { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
      { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
      { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
      { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
      { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
      { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
      { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
      .
      .
      .
      { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
      { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
      { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
      { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
      .
      .
      .
      { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
      { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
      { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
      { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
      { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
      { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6

      Totals:
          Hits: 31536
          Entries: 323
          Dropped: 0

  The above list does give us a breakdown of the ioctl syscall by
  pid, but it also gives us quite a bit more than that, which we
  don't really care about at the moment.  Since we know the syscall
  id for sys_ioctl (16, displayed next to the sys_ioctl name), we
  can use that to filter out all the other syscalls::

      # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
             /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

      # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
      # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]

      { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
      { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
      .
      .
      .
      { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
      { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
      { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
      { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
      { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
      { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443

      Totals:
          Hits: 101162
          Entries: 103
          Dropped: 0

  The above output shows that 'compiz' and 'Xorg' are far and away
  the heaviest ioctl callers (which might lead to questions about
  whether they really need to be making all those calls and to
  possible avenues for further investigation.)

  The compound key examples used a key and a sum value (hitcount) to
  sort the output, but we can just as easily use two keys instead.
  Here's an example where we use a compound key composed of the the
  common_pid and size event fields.  Sorting with pid as the primary
  key and 'size' as the secondary key allows us to display an
  ordered summary of the recvfrom sizes, with counts, received by
  each process::

      # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
             /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger

      # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
      # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]

      { common_pid: smbd            [       784], size:          4 } hitcount:          1
      { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
      { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
      { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
      { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
      { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
      { common_pid: compiz          [      2994], size:          8 } hitcount:          1
      { common_pid: compiz          [      2994], size:         20 } hitcount:         11
      { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
      { common_pid: firefox         [      8817], size:          4 } hitcount:          1
      { common_pid: firefox         [      8817], size:          8 } hitcount:          5
      { common_pid: firefox         [      8817], size:        588 } hitcount:          2
      { common_pid: firefox         [      8817], size:        628 } hitcount:          1
      { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
      { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
      { common_pid: firefox         [      8822], size:          8 } hitcount:          2
      { common_pid: firefox         [      8822], size:        160 } hitcount:          2
      { common_pid: firefox         [      8822], size:        320 } hitcount:          2
      { common_pid: firefox         [      8822], size:        352 } hitcount:          1
      .
      .
      .
      { common_pid: pool            [      8923], size:       1960 } hitcount:         10
      { common_pid: pool            [      8923], size:       2048 } hitcount:         10
      { common_pid: pool            [      8924], size:       1960 } hitcount:         10
      { common_pid: pool            [      8924], size:       2048 } hitcount:         10
      { common_pid: pool            [      8928], size:       1964 } hitcount:          4
      { common_pid: pool            [      8928], size:       1965 } hitcount:          2
      { common_pid: pool            [      8928], size:       2048 } hitcount:          6
      { common_pid: pool            [      8929], size:       1982 } hitcount:          1
      { common_pid: pool            [      8929], size:       2048 } hitcount:          1

      Totals:
          Hits: 2016
          Entries: 224
          Dropped: 0
1351 1352 1353 1354 1355 1356 1357 1358 1359

  The above example also illustrates the fact that although a compound
  key is treated as a single entity for hashing purposes, the sub-keys
  it's composed of can be accessed independently.

  The next example uses a string field as the hash key and
  demonstrates how you can manually pause and continue a hist trigger.
  In this example, we'll aggregate fork counts and don't expect a
  large number of entries in the hash table, so we'll drop it to a
1360
  much smaller number, say 256::
1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395

    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: whoopsie                            } hitcount:          1
    { child_comm: smbd                                } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: postgres                            } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: dhclient                            } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: nm-dispatcher.a                     } hitcount:          8
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dbus-daemon                         } hitcount:          8
    { child_comm: glib-pacrunner                      } hitcount:         10
    { child_comm: evolution                           } hitcount:         23

    Totals:
        Hits: 89
        Entries: 20
        Dropped: 0

  If we want to pause the hist trigger, we can simply append :pause to
  the command that started the trigger.  Notice that the trigger info
1396
  displays as [paused]::
1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432

    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         10
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         20
    { child_comm: nm-dispatcher.a                     } hitcount:         20
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 199
        Entries: 21
        Dropped: 0

  To manually continue having the trigger aggregate events, append
  :cont instead.  Notice that the trigger info displays as [active]
1433
  again, and the data has changed::
1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486

    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: bash                                } hitcount:          5
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         11
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         22
    { child_comm: nm-dispatcher.a                     } hitcount:         22
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 206
        Entries: 21
        Dropped: 0

  The previous example showed how to start and stop a hist trigger by
  appending 'pause' and 'continue' to the hist trigger command.  A
  hist trigger can also be started in a paused state by initially
  starting the trigger with ':pause' appended.  This allows you to
  start the trigger only when you're ready to start collecting data
  and not before.  For example, you could start the trigger in a
  paused state, then unpause it and do something you want to measure,
  then pause the trigger again when done.

  Of course, doing this manually can be difficult and error-prone, but
  it is possible to automatically start and stop a hist trigger based
  on some condition, via the enable_hist and disable_hist triggers.

  For example, suppose we wanted to take a look at the relative
  weights in terms of skb length for each callpath that leads to a
  netif_receieve_skb event when downloading a decent-sized file using
  wget.

  First we set up an initially paused stacktrace trigger on the
1487
  netif_receive_skb event::
1488 1489 1490 1491 1492 1493 1494 1495 1496 1497

    # echo 'hist:key=stacktrace:vals=len:pause' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Next, we set up an 'enable_hist' trigger on the sched_process_exec
  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
  this new trigger is that it will 'unpause' the hist trigger we just
  set up on netif_receive_skb if and only if it sees a
  sched_process_exec event with a filename of '/usr/bin/wget'.  When
  that happens, all netif_receive_skb events are aggregated into a
1498
  hash table keyed on stacktrace::
1499 1500 1501 1502 1503 1504 1505

    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

  The aggregation continues until the netif_receive_skb is paused
  again, which is what the following disable_hist event does by
  creating a similar setup on the sched_process_exit event, using the
1506
  filter 'comm==wget'::
1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517

    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  Whenever a process exits and the comm field of the disable_hist
  trigger filter matches 'comm==wget', the netif_receive_skb hist
  trigger is disabled.

  The overall effect is that netif_receive_skb events are aggregated
  into the hash table for only the duration of the wget.  Executing a
  wget command and then listing the 'hist' file will display the
1518
  output generated by the wget command::
1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_receive+0xc8/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:         85  len:      28884
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         dev_gro_receive+0x23a/0x360
         napi_gro_receive+0x30/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
    } hitcount:         98  len:     664329
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         process_backlog+0xa8/0x150
         net_rx_action+0x15d/0x340
         __do_softirq+0x114/0x2c0
         do_softirq_own_stack+0x1c/0x30
         do_softirq+0x65/0x70
         __local_bh_enable_ip+0xb5/0xc0
         ip_finish_output+0x1f4/0x840
         ip_output+0x6b/0xc0
         ip_local_out_sk+0x31/0x40
         ip_send_skb+0x1a/0x50
         udp_send_skb+0x173/0x2a0
         udp_sendmsg+0x2bf/0x9f0
         inet_sendmsg+0x64/0xa0
         sock_sendmsg+0x3d/0x50
    } hitcount:        115  len:      13030
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         napi_gro_flush+0x6d/0x90
         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:        934  len:    5512212

    Totals:
        Hits: 1232
        Entries: 4
        Dropped: 0

  The above shows all the netif_receive_skb callpaths and their total
  lengths for the duration of the wget command.

  The 'clear' hist trigger param can be used to clear the hash table.
  Suppose we wanted to try another run of the previous example but
  this time also wanted to see the complete list of events that went
  into the histogram.  In order to avoid having to set everything up
1603
  again, we can just clear the histogram first::
1604 1605 1606 1607 1608

    # echo 'hist:key=stacktrace:vals=len:clear' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Just to verify that it is in fact cleared, here's what we now see in
1609
  the hist file::
1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0

  Since we want to see the detailed list of every netif_receive_skb
  event occurring during the new run, which are in fact the same
  events being aggregated into the hash table, we add some additional
  'enable_event' events to the triggering sched_process_exec and
1623
  sched_process_exit events as such::
1624 1625 1626 1627 1628 1629 1630 1631 1632 1633

    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  If you read the trigger files for the sched_process_exec and
  sched_process_exit triggers, you should see two triggers for each:
  one enabling/disabling the hist aggregation and the other
1634
  enabling/disabling the logging of events::
1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
    enable_event:net:netif_receive_skb:unlimited if comm==wget
    disable_hist:net:netif_receive_skb:unlimited if comm==wget

  In other words, whenever either of the sched_process_exec or
  sched_process_exit events is hit and matches 'wget', it enables or
  disables both the histogram and the event log, and what you end up
  with is a hash table and set of events just covering the specified
1648
  duration.  Run the wget command again::
1649 1650 1651 1652 1653

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

  Displaying the 'hist' file should show something similar to what you
  saw in the last run, but this time you should also see the
1654
  individual events in the trace file::
1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678

    # cat /sys/kernel/debug/tracing/trace

    # tracer: nop
    #
    # entries-in-buffer/entries-written: 183/1426   #P:4
    #
    #                              _-----=> irqs-off
    #                             / _----=> need-resched
    #                            | / _---=> hardirq/softirq
    #                            || / _--=> preempt-depth
    #                            ||| /     delay
    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
    #              | |       |   ||||       |         |
                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
    ##### CPU 2 buffer started ####
      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
1679 1680
    ....

1681 1682 1683 1684 1685 1686

  The following example demonstrates how multiple hist triggers can be
  attached to a given event.  This capability can be useful for
  creating a set of different summaries derived from the same set of
  events, or for comparing the effects of different filters, among
  other things.
1687
  ::
1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707

    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=len:vals=common_preempt_count' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  The above set of commands create four triggers differing only in
  their filters, along with a completely different though fairly
  nonsensical trigger.  Note that in order to append multiple hist
  triggers to the same file, you should use the '>>' operator to
  append them ('>' will also add the new hist trigger, but will remove
  any existing hist triggers beforehand).

  Displaying the contents of the 'hist' file for the event shows the
1708
  contents of all five histograms::
1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist

    # event histogram
    #
    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
    #

    { len:        176 } hitcount:          1  common_preempt_count:          0
    { len:        223 } hitcount:          1  common_preempt_count:          0
    { len:       4854 } hitcount:          1  common_preempt_count:          0
    { len:        395 } hitcount:          1  common_preempt_count:          0
    { len:        177 } hitcount:          1  common_preempt_count:          0
    { len:        446 } hitcount:          1  common_preempt_count:          0
    { len:       1601 } hitcount:          1  common_preempt_count:          0
    .
    .
    .
    { len:       1280 } hitcount:         66  common_preempt_count:          0
    { len:        116 } hitcount:         81  common_preempt_count:         40
    { len:        708 } hitcount:        112  common_preempt_count:          0
    { len:         46 } hitcount:        221  common_preempt_count:          0
    { len:       1264 } hitcount:        458  common_preempt_count:          0

    Totals:
        Hits: 1428
        Entries: 147
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
    .
    .
    .
    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032

    Totals:
        Hits: 1451
        Entries: 318
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
    #

    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996

    Totals:
        Hits: 14
        Entries: 12
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0
1822 1823 1824 1825 1826 1827

  Named triggers can be used to have triggers share a common set of
  histogram data.  This capability is mostly useful for combining the
  output of events generated by tracepoints contained inside inline
  functions, but names can be used in a hist trigger on any event.
  For example, these two triggers when hit will update the same 'len'
1828
  field in the shared 'foo' histogram data::
1829 1830 1831 1832 1833 1834 1835

    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  You can see that they're updating common histogram data by reading
1836
  each event's hist files at the same time::
1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0
    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0

  And here's an example that shows how to combine histogram data from
  any two events even if they don't share any 'compatible' fields
  other than 'hitcount' and 'stacktrace'.  These commands create a
1949
  couple of triggers named 'bar' using those fields::
1950 1951 1952 1953 1954 1955 1956

    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
          /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  And displaying the output of either shows some interesting if
1957
  somewhat confusing output::
1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066 2067 2068 2069

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
    #

    { stacktrace:
             _do_fork+0x18e/0x330
             kernel_thread+0x29/0x30
             kthreadd+0x154/0x1b0
             ret_from_fork+0x3f/0x70
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x126/0x240
             ip_local_out_sk+0x31/0x40
             igmp_send_report+0x1e9/0x230
             igmp_timer_expire+0xe9/0x120
             call_timer_fn+0x39/0xf0
             run_timer_softirq+0x1e1/0x290
             __do_softirq+0xfd/0x290
             irq_exit+0x98/0xb0
             smp_apic_timer_interrupt+0x4a/0x60
             apic_timer_interrupt+0x6d/0x80
             cpuidle_enter+0x17/0x20
             call_cpuidle+0x3b/0x60
             cpu_startup_entry+0x22d/0x310
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x17f/0x240
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x13e/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
             SyS_sendto+0xe/0x10
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:          2
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x14e/0x270
    } hitcount:         76
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x269/0x270
    } hitcount:         77
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
    } hitcount:         88
    { stacktrace:
             _do_fork+0x18e/0x330
             SyS_clone+0x19/0x20
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:        244

    Totals:
        Hits: 489
        Entries: 7
        Dropped: 0