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ftrace - Linux kernel internal tracer

Introduction
------------

Ftrace is an internal tracer for the Linux kernel. It is designed to
follow the processing of what happens within the kernel as that is
normally a black box. It allows the user to trace kernel functions
that are called in real time, as well as to see various events like
tasks scheduling, interrupts, disk activity and other services that
the kernel provides.

Ftrace was intorduced to Linux in the 2.6.27 kernel, and has increased
in functionality ever since. It is not meant to trace what is happening
inside user applications, but can be used to trace within system calls
that user applications make.


The Debug File System
---------------------

The user interface for ftrace is a series of files within the debug
file system that is usually mounted at /sys/kernel/debug. The ftrace
files are in the tracing directory that can be accessed at
/sys/kernel/debug/tracing.

Note, there is also a user interface tool called trace-cmd. See later
in this document for more information about that tool.

In order to mount the debug filesystem, perform the following:

 mount -t debugfs nodev /sys/kernel/debug

Then you can change directory into the ftrace tracing location:

 cd /sys/kernel/debug/tracing

Note, all these files can only be modified by root user, as enabling
tracing can have an impact on the performance of the system.


Ftrace files
------------

The main files within this directory are:

 trace - the file that shows the output of a ftrace trace. This is
       really a snapshot of the trace in time, as it stops tracing as
       this file is read, and it does not consume the events read.
       That is, if the user disabled tracing and read this file, it
       will always report the same thing every time its read.

       Also, to clear the trace buffer, simply write into this file.

          ># echo > trace

       This will erase the entire contents of the trace buffer.

 trace_pipe - like "trace" but is used to read the trace live. It is
       a producer / consumer trace, where each read will consume the
       event that is read. But this can be used to see an active trace
       without stopping the trace as it is read.

 available_tracers - a list of ftrace tracers that have been compiled
       into the kernel.

 current_tracer   - enables or disables a ftrace tracer

 events - a directory that contains events to trace and can be used
       to enable or disable events as well as set filters for the events

 tracing_on - disable and enable recording to the ftrace buffer.
       Note, disabling tracing via the tracing_on file does not disable
       the actual tracing that is happening inside the kernel. It only
       disables writing to the buffer. The work to do the trace still
       happens, but the data does not go anywhere.

There are several other files, but we will get to them as they come
up with functionalities of the tracers.


Tracers and Events
------------------

Tracers have specific functionality within the kernel, where as events
are just some kind of data that is recorded into the ftrace buffer.
To understand this more, we need to take a look at the tracers themselves
and the events as well.

nop
---

The default tracer is called "nop". It is just a nop tracer, and does not
provide any tracing facility itself. But, as events may interleave into
any tracer, the "nop" tracer is what is used if you are only interested
in tracing events.

When the "nop" tracer is active and the trace buffer is empty, the "trace"
file shows the following:

># cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 0/0   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |


It starts with what tracer is active and then gives a default header.

Now to enable an event, you must write an ASCII '1' into the "enable"
file for the particular event.

># echo 1 > events/sched/sched_switch/enable
># cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 463/463   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
            bash-1367  [007] d...... 11927.750484: sched_switch: prev_comm=bash prev_pid=1367 prev_prio=120 prev_state=S ==> next_comm=kworker/7:1 next_pid=121 next_prio=120
     kworker/7:1-121   [007] d...... 11927.750514: sched_switch: prev_comm=kworker/7:1 prev_pid=121 prev_prio=120 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
          <idle>-0     [000] d...... 11927.750531: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=sshd next_pid=1365 next_prio=120
          <idle>-0     [007] d...... 11927.750555: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/7:1 next_pid=121 next_prio=120
     kworker/7:1-121   [007] d...... 11927.750575: sched_switch: prev_comm=kworker/7:1 prev_pid=121 prev_prio=120 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
            sshd-1365  [000] d...... 11927.750673: sched_switch: prev_comm=sshd prev_pid=1365 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
          <idle>-0     [001] d...... 11927.752568: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/1:1 next_pid=57 next_prio=120
          <idle>-0     [002] d...... 11927.752589: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_sched next_pid=10 next_prio=120
     kworker/1:1-57    [001] d...... 11927.752590: sched_switch: prev_comm=kworker/1:1 prev_pid=57 prev_prio=120 prev_state=S ==> next_comm=swapper/1 next_pid=0 next_prio=120
       rcu_sched-10    [002] d...... 11927.752610: sched_switch: prev_comm=rcu_sched prev_pid=10 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
          <idle>-0     [007] d...... 11927.753548: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_sched next_pid=10 next_prio=120
       rcu_sched-10    [007] d...... 11927.753568: sched_switch: prev_comm=rcu_sched prev_pid=10 prev_prio=120 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
          <idle>-0     [007] d...... 11927.755538: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/7:1 next_pid=121 next_prio=120


As you can see there is quite a lot of information that is displayed
by simply enabling the sched_switch event.

Events
------

The events are broken up into "systems". Each system of events has its
own directory under the "events" directory located in the ftrace "tracing"
directory in the debug file system.

># ls -F events
block/       header_event  lock/     printk/        skb/       vsyscall/
compaction/  header_page   mce/      random/        sock/      workqueue/
drm/         i915/         migrate/  raw_syscalls/  sunrpc/    writeback/
enable       irq/          module/   rcu/           syscalls/
ext4/        jbd2/         napi/     rpm/           task/
ftrace/      kmem/         net/      sched/         timer/
hda/         kvm/          oom/      scsi/          udp/
hda_intel/   kvmmmu/       power/    signal/        vmscan/

Each of these directories represent a system or group of events. Notice that
there's three files in this directory:

enable
header_event
header_page

The only one you should be concerned about is the "enable" file, as that
will enable all events when an ASCII '1' is written into it and disable
all events when an ASCII '0' is written into it.

The header_event and header_page provides information necessary for
the trace-cmd tool.

Each of these directories shows the events that are within that system:

># ls -F events/sched
enable                   sched_process_exit/  sched_stat_sleep/
filter                   sched_process_fork/  sched_stat_wait/
sched_kthread_stop/      sched_process_free/  sched_switch/
sched_kthread_stop_ret/  sched_process_wait/  sched_wait_task/
sched_migrate_task/      sched_stat_blocked/  sched_wakeup/
sched_pi_setprio/        sched_stat_iowait/   sched_wakeup_new/
sched_process_exec/      sched_stat_runtime/


Each directory here represents a single event. Notice that there's two
files in the system directory:

enable
filter

The "enable" file here can enable or disable all events within the system
when an ASCII '1' or '0', respectively, is written to this file.

The "filter" file will be described shortly.


Within the individual event directories exist control files:

># ls -F events/sched/sched_wakeup/
enable  filter  format  id


We already used the "enable" file. Now to explain the other files.

The "format" file shows the fields that are written when the event
is enabled, as well as the fields that can be used for the filter.

The "id" file is used by the perf tool and is not something that needs
to be delt with here.


># cat events/sched/sched_wakeup/format
name: sched_wakeup
ID: 249
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 short common_migrate_disable;        offset:8;        size:2;        signed:0;
        field:unsigned short common_padding;        offset:10;        size:2;        signed:0;

        field:char comm[16];        offset:16;        size:16;        signed:1;
        fieldid_t pid;        offset:32;        size:4;        signed:1;
        field:int prio;        offset:36;        size:4;        signed:1;
        field:int success;        offset:40;        size:4;        signed:1;
        field:int target_cpu;        offset:44;        size:4;        signed:1;

print fmt: "comm=%s pid=%d prio=%d success=%d target_cpu=%03d", REC->comm, REC->pid, REC->prio, REC->success, REC->target_cpu


This file is also used by perf and trace-cmd to tell how to read the
raw binary output from the tracing buffers for the event. But what you
need to know is the field names, as they are used by the filtering.

The first set of fields before the blank line are the common fields that
exist for all events. The specific fields for the event come after the
blank line and here it starts with "comm".


Filtering events
----------------

There are times when you may not want to trace all events, but only
events where one of the event's fields contains a certain value.
The "filter" file allows for this.

The filter provides the following predicates:

For numerical fields:

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


For string fields:

 ==, !=, ~


Logical && and || as well as parenthesis are also acceptable.

The syntax is

 <filter> = FIELD <pred-num> | FIELD <pred-string> |
    '(' <filter> ')' | <filter> '&&' <filter> | <filter> '||' <filter>

 <pred-num> = <num-op> <number>

 <pred-string> = <string-op> <string>

 <num-op> = '==' | '!=' | '<' | '<=' | '>' | '>='

 <string-op> = '==' | '!=' | '~'

 <number> = <digits> | '0x'<hex-number> 

 <digits> = [0-9] | <digits><digits>

 <hex-number> = [0-9] | [a-f] | [A-F] | <hex-number><hex-number>

 <string> = '"' VALUE '"'

The glob expression '~' is a very simple glob. it can only be:

 <glob> = VALUE | '*' VALUE | VALUE '*' | '*' VALUE '*'

That is, anything more complex will not be valid. Such as:

  VALUE '*' VALUE


What the glob does is to match a string with wild cards at the beginning
or end or both, of a value:

  comm ~ "kwork*"


Example:

To trace all schedule switches to a real time task:

># echo 'next_prio < 100' > events/sched/sched_switch/filter
># cat events/sched/sched_switch/filter
next_prio < 100
># cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 11/11   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
          <idle>-0     [001] d...... 14331.192687: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rtkit-daemon next_pid=992 next_prio=0
          <idle>-0     [001] d...... 14333.737030: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/1 next_pid=12 next_prio=0
          <idle>-0     [000] d...... 14333.738023: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/0 next_pid=11 next_prio=0
          <idle>-0     [002] d...... 14333.751985: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/2 next_pid=17 next_prio=0
          <idle>-0     [003] d...... 14333.765947: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/3 next_pid=22 next_prio=0
          <idle>-0     [004] d...... 14333.779933: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/4 next_pid=27 next_prio=0
          <idle>-0     [005] d...... 14333.794114: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=watchdog/5 next_pid=32 next_prio=0



Task priorities
---------------

This is a good time to explain task priorities, as the tracer reports them
differently than the way user processes see priorities. A task has priority
policies that are SCHED_OTHER, SCHED_FIFO and SCHED_RR. By default
tasks are assigned SCHED_OTHER which runs under the kernels Completely
Fail Scheduler (CFS), where as SCHED_FIFO and SCHED_RR runs under
the real-time scheduler. The real-time scheduler has 99 different priorities
ranging from 1 - 99, where 99 is the highest priority and 1 is the lowest.
This is set by sched_setscheduler(2).

If you noticed above, to show real time tasks, the filter used
"next_prio < 100". Ftrace reports the internal kernel version of priorities
for tasks and not the priority that a task sees. This can be a little
confusing. For user real-time priorities of 1 through 99 are mapped
internally as 98 to 0, where 0 is the highest priority and 98 is the lowest
of the real time priorities. All non real-time tasks show a priority of 120,
as CFS does not use the priority to determine which tasks to run, although
it does use a nice value, but that's not represented by the prio field
reported in the traces.



Tracers
-------

Depending on how the kernel was configured, not all tracers may be available
for a given kernel.For the Red Hat Enterprise Linux for Real Time kernels, the trace and debug kernels have
different tracers than the production kernel does. This is because some
of the tracers have a noticeable overhead when the tracer is configured
into the kernel but not active. Those tracers are only enabled for
the trace and debug kernels.


To see what tracers are available for the kernel, cat out the contents
of "available_tracers":

># cat available_tracers 
function_graph wakeup_rt wakeup preemptirqsoff preemptoff irqsoff function nop


The "nop" tracer has already been discussed and is available in all
kernels.


The "function" tracer
---------------------

The most popular tracer aside from the "nop" tracer is the "function"
tracer. This tracer traces the function calls within the kernel.
Depending on how many functions are tracer or which specific functions,
it can cause a very noticeable overhead when tracing is active.

Note, due to a clever trick with code modification, the function tracer
induces very little overhead when not active. This is because the
hooks in the function calls to be traced are converted into nops on
boot, and are only converted back to hooks into the tracer when activated.

># echo function > current_tracer
># cat trace
# tracer: function
#
# entries-in-buffer/entries-written: 319338/253106705   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
     kworker/5:1-58    [005] ....... 32462.200700: smp_call_function_single <-cpufreq_get_measured_perf
     kworker/5:1-58    [005] d...... 32462.200700: read_measured_perf_ctrs <-smp_call_function_single
     kworker/5:1-58    [005] ....... 32462.200701: cpufreq_cpu_put <-__cpufreq_driver_getavg
     kworker/5:1-58    [005] ....... 32462.200702: module_put <-cpufreq_cpu_put
     kworker/5:1-58    [005] ....... 32462.200702: od_check_cpu <-dbs_check_cpu
     kworker/5:1-58    [005] ....... 32462.200702: usecs_to_jiffies <-od_dbs_timer
     kworker/5:1-58    [005] ....... 32462.200703: schedule_delayed_work_on <-od_dbs_timer
     kworker/5:1-58    [005] ....... 32462.200703: queue_delayed_work_on <-schedule_delayed_work_on
     kworker/5:1-58    [005] d...... 32462.200704: __queue_delayed_work <-queue_delayed_work_on
     kworker/5:1-58    [005] d...... 32462.200704: get_work_gcwq <-__queue_delayed_work
     kworker/5:1-58    [005] d...... 32462.200704: get_cwq <-__queue_delayed_work
     kworker/5:1-58    [005] d...... 32462.200705: add_timer_on <-__queue_delayed_work
     kworker/5:1-58    [005] d...... 32462.200705: _raw_spin_lock_irqsave <-add_timer_on
     kworker/5:1-58    [005] d...... 32462.200705: internal_add_timer <-add_timer_on


Filtering on functions
----------------------

As tracing all functions can be induce a substantial overhead, as well
as adding a lot of noise to the trace (you may not be interested in every
function call), ftrace provides a way to limit what functions can be
traced. There are two files for this purpose:

 set_ftrace_filter

 set_ftrace_notrace


For a list of functions that can be traced, as well as added to these files:

 available_filter_functions


By writing a name of a function into the "set_ftrace_filter" file, the
function tracer will only trace that function.

># echo schedule_delayed_work > set_ftrace_filter
># cat set_ftrace_filter
schedule_delayed_work
># cat trace
# tracer: function
#
# entries-in-buffer/entries-written: 8/8   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
     kworker/0:2-1586  [000] ....... 32820.361913: schedule_delayed_work <-vmstat_update
     kworker/2:1-62    [002] ....... 32820.370891: schedule_delayed_work <-vmstat_update
     kworker/3:2-5004  [003] ....... 32820.373881: schedule_delayed_work <-vmstat_update
     kworker/0:2-1586  [000] ....... 32820.448658: schedule_delayed_work <-do_cache_clean
     kworker/4:1-61    [004] ....... 32820.537541: schedule_delayed_work <-vmstat_update
     kworker/4:1-61    [004] ....... 32820.537546: schedule_delayed_work <-sync_cmos_clock
     kworker/7:1-121   [007] ....... 32820.897372: schedule_delayed_work <-vmstat_update
     kworker/1:1-57    [001] ....... 32820.898361: schedule_delayed_work <-vmstat_update


Note, modifications to these files follows shell concatenation rules:

># cat set_ftrace_filter
schedule_delayed_work
># echo do_IRQ > set_ftrace_filter
># cat set_ftrace_filter
do_IRQ

Notice that writing with '>' into set_ftrace_filter cleared what was
currently in the file and replaced it with the new contents. Just
writing into the file will clear it:

># cat set_ftrace_filter
do_IRQ
># echo > set_ftrace_filter
># cat set_ftrace_filter
#### all functions enabled ####

To append to the list, use the shell append operation '>>':

># cat set_ftrace_filter
do_IRQ
># echo schedule_delayed_work >> set_ftrace_filter
># cat set_ftrace_filter
schedule_delayed_work
do_IRQ


Note, the order of functions displayed has nothing to do with how they
were added. Their order is dependent upon how the functions are layed
out in the kernel internal function list table.


Globs
-----

Functions can be added to these files with the same type of glob
expressions described in the event filtering section. The format is
identical:

 <glob> = VALUE | '*' VALUE | VALUE '*' | '*' VALUE '*'


If you want to trace all functions that start with "sched":

># echo 'sched*' > set_ftrace_filter
># cat set_ftrace_filter
schedule_delayed_work_on
schedule_delayed_work
schedule_work_on
schedule_work
schedule_on_each_cpu
sched_feat_open
sched_feat_show
[...]
># echo function > current_tracer
># cat trace
# tracer: function
#
# entries-in-buffer/entries-written: 1270/1270   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
            bash-1367  [001] ....... 34240.654888: schedule_work <-tty_flip_buffer_push
            bash-1367  [001] .N..... 34240.654902: schedule <-sysret_careful
     kworker/1:1-57    [001] ....... 34240.654921: schedule <-worker_thread
          <idle>-0     [000] .N..... 34240.654949: schedule <-cpu_idle
            bash-1367  [001] ....... 34240.655069: schedule_work <-tty_flip_buffer_push
            bash-1367  [001] .N..... 34240.655079: schedule <-sysret_careful
            sshd-1365  [000] ....... 34240.655087: schedule_timeout <-wait_for_common
            sshd-1365  [000] ....... 34240.655088: schedule <-schedule_timeout



set_ftrace_notrace
------------------

There are cases were you may want to trace everything except for various
functions that you don't care about. Perhaps there's functions that cause
too much noise in the trace, for example, perhaps locks are showing
up in the trace and you don't care about them:

># echo '*lock*' > set_ftrace_notrace
># cat set_ftrace_notrace
update_persistent_clock
read_persistent_clock
set_task_blockstep
user_enable_block_step
read_hv_clock
__acpi_acquire_global_lock
__acpi_release_global_lock
cpu_hotplug_driver_lock
cpu_hotplug_driver_unlock
[...]

But notice that you also included functions that have "clock" and "block"
in their names. To remove them but still keep the "lock" functions, use
the '!' symbol:


># echo '!*clock*' >> set_ftrace_notrace
># echo '!*block*' >> set_ftrace_notrace
># cat set_ftrace_notrace
__acpi_acquire_global_lock
__acpi_release_global_lock
cpu_hotplug_driver_lock
cpu_hotplug_driver_unlock
lock_vector_lock
unlock_vector_lock
console_lock
console_trylock
console_unlock
is_console_locked
kmsg_dump_get_line_nolock
[...]

But remember to use '>>' instead of '>', as that will clear out all
functions in the file.



Latency tracers
---------------

As stated, the difference between events and tracers, is that events
just enable recording some specific information within the kernel.
Traces have a bit more impact. Function tracing, in essence, also
just records information, but it requires a bit more work than enabling
a static tracepoint (event). Also, to limit what function tracing can
trace, requires writing into control files for the function tracer.

Another type of tracer is the latency tracers. These record a snapshot
of the trace when the latency is greater than the previously recorded
latency. There are two types of latency tracers, one kind records the
length of time when activities within the kernel are disabled, and the
other records the time it takes from when a task is woken from sleep
to the time it gets scheduled.

tracing_max_latency
-------------------

A latency tracer will just keep track of a snapshot of a trace when a new
max latency is hit. To see the current max latency time, cat the contents
of the file "tracing_max_latency". This file can also be used to set
the max time. Either to reset it back to zero or some lesser number to
trigger new snapshots of latencies, or to set it to a greater number to
not record anything unless a latency has exceeded some given time.

The unit of time that "tracing_max_latency" uses (as well as all other
tracing files, unless otherwise specified) is microseconds.


irqsoff tracer
--------------

A common use of the tracing facility is to see how long interrupts have
been disabled for. When interrupts are disabled, the system cannot
respond to external events, which can include a packet coming in on the
network card, or perhaps a task on another CPU woke up a task on the current
CPU and sent an interprocessor interrupt (IPI) to tell the current CPU
to run the new task. With interrupts disabled, the current CPU will
ignore all external events, which is a source of latencies. This is why
monitorying how long interrupts are disabled can show why the system
did not react in a proper time that was expected.

The irqsoff tracer traces the time interrupts are disabled to the time
they are enabled again. If the time interrupts were disabled is larger
than the time specified by "tracing_max_latency" has, then it will
save the current trace off to a "snapshot" buffer, reset the current
buffer and continue tracing looking for the next time interrupts
are off for a long time.

Here's an example of how to use irqsoff tracer:

># echo 0 > tracing_max_latency
># echo irqsoff > current_tracer
># sleep 10
># cat trace
# tracer: irqsoff
#
# irqsoff latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 523 us, #1301/1301, CPU#2 | (Mreempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: swapper/2-0 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: cpu_idle
#  => ended at:   cpu_idle
#
#
#                  _--------=> CPU#              
#                 / _-------=> irqs-off          
#                | / _------=> need-resched      
#                || / _-----=> need-resched_lazy 
#                ||| / _----=> hardirq/softirq   
#                |||| / _---=> preempt-depth     
#                ||||| / _--=> preempt-lazy-depth
#                |||||| / _-=> migrate-disable   
#                ||||||| /     delay             
#  cmd     pid   |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       2dN..1..    0us : tick_nohz_idle_exit <-cpu_idle
  <idle>-0       2dN..1..    1us : menu_hrtimer_cancel <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    1us : ktime_get <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    1us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    2us : update_cpu_load_nohz <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    2us : _raw_spin_lock <-update_cpu_load_nohz
  <idle>-0       2dN..1..    3us : add_preempt_count <-_raw_spin_lock
  <idle>-0       2dN..2..    3us : __update_cpu_load <-update_cpu_load_nohz
  <idle>-0       2dN..2..    4us : sub_preempt_count <-update_cpu_load_nohz
  <idle>-0       2dN..1..    4us : calc_load_exit_idle <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    5us : touch_softlockup_watchdog <-tick_nohz_idle_exit
  <idle>-0       2dN..1..    5us : hrtimer_cancel <-tick_nohz_idle_exit

[...]

  <idle>-0       2dN..1..  521us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  521us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       2dN..1..  521us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  522us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  522us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  522us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       2dN..1..  522us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  523us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  523us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       2dN..1..  523us : tick_nohz_idle_exit <-cpu_idle
  <idle>-0       2dN..1..  524us+: trace_hardirqs_on <-cpu_idle
  <idle>-0       2dN..1..  537us : <stack trace>
 => tick_nohz_idle_exit
 => cpu_idle
 => start_secondary


By default, the irqsoff tracer enables function tracing to show what functions
are being called while interrupts were disabled. But as you can see, it
can produce a lot of output (the total line count of the above trace
was 1,327 lines. Most of that was cut to not waste space in this document).
The problem with the function tracer is that it incurs a substantial overhead
and exagerates the actual latency.

The reported latency above is 523 microseconds. The trace ends at 537
microseconds, but that's because it took 14 microseconds to produce the
stack trace.

The end of the trace does a stack dump to show where the latency occurred.
The above happened in tick_nohz_idle_exit(), and even though we can blame
the function tracer for exagerating the latency, this trace shows
that using NO HZ idle can have issues with a real time system. When a
system with NO HZ set is idle, the timer tick is stopped. When the system
resumes from idle, the timer must catch up to the current time and executes
all the ticks it missed in the loop. This is done with interrupts disabled.

Looking at the latency field "2dN..1.." you can see that this loop
ran on CPU 2, had interrupts disabled "d". The scheduler needed to run
"N" (for NEED_RESCHED). Preemption was disabled, as the preempt_count
counter was set to "1".

Ideally, when coming out of NO HZ, the accounting could be done in a single
step, but as that is tricky to get right, the current method is to just
run the current code in a loop as if the timer went off each time.


No function tracing
-------------------

As function tracing can exaggerate the latency, you can either
limit what functions are traced via the "set_ftrace_filter" and
"set_ftrace_notrace" files as described above in the function tracing
section. But you can also disable tracing totally via the tracing
option function-trace.


># echo 0 > /sys/kernel/debug/tracing/options/function-trace


This disables function tracing by all the ftrace tracers. Including
the function tracer, which would make it rather pointless because
the function tracer would act just like the "nop" tracer.

># echo 0 > options/function-trace
># echo 0 > tracing_max_latency
># echo irqsoff > current_tracer
># sleep 10
># cat trace
# tracer: irqsoff
#
# irqsoff latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 80 us, #4/4, CPU#6 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: swapper/6-0 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: cpu_idle
#  => ended at:   cpu_idle
#
#
#                  _--------=> CPU#              
#                 / _-------=> irqs-off          
#                | / _------=> need-resched      
#                || / _-----=> need-resched_lazy 
#                ||| / _----=> hardirq/softirq   
#                |||| / _---=> preempt-depth     
#                ||||| / _--=> preempt-lazy-depth
#                |||||| / _-=> migrate-disable   
#                ||||||| /     delay             
#  cmd     pid   |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       6dN..1..    0us+: tick_nohz_idle_exit <-cpu_idle
  <idle>-0       6dN..1..   81us : tick_nohz_idle_exit <-cpu_idle
  <idle>-0       6dN..1..   81us+: trace_hardirqs_on <-cpu_idle
  <idle>-0       6dN..1..   87us : <stack trace>
 => tick_nohz_idle_exit
 => cpu_idle
 => start_secondary


This time the latency is much more compact and accurate (80 microseconds
is still a lot, but much lower than 523). Here the backtrace is much more
important as its now the only real information to know where the latency
occurred.


preemptoff tracer
-----------------

There are points in the kernel that disables preemption but not interrupts.
That is, an interrupt can still interrupt the current process but that
process cannot be scheduled out for a higher priority process.

This tracer records the time that preemption is disabed via the
kernel internal "preempt_disable()" function.

># echo 0 > /sys/kernel/debug/tracing/options/function-trace
># echo 0 > tracing_max_latency
># echo preemptoff > current_tracer
># sleep 10
># cat trace
# tracer: preemptoff
#
# preemptoff latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 65 us, #4/4, CPU#6 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: swapper/6-0 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: cpuidle_enter
#  => ended at:   start_secondary
#
#
#                   _--------=> CPU#              
#                  / _-------=> irqs-off          
#                 | / _------=> need-resched      
#                 || / _-----=> need-resched_lazy 
#                 ||| / _----=> hardirq/softirq   
#                 |||| / _---=> preempt-depth     
#                 ||||| / _--=> preempt-lazy-depth
#                 |||||| / _-=> migrate-disable   
#                 ||||||| /     delay             
#  cmd     pid    |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       6d...1..    1us+: intel_idle <-cpuidle_enter
  <idle>-0       6.N..1..   65us : cpu_idle <-start_secondary
  <idle>-0       6.N..1..   66us+: trace_preempt_on <-start_secondary
  <idle>-0       6.N..1..   71us : <stack trace>
 => sub_preempt_count
 => cpu_idle
 => start_secondary


There's not much interesting in this trace except that preemption was
disabled for 65 microseconds.


preemptirqsoff tracer
---------------------

Knowing when interrupts are disabled or how long preemption is disabled
via the preempt_disable() kernel interface is not as interesting as
knowing how long true preemption is disabled. That is, if we have the
following scenario:

A) preempt_disable()

[...]

B) irqs_disable()

[...]

C) preempt_enable();

[...]

D) irqs_enable();


"irqsoff" tracer will give you the time from B to D
"preemptoff" tracer will give you the time from A to C.

But the current task cannot be preempted from A to D which is what we
really care about. When a task cannot be preempted, a new task can
no execute when it is woken up if it is to run on the same CPU as the
task that has true preemption disabled (either interrupts disabled or
preemption disabled). The "preemptirqsoff" tracer will handle this.

"preemptirqsoff" tracer will give you the time from A to D


># echo 1 > /sys/kernel/debug/tracing/options/function-trace
># echo 0 > tracing_max_latency
># echo preemptirqsoff > current_tracer
># sleep 10
># cat trace
# tracer: preemptirqsoff
#
# preemptirqsoff latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 377 us, #1289/1289, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: swapper/1-0 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: cpuidle_enter
#  => ended at:   start_secondary
#
#
#                  _--------=> CPU#              
#                 / _-------=> irqs-off          
#                | / _------=> need-resched      
#                || / _-----=> need-resched_lazy 
#                ||| / _----=> hardirq/softirq   
#                |||| / _---=> preempt-depth     
#                ||||| / _--=> preempt-lazy-depth
#                |||||| / _-=> migrate-disable   
#                ||||||| /     delay             
#  cmd     pid   |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       1d...1..    0us : intel_idle <-cpuidle_enter
  <idle>-0       1d...1..    1us : ktime_get <-cpuidle_wrap_enter
  <idle>-0       1d...1..    2us : smp_reschedule_interrupt <-reschedule_interrupt
  <idle>-0       1d...1..    3us : scheduler_ipi <-smp_reschedule_interrupt
  <idle>-0       1d...1..    3us : irq_enter <-scheduler_ipi
  <idle>-0       1d...1..    4us : rcu_irq_enter <-irq_enter
  <idle>-0       1d...1..    4us : rcu_eqs_exit_common.isra.45 <-rcu_irq_enter
  <idle>-0       1d...1..    5us : tick_check_idle <-irq_enter
  <idle>-0       1d...1..    5us : tick_check_oneshot_broadcast <-tick_check_idle
  <idle>-0       1d...1..    5us : ktime_get <-tick_check_idle
  <idle>-0       1d...1..    6us : tick_nohz_stop_idle <-tick_check_idle
  <idle>-0       1d...1..    6us : update_ts_time_stats <-tick_nohz_stop_idle
  <idle>-0       1d...1..    7us : nr_iowait_cpu <-update_ts_time_stats
  <idle>-0       1d...1..    7us : touch_softlockup_watchdog <-sched_clock_idle_wakeup_event
  <idle>-0       1d...1..    7us : tick_do_update_jiffies64 <-tick_check_idle
  <idle>-0       1d...1..    8us : touch_softlockup_watchdog <-tick_check_idle
  <idle>-0       1d...1..    8us : irqtime_account_irq <-irq_enter
  <idle>-0       1d...1..    9us : in_serving_softirq <-irqtime_account_irq
  <idle>-0       1d...1..    9us : add_preempt_count <-irq_enter
  <idle>-0       1d..h1..    9us : sched_ttwu_pending <-scheduler_ipi
  <idle>-0       1d..h1..   10us : _raw_spin_lock <-sched_ttwu_pending
  <idle>-0       1d..h1..   10us : add_preempt_count <-_raw_spin_lock
  <idle>-0       1d..h2..   11us : sub_preempt_count <-sched_ttwu_pending
  <idle>-0       1d..h1..   11us : raise_softirq_irqoff <-scheduler_ipi
  <idle>-0       1d..h1..   12us : do_raise_softirq_irqoff <-raise_softirq_irqoff
  <idle>-0       1d..h1..   12us : irq_exit <-scheduler_ipi
  <idle>-0       1d..h1..   12us : irqtime_account_irq <-irq_exit
  <idle>-0       1d..h1..   13us : sub_preempt_count <-irq_exit
  <idle>-0       1d...2..   13us : wakeup_softirqd <-irq_exit
  <idle>-0       1d...2..   14us : wake_up_process <-wakeup_softirqd
  <idle>-0       1d...2..   14us : try_to_wake_up <-wake_up_process

[...]

  <idle>-0       1d...4..   18us : dequeue_rt_stack <-enqueue_task_rt
  <idle>-0       1d...4..   19us : cpupri_set <-enqueue_task_rt
  <idle>-0       1d...4..   20us : update_rt_migration <-enqueue_task_rt
  <idle>-0       1d...4..   20us : ttwu_do_wakeup <-ttwu_do_activate.constprop.90
  <idle>-0       1d...4..   20us : check_preempt_curr <-ttwu_do_wakeup
  <idle>-0       1d...4..   21us : resched_task <-check_preempt_curr
  <idle>-0       1dN..4..   21us : task_woken_rt <-ttwu_do_wakeup
  <idle>-0       1dN..4..   22us : sub_preempt_count <-try_to_wake_up
  <idle>-0       1dN..3..   22us : ttwu_stat <-try_to_wake_up
  <idle>-0       1dN..3..   23us : _raw_spin_unlock_irqrestore <-try_to_wake_up
  <idle>-0       1dN..3..   23us : sub_preempt_count <-_raw_spin_unlock_irqrestore

[...]

  <idle>-0       1dN..1..  376us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       1dN..1..  376us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       1dN..1..  376us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       1dN..1..  377us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       1dN..1..  377us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       1dN..1..  377us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       1dN..1..  377us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       1.N..1..  378us : cpu_idle <-start_secondary
  <idle>-0       1.N..1..  378us+: trace_preempt_on <-start_secondary
  <idle>-0       1.N..1..  391us : <stack trace>
 => sub_preempt_count
 => cpu_idle
 => start_secondary

The above is a much more interesting trace. Although we enabled function
tracing again, it allows us to see more of what is happening during
the trace.

The trace starts out at intel_idle() which on the box the trace was run on
is the idle function. Idle function usually disable preemption and
sometimes interrupts when the system is put to sleep, although an
interrupt will wake up the processor, the interrupt will not be serviced
until the processor re-enables interrupts again.

As interrupts and preemption is disabled across a full idle, the tracer
must account for this, as it is pretty useless to trace how long the
CPU has been idle. Thus, immediately exiting the idle state, the
latency tracers are re-enabled. This is where the start of the trace
occurred.

Then we can see that an interrupt is triggered after interrupts were
enabled (schedule_ipi). An interprocessor interrupt happened to wake up
a process that is on the current CPU.

Next the irq_enter() is called. This tells the system (including the
tracing system) that the kernel is now int interrupt mode. Notice that
'h' is not set until after "add_preempt_count" is called. That's because
the irq accounting is shared with the preempt_count code. A lot has happened
before that got set, as NO HZ and RCU must perform activities immediately
when coming out of idle via an interrupt.

A softirq was raised while in the interrupt and as the Red Hat Enterprise Linux for Real Time kernel runs
soft interrupts as threads, the corresponding softirq was woken up
on exiting the interrupt (irq_exit).

This wakeup also triggered the NEED_RESCHED flag "N" to be set, to let
the system know that the kernel needs to call schedule as soon as
preemption is re-enabled.

Finally the NO HZ accounting ran again with interrupts and preemption
disabled. Finally, interrupts were enabled and so was the preemption.



wakeup tracer
-------------

The previous tracers ("irqsoff", "preemptoff", and "preemptirqsoff")
were single CPU tracers. That is, they only reported the activities
on a single CPU, as interrupts only occurred there.

Both "wakeup" and "wakeup_rt" tracers are full CPU tracers. That is,
they report the activities of what happens across all CPUs. This is
because a task may be woken from one CPU but get scheduled on another
CPU.

The "wakeup" tracer is not that interresting from a real-time perspective,
as it records the time it takes to wake up the highest priority task
in the system even if that task does not happen to be a real time task.
Non real-time tasks may be delayed due scheduling balacing, and not
immediately scheduled for throughput reasons. Real-time tasks are scheduled
immediately after they are woken. Recording the max time it takes to
wake up a non real-time task will hide the times it takes to wake up
a real-time task. Because of this, we will focus on the "wakeup_rt" tracer
instead.


wakeup_rt tracer
----------------

The "wakeup" tracer records the time it takes from the current highest
priority task to wake up to the time it is scheduled. Because non real-time
tasks may take much longer to wake up than a real-time task, and that
the latency tracers only record the longest time, "wakeup" tracer is not
that suitable for seeing how long a real-time task takes to be scheduled
from the time it is woken. For that, we use the "wakeup_rt" tracer.

The "wakeup_rt" tracer only records the time for real-time tasks and
ignores the time for non real-time tasks.

># echo 0 > tracing_max_latency
># echo preemptirqsoff > current_tracer
># sleep 10
># cat trace
# tracer: wakeup_rt
#
# wakeup_rt latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 385 us, #1339/1339, CPU#7 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: ksoftirqd/7-51 (uid:0 nice:0 policy:1 rt_prio:1)
#    -----------------
#
#                  _--------=> CPU#              
#                 / _-------=> irqs-off          
#                | / _------=> need-resched      
#                || / _-----=> need-resched_lazy 
#                ||| / _----=> hardirq/softirq   
#                |||| / _---=> preempt-depth     
#                ||||| / _--=> preempt-lazy-depth
#                |||||| / _-=> migrate-disable   
#                ||||||| /     delay             
#  cmd     pid   |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       7d...5..    0us :      0:120:R   + [007]    51: 98:R ksoftirqd/7
  <idle>-0       7d...5..    2us : ttwu_do_activate.constprop.90 <-try_to_wake_up
  <idle>-0       7d...4..    2us : check_preempt_curr <-ttwu_do_wakeup
  <idle>-0       7d...4..    3us : resched_task <-check_preempt_curr
  <idle>-0       7dN..4..    3us : task_woken_rt <-ttwu_do_wakeup
  <idle>-0       7dN..4..    4us : sub_preempt_count <-try_to_wake_up
  <idle>-0       7dN..3..    4us : ttwu_stat <-try_to_wake_up
  <idle>-0       7dN..3..    4us : _raw_spin_unlock_irqrestore <-try_to_wake_up
  <idle>-0       7dN..3..    5us : sub_preempt_count <-_raw_spin_unlock_irqrestore
  <idle>-0       7dN..2..    5us : idle_cpu <-irq_exit
  <idle>-0       7dN..2..    5us : rcu_irq_exit <-irq_exit
  <idle>-0       7dN..2..    6us : rcu_eqs_enter_common.isra.47 <-rcu_irq_exit

[...]

  <idle>-0       7dN..1..   53us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   53us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   54us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   54us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       7dN..1..   54us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   54us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   55us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   55us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       7dN..1..   55us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   55us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   56us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   56us : irqtime_account_process_tick.isra.2 <-account_idle_ticks
  <idle>-0       7dN..1..   56us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   56us : nsecs_to_jiffies64 <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   57us : account_idle_time <-irqtime_account_process_tick.isra.2
  <idle>-0       7dN..1..   57us : irqtime_account_process_tick.isra.2 <-account_idle_ticks

[...]

  <idle>-0       7dN.h1..  377us : tick_program_event <-hrtimer_interrupt
  <idle>-0       7dN.h1..  378us : clockevents_program_event <-tick_program_event
  <idle>-0       7dN.h1..  378us : ktime_get <-clockevents_program_event
  <idle>-0       7dN.h1..  378us : lapic_next_deadline <-clockevents_program_event
  <idle>-0       7dN.h1..  379us : irq_exit <-smp_apic_timer_interrupt
  <idle>-0       7dN.h1..  379us : irqtime_account_irq <-irq_exit
  <idle>-0       7dN.h1..  379us : sub_preempt_count <-irq_exit
  <idle>-0       7dN..2..  379us : wakeup_softirqd <-irq_exit
  <idle>-0       7dN..2..  380us : idle_cpu <-irq_exit
  <idle>-0       7dN..2..  380us : rcu_irq_exit <-irq_exit
  <idle>-0       7dN..2..  380us : sub_preempt_count <-irq_exit
  <idle>-0       7.N..1..  381us : sub_preempt_count <-cpu_idle
  <idle>-0       7.N.....  381us : __schedule <-preempt_schedule
  <idle>-0       7.N.....  382us : add_preempt_count <-__schedule
  <idle>-0       7.N..1..  382us : rcu_note_context_switch <-__schedule
  <idle>-0       7.N..1..  382us : _raw_spin_lock_irq <-__schedule
  <idle>-0       7dN..1..  382us : add_preempt_count <-_raw_spin_lock_irq
  <idle>-0       7dN..2..  383us : update_rq_clock <-__schedule
  <idle>-0       7dN..2..  383us : put_prev_task_idle <-__schedule
  <idle>-0       7dN..2..  383us : pick_next_task_stop <-__schedule
  <idle>-0       7dN..2..  384us : pick_next_task_rt <-__schedule
  <idle>-0       7dN..2..  384us : dequeue_pushable_task <-pick_next_task_rt
  <idle>-0       7d...3..  385us : __schedule <-preempt_schedule
  <idle>-0       7d...3..  385us :      0:120:R ==> [007]    51: 98:R ksoftirqd/7


And once again we can see that NO HZ affects the wake up time of a
real time task (this case it was ksoftirqd).

Notice the first traced item:

       0:120:R   + [007]    51: 98:R ksoftirqd/7

This is in the format of:

  <pid>:<prio>:<process-state>    + [<CPU#>]   <pid>:<prio>:<process-state>

The first pid, prio and process-state is for the task performing the
wake up. Again, the prio is the internal kernel prio, where 120 is for
SCHED_OTHER. The "+" represents a wake up is happening. The CPU# the
CPU waking task in currently assigned to (and being woken up on).
The second set of pid, prio and process-state is for the task being
woken up. The prio of 98 is internal to the kernel, and to get the real
real-time priority for the task you must subtract it from 99.
(99 - 98 = real-time priority of 1 - low priority)

The process-state should be always in the "R" (running) state, and
can be ignored. The original location to record the trace when waking
up was before the task was actually woken. Due to changes in the wake
up code, the trace hook had to be moved to after the wake up, which
means the task being woken up will have already been set to running
and the trace will reflect that.

The last line of the trace:

      0:120:R ==> [007]    51: 98:R ksoftirqd/7

Represents the scheduling of a task.

  <pid>:<prio>:<process-state> ==> [CPU#] <pid>:<prio><process-state>

The first set of pid, prio and process-state belongs to the task that
is being scheduled out. The second set is for the task that is being
scheduled in. The "==>" represents a task scheduling switch, and the
CPU# should always match the current CPU that is on (7 in this case).

The first process-state here is of more importance than that of the
wake up trace. If the previous task is in the running state (as it is
in this case), that means it has been preempted (still wants to run
but must yield for the new task).



Using events in tracers
-----------------------

With the "wakeup_rt" tracer, as with all tracers, function tracing can
exaggerate the latency times. But disabling the function tracing for
"wakeup_rt" is not very useful.

># echo 0 > /sys/kernel/debug/tracing/options/function-trace
># echo 0 > tracing_max_latency
># echo wakeup_rt > current_tracer
># sleep 10
># cat trace
# tracer: wakeup_rt
#
# wakeup_rt latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 64 us, #18446744073709512109/18446744073709512109, CPU#5 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: irq/43-em1-878 (uid:0 nice:0 policy:1 rt_prio:50)
#    -----------------
#
#                  _--------=> CPU#              
#                 / _-------=> irqs-off          
#                | / _------=> need-resched      
#                || / _-----=> need-resched_lazy 
#                ||| / _----=> hardirq/softirq   
#                |||| / _---=> preempt-depth     
#                ||||| / _--=> preempt-lazy-depth
#                |||||| / _-=> migrate-disable   
#                ||||||| /     delay             
#  cmd     pid   |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       0d..h4..    0us :      0:120:R   + [005]   878: 49:R irq/43-em1
  <idle>-0       0d..h4..    2us+: ttwu_do_activate.constprop.90 <-try_to_wake_up
  <idle>-0       5d...3..   63us : __schedule <-preempt_schedule
  <idle>-0       5d...3..   64us :      0:120:R ==> [005]   878: 49:R irq/43-em1

The irq thread was woken up by a task on CPU 0, and it scheduled on
CPU 5.


As function tracing causes a large overhead, with the wakeup tracers, you
can still get information by using events, and events are sparse enough
to not cause much overhead even when enabled.

># echo 0 > /sys/kernel/debug/tracing/options/function-trace
># echo 1 > events/enable
># echo 0 > tracing_max_latency
># echo wakeup_rt > current_tracer
># sleep 10
># cat trace
# tracer: wakeup_rt
#
# wakeup_rt latency trace v1.1.5 on 3.8.13-test-mrg-rt9+
# --------------------------------------------------------------------
# latency: 67 us, #15/15, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
#    -----------------
#    | task: irq/43-em1-878 (uid:0 nice:0 policy:1 rt_prio:50)
#    -----------------
#
#                   _--------=> CPU#              
#                  / _-------=> irqs-off          
#                 | / _------=> need-resched      
#                 || / _-----=> need-resched_lazy 
#                 ||| / _----=> hardirq/softirq   
#                 |||| / _---=> preempt-depth     
#                 ||||| / _--=> preempt-lazy-depth
#                 |||||| / _-=> migrate-disable   
#                 ||||||| /     delay             
#  cmd     pid    |||||||| time  |   caller       
#     \   /      ||||||||  \   |   /            
  <idle>-0       0d..h4..    0us :      0:120:R   + [001]   878: 49:R irq/43-em1
  <idle>-0       0d..h4..    1us : ttwu_do_activate.constprop.90 <-try_to_wake_up
  <idle>-0       0d..h4..    1us+: sched_wakeup: comm=irq/43-em1 pid=878 prio=49 success=1 target_cpu=001
  <idle>-0       0....2..    5us : power_end: cpu_id=0
  <idle>-0       0....2..    6us+: cpu_idle: state=4294967295 cpu_id=0
  <idle>-0       0d...2..    9us : power_start: type=1 state=3 cpu_id=0
  <idle>-0       0d...2..   10us+: cpu_idle: state=3 cpu_id=0
  <idle>-0       1.N..2..   25us+: power_end: cpu_id=1
  <idle>-0       1.N..2..   27us+: cpu_idle: state=4294967295 cpu_id=1
  <idle>-0       1dN..3..   30us : hrtimer_cancel: hrtimer=ffff88011ea4cf40
  <idle>-0       1dN..3..   31us+: hrtimer_start: hrtimer=ffff88011ea4cf40 function=tick_sched_timer expires=9670689000000 softexpires=9670689000000
  <idle>-0       1.N..2..   64us : rcu_utilization: Start context switch
  <idle>-0       1.N..2..   65us+: rcu_utilization: End context switch
  <idle>-0       1d...3..   66us : __schedule <-preempt_schedule
  <idle>-0       1d...3..   67us :      0:120:R ==> [001]   878: 49:R irq/43-em1


The above trace is much more accurate to a real latency, but this time
we get a lot more information. The task being woken up in on CPU 1, and
the first time we see CPU 1 is at the 25 microsecond time. The "power_end"
trace point shows that the CPU is coming out of a deep power state, which
explains why the time took so long. The high resolution timer has been
reinitialized, and we can assume from our other traces that the NO HZ
code is running again to catch up on the tick, although no trace points
currently represent that. This process took 33 microseconds, where we
see RCU handling a context switch, and eventually the schedule takes place.



function_graph
--------------

The "function" tracer is extremely informative, albeit invasive, but
it is a bit difficult for a human to read.

          <idle>-0     [000] ....1.. 10698.878897: sub_preempt_count <-__schedule
            less-3062  [006] ....... 10698.878897: add_preempt_count <-migrate_disable
             cat-3061  [007] d...... 10698.878897: add_preempt_count <-_raw_spin_lock
          <idle>-0     [000] ....... 10698.878897: add_preempt_count <-cpu_idle
            less-3062  [006] ....11. 10698.878897: pin_current_cpu <-migrate_disable
          <idle>-0     [000] ....1.. 10698.878898: tick_nohz_idle_enter <-cpu_idle
             cat-3061  [007] d...1.. 10698.878898: sub_preempt_count <-__raw_spin_unlock
            less-3062  [006] ....111 10698.878898: sub_preempt_count <-migrate_disable
          <idle>-0     [000] ....1.. 10698.878898: set_cpu_sd_state_idle <-tick_nohz_idle_enter
             cat-3061  [007] ....... 10698.878898: free_delayed <-__slab_alloc.isra.60
            less-3062  [006] .....11 10698.878898: migrate_disable <-get_page_from_freelist
            less-3062  [006] .....11 10698.878898: add_preempt_count <-migrate_disable
          <idle>-0     [000] d...1.. 10698.878898: __tick_nohz_idle_enter <-tick_nohz_idle_enter
            less-3062  [006] ....112 10698.878898: sub_preempt_count <-migrate_disable
          <idle>-0     [000] d...1.. 10698.878898: ktime_get <-__tick_nohz_idle_enter
             cat-3061  [007] ....... 10698.878898: __rt_mutex_init <-tracing_open


The "function_graph" tracer is a bit more easy on the eyes, and lets
the developer follow the code in much more detail.

># echo function_graph > current_tracer
># cat trace
# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 5)   0.125 us    |            source_load();
 5)   0.137 us    |            idle_cpu();
 5)   0.105 us    |            source_load();
 5)   0.110 us    |            idle_cpu();
 5)   0.132 us    |            source_load();
 5)   0.134 us    |            idle_cpu();
 5)   0.127 us    |            source_load();
 5)   0.144 us    |            idle_cpu();
 5)   0.132 us    |            source_load();
 5)   0.112 us    |            idle_cpu();
 5)   0.120 us    |            source_load();
 5)   0.130 us    |            idle_cpu();
 5) + 20.812 us   |          } /* find_busiest_group */
 5) + 21.905 us   |        } /* load_balance */
 5)   0.099 us    |        msecs_to_jiffies();
 5)   0.120 us    |        __rcu_read_unlock();
 5)               |        _raw_spin_lock() {
 5)   0.115 us    |          add_preempt_count();
 5)   1.115 us    |        }
 5) + 46.645 us   |      } /* idle_balance */
 5)               |      put_prev_task_rt() {
 5)               |        update_curr_rt() {
 5)               |          cpuacct_charge() {
 5)   0.110 us    |            __rcu_read_lock();
 5)   0.110 us    |            __rcu_read_unlock();
 5)   2.111 us    |          }
 5)   0.100 us    |          sched_avg_update();
 5)               |          _raw_spin_lock() {
 5)   0.116 us    |            add_preempt_count();
 5)   1.151 us    |          }
 5)   0.122 us    |          balance_runtime();
 5)   0.110 us    |          sub_preempt_count();
 5)   8.165 us    |        }
 5)   9.152 us    |      }
 5)   0.148 us    |      pick_next_task_fair();
 5)   0.112 us    |      pick_next_task_stop();
 5)   0.117 us    |      pick_next_task_rt();
 5)   0.123 us    |      pick_next_task_fair();
 5)   0.138 us    |      pick_next_task_idle();
 ------------------------------------------
 5)   ksoftir-39   =>    <idle>-0   
 ------------------------------------------

 5)               |      finish_task_switch() {
 5)               |        _raw_spin_unlock_irq() {
 5)   0.260 us    |          sub_preempt_count();
 5)   1.289 us    |        }
 5)   2.309 us    |      }
 5)   0.132 us    |      sub_preempt_count();
 5) ! 151.784 us  |    } /* __schedule */
 5)   0.272 us    |  } /* sub_preempt_count */


The "function" tracer only traces the start of the function where as the
"function_graph" tracer also traces the exit of the function, allowing
to show a flow of function calls in the kernel. As one function calls
the next function, it is indented in the trace and C code curly brackets
are placed around them. When there's a leaf function (a function that
does not call any other function, or any function that happens to be
traced), it is simply finished with a ";".

This tracer has a different format than the other tracers, to help
ease the reading of the trace. The first number "5)" represents the
CPU that the trace happened on. The second number is the time the
function took to execute. Note, this time also include the overhead
of the "function_graph" tracer itself, so for functions that have
several other functions traced within it, its time will be rather
exaggerated. For leaf functions, the time is rather accurate.

When a schedule switch is detected (does not require the sched_switch
event enabled, as all traces record the pid), it shows up as separately
displayed.

 ------------------------------------------
 5)   ksoftir-39   =>    <idle>-0   
 ------------------------------------------

The name is cropped to 7 characters (from "ksoftirqd" to "ksoftir").


Follow a function
-----------------

Because the "function_graph" tracer records both the start and exit
of a function, several more features are possible. One of these features
is to graph only a specific function. That is, to see what a specific
function calls and ignore all other functions.

For example, if you are interested in what the sys_read() function
calls, you can use the "set_graph_function" file in the tracing
debug file system.

># echo sys_read > set_graph_function
># echo function_graph > current_tracer
># sleep 10
># cat trace
# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 0)               |  sys_read() {
 0)   0.126 us    |    fget_light();
 0)               |    vfs_read() {
 0)               |      rw_verify_area() {
 0)               |        security_file_permission() {
 0)   0.077 us    |          cap_file_permission();
 0)   0.076 us    |          __fsnotify_parent();
 0)   0.100 us    |          fsnotify();
 0)   2.001 us    |        }
 0)   2.608 us    |      }
 0)               |      tty_read() {
 0)   0.070 us    |        tty_paranoia_check();
 0)               |        tty_ldisc_ref_wait() {
 0)               |          tty_ldisc_try() {
 0)               |            _raw_spin_lock_irqsave() {
 0)   0.130 us    |              add_preempt_count();
 0)   0.759 us    |            }
 0)               |            _raw_spin_unlock_irqrestore() {
 0)   0.132 us    |              sub_preempt_count();
 0)   0.774 us    |            }
 0)   2.576 us    |          }
 0)   3.161 us    |        }
 0)               |        n_tty_read() {
 0)               |          _mutex_lock_interruptible() {
 0)   0.087 us    |            rt_mutex_lock_interruptible();
 0)   0.694 us    |          }
 0)               |          add_wait_queue() {
 0)               |            migrate_disable() {
 0)   0.100 us    |              add_preempt_count();
 0)   0.073 us    |              pin_current_cpu();
 0)   0.085 us    |              sub_preempt_count();
 0)   1.829 us    |            }
 0)   0.060 us    |            rt_spin_lock();
 0)   0.065 us    |            rt_spin_unlock();
 0)               |            migrate_enable() {
 0)   0.077 us    |              add_preempt_count();
 0)   0.070 us    |              unpin_current_cpu();
 0)   0.077 us    |              sub_preempt_count();
 0)   1.847 us    |            }
 0)   5.899 us    |          }


The above shows the flow of functions called by sys_read().


To reset the "set_graph_function" simply write into that file like
the "set_ftrace_filter" file is done.

># echo > set_graph_function


Time a function
---------------

As the "function_graph" tracer is associated to the "function" tracer
it is also affected by the "set_ftrace_filter", "set_ftrace_notrace"
as well as the sysctl feature "kernel.ftrace_enabled".

As mentioned previously, only the leaf functions contain the most accurate
times of execution. By filtering on a specific function, you can see
the time it takes to execute a single function.


># echo do_IRQ > set_ftrace_filter
># echo function_graph > current_tracer
># sleep 10
># cat trace
# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 4)   ==========> |
 4)   6.486 us    |  do_IRQ();
 0)   ==========> |
 0)   3.801 us    |  do_IRQ();
 4)   ==========> |
 4)   3.221 us    |  do_IRQ();
 0)   ==========> |
 0) + 11.153 us   |  do_IRQ();
 0)   ==========> |
 0) + 10.968 us   |  do_IRQ();
 6)   ==========> |
 6)   9.280 us    |  do_IRQ();
 0)   ==========> |
 0)   9.467 us    |  do_IRQ();
 0)   ==========> |
 0) + 11.238 us   |  do_IRQ();


The "==========>" show when an interrupt entered. The "<==========" is
missing because it is associated with the exit part of the trace.
As "do_IRQ" is a leaf function here, the exit arrow was folded into
the function and does not appear in the trace.


Events in function graph tracer
-------------------------------

As explained previously, events can be enabled with all tracers.
But with the "function_graph" tracer, they are displayed a little
differently.

># echo 1 > events/irq/enable
># echo do_IRQ > set_ftrace_filter
># echo function_graph > current_tracer
># sleep 10
># cat trace
# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 5)   ==========> |
 5)               |  do_IRQ() {
 5)               |  /* irq_handler_entry: irq=43 name=em1 */
 5)               |  /* irq_handler_exit: irq=43 ret=handled */
 5) + 15.721 us   |  }
 5)   <========== |
 3)               |  /* softirq_raise: vec=3 [action=NET_RX] */
 3)               |  /* softirq_entry: vec=3 [action=NET_RX] */
 3)               |  /* softirq_exit: vec=3 [action=NET_RX] */
 0)   ==========> |
 0)               |  do_IRQ() {
 0)               |  /* irq_handler_entry: irq=43 name=em1 */
 0)               |  /* irq_handler_exit: irq=43 ret=handled */
 0)   8.915 us    |  }
 0)   <========== |
 3)               |  /* softirq_raise: vec=3 [action=NET_RX] */
 3)               |  /* softirq_entry: vec=3 [action=NET_RX] */
 3)               |  /* softirq_exit: vec=3 [action=NET_RX] */
 0)               |  /* softirq_raise: vec=1 [action=TIMER] */
 0)               |  /* softirq_raise: vec=9 [action=RCU] */
 ------------------------------------------
 0)    <idle>-0    =>   ksoftir-3   
 ------------------------------------------

 0)               |  /* softirq_entry: vec=1 [action=TIMER] */
 0)               |  /* softirq_exit: vec=1 [action=TIMER] */
 0)               |  /* softirq_entry: vec=9 [action=RCU] */
 0)               |  /* softirq_exit: vec=9 [action=RCU] */
 ------------------------------------------
 0)   ksoftir-3    =>    <idle>-0   
 ------------------------------------------

Keeping with the C formatting, events in the "function_graph" tracer
appear as comments. Recording the interrupt events gives more detail
to what interrupts are occurring when "do_IRQ()" is called. As the
"do_IRQ()" exit trace is not folded, the "<==========" appears to
display that the interrupt is over.


Annotations
-----------

In the traces, including the "function_graph" tracer, you may see
annotations around the times. "+" and "!". A "+" appears when the
time between events is greater than 10 microseconds, and a "!" appears
when that time is greater than 100 microseconds. You can see this in the
above tracers:

  <idle>-0       0d..h4..    2us+: ttwu_do_activate.constprop.90 <-try_to_wake_up
  <idle>-0       5d...3..   63us : __schedule <-preempt_schedule


 5) + 20.812 us   |          } /* find_busiest_group */
 5) + 21.905 us   |        } /* load_balance */

 5) ! 151.784 us  |    } /* __schedule */



Buffer size
-----------

When tracing functions, you will almost always use events. This is because
the amount of functions being traced will quickly fill the ring buffer
faster than anything can read from it. The amount lost can be minimized
with filtering the trace as well as increasing the size of the buffer.

The size of the buffer is controlled by the "buffer_size_kb" file.
As the name suggests, the size is in kilobytes. When you first boot up,
as tracing is used by only a small minority of users, the trace buffer
is compressed. The first time you use any of the tracing features,
the tracing buffer will automatically increase to a decent size.

># cat buffer_size_kb 
7 (expanded: 1408)

Note, for efficiency reasons, the buffer is split into multiple buffers
per CPU. The size displayed by "buffer_size_kb" is the size of each
CPU buffer. To see the total size of all buffers look at
"buffer_total_size_kb"

># cat buffer_total_size_kb
56 (expanded: 11264)

After running any trace, the buffer will expand to the size that is
denoted by the "expanded" value.

># echo 1 > events/enable
># cat buffer_size_kb 
1408

To change the size of the buffer, simply echo in a number.

># echo 10000 > buffer_size_kb
># cat buffer_size_kb
10000

Note, if you change the size before using any tracer, the buffers
will go to that size, and the expanded value will then be ignored.


Buffer size per CPU
-------------------

If there's a case you care about activity on one CPU more than another
CPU, and you need to save memory, you can change the sizes of the
ring buffers per CPU. These files exist in a "per_cpu/cpuX/" directory.

># cat per_cpu/cpu1/buffer_size_kb
10000

># echo 100 > per_cpu/cpu1/buffer_size_kb
># cat per_cpu/cpu1/buffer_size_kb
100

When the per CPU buffers differ in size, the top level buffer_size_kb
will display an "X".

># cat buffer_size_kb
X

But the total size will still display the amount allocated.

># cat buffer_total_size_kb 
70100


Trace Marker
------------

It is sometimes useful to synchronize actions in userspace with events
within the kernel. The "trace_marker" allows userspace to write into
the ftrace buffer.

># echo hello world > trace_marker 
># cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 1/1   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
            bash-1086  [001] .....11 21351.346541: tracing_mark_write: hello world

Writing into the kernel is very light weight. User programs can take
advantage of this with the following C code:

        static int trace_fd = -1;

        void trace_write(const char *fmt, ...)
        {
                va_list ap;
                char buf[256];
                int n;

                if (trace_fd < 0)
                        return;

                va_start(ap, fmt);
                n = vsnprintf(buf, 256, fmt, ap);
                va_end(ap);

                write(trace_fd, buf, n);
        }


        [...]

        trace_fd = open("trace_marker", WR_ONLY);


and later use the "trace_write()" function to record into the ftrace
buffer.

        trace_write("record this event\n");


tracer options
--------------

There are several options that can affect the formating of the trace
output as well as how the tracers behave. Some trace options only exist
for a given tracer and their control file appears only when the tracer
is activated.

The trace option control files exist in the "options" directory.

># ls options
annotate         graph-time       print-parent  sym-userobj
bin              hex              raw           test_nop_accept
block            irq-info         record-cmd    test_nop_refuse
branch           latency-format   sleep-time    trace_printk
context-info     markers          stacktrace    userstacktrace
disable_on_free  overwrite        sym-addr      verbose
ftrace_preempt   printk-msg-only  sym-offset

The "function_graph" tracer adds several of its own.

># echo function_graph > current_tracer
># ls options
annotate           funcgraph-cpu       irq-info         sleep-time
bin                funcgraph-duration  latency-format   stacktrace
block              funcgraph-irqs      markers          sym-addr
branch             funcgraph-overhead  overwrite        sym-offset
context-info       funcgraph-overrun   printk-msg-only  sym-userobj
disable_on_free    funcgraph-proc      print-parent     trace_printk
ftrace_preempt     graph-time          raw              userstacktrace
funcgraph-abstime  hex                 record-cmd       verbose



  annotate - It is sometimes confusing when the CPU buffers are full
               and one CPU buffer had a lot of events recently, thus
             a shorter time frame, were another CPU may have only had
             a few events, which lets it have older events. When
             the trace is reported, it shows the oldest events first,
             and it may look like only one CPU ran (the one with the
             oldest events). When the annotate option is set, it will
             display when a new CPU buffer started:

          <idle>-0     [005] d...1..   910.328077: cpuidle_wrap_enter <-cpuidle_enter_tk
          <idle>-0     [005] d...1..   910.328077: ktime_get <-cpuidle_wrap_enter
          <idle>-0     [005] d...1..   910.328078: intel_idle <-cpuidle_enter
          <idle>-0     [005] d...1..   910.328078: leave_mm <-intel_idle
##### CPU 7 buffer started ####
          <idle>-0     [007] d...1..   910.360866: tick_do_update_jiffies64 <-tick_check_idle
          <idle>-0     [007] d...1..   910.360866: _raw_spin_lock <-tick_do_update_jiffies64
          <idle>-0     [007] d...1..   910.360866: add_preempt_count <-_raw_spin_lock


  bin - This will print out the formats in raw binary.

  block - When set, reading trace_pipe will not block when polled.

  context-info - Show only the event data. Hides the comm, PID,
                   timestamp, CPU, and other useful data.

  disable_on_free - When the free_buffer is closed, tracing will
                      stop (tracing_on set to 0).

  ftrace_preempt - Normally the function tracer disables interrupts as
                   the recursion protection will hide interrupts from being
                 traced if the interrupt happened while another function
                 was being traced. If this option is enabled, then it
                 will not disable interrupts but will only disable
                 preemption. But note, if an interrupt were to arrive
                 when another function is being traced, all functions
                 within that interrupt will not be traced, as function
                 tracing is temporarily disablde for recursion protection.

  graph-time - When running function graph tracer, to include the
                 time to call nested functions. When this is not set,
               the time reported for the function will only include
               the time the function itself executed for, not the time
               for functions that it called.

  hex - Similar to raw, but the numbers will be in a hexadecimal
        format.

  irq-info - Shows the interrupt, preempt count, need resched data.
               When disabled, the trace looks like:

# tracer: function
#
# entries-in-buffer/entries-written: 319494/4972382   #P:8
#
#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
#              | |       |          |         |
          <idle>-0     [004]    983.062800: lock_hrtimer_base.isra.25 <-__hrtimer_start_range_ns
          <idle>-0     [004]    983.062801: _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.25
          <idle>-0     [004]    983.062801: add_preempt_count <-_raw_spin_lock_irqsave
          <idle>-0     [004]    983.062801: __remove_hrtimer <-__hrtimer_start_range_ns
          <idle>-0     [004]    983.062801: hrtimer_force_reprogram <-__remove_hrtimer


  latency-format - This option changes the trace. When
                   it is enabled, the trace displays
                   additional information about the
                   latencies, as described in "Latency
                   trace format".

  markers - When set, the trace_marker is writable (only by root).
              When disabled, the trace_marker will error with EINVAL
            on write.

  overwrite - This controls what happens when the trace buffer is
              full. If "1" (default), the oldest events are
              discarded and overwritten. If "0", then the newest
              events are discarded.
                (see per_cpu/cpu0/stats for overrun and dropped)

  printk-msg-only - When set, trace_printk()s will only show the format
                      and not their parameters (if trace_bprintk() or
                    trace_bputs() was used to save the trace_printk()).

  print-parent - On function traces, display the calling (parent)
                 function as well as the function being traced.

    print-parent:
       bash-1423  [006]   1755.774709: msecs_to_jiffies <-idle_balance

    noprint-parent:
       bash-1423  [006]   1755.774709: msecs_to_jiffies


  raw - This will display raw numbers. This option is best for
        use with user applications that can translate the raw
        numbers better than having it done in the kernel.

  record-cmd - When any event or tracer is enabled, a hook is enabled
                 in the sched_switch trace point to fill comm cache
               with mapped pids and comms. But this may cause some
               overhead, and if you only care about pids, and not the
               name of the task, disabling this option can lower the
               impact of tracing.

  sleep-time - When running function graph tracer, to include
                 the time a task schedules out in its function.
               When enabled, it will account time the task has been
               scheduled out as part of the function call.

  stacktrace - This is one of the options that changes the trace
               itself. When a trace is recorded, so is the stack
               of functions. This allows for back traces of
               trace sites.

  sym-addr - this will also display the function address as well
             as the function name.

  sym-offset - Display not only the function name, but also the
               offset in the function. For example, instead of
               seeing just "ktime_get", you will see
               "ktime_get+0xb/0x20".

    sym-offset:
      bash-1423  [006]   1755.774709: msecs_to_jiffies+0x0/0x20

    sym-addr:
      bash-1423  [006]   1755.774709: msecs_to_jiffies <ffffffff8106b5f0>


  sym-userobj - when user stacktrace are enabled, look up which
                object the address belongs to, and print a
                relative address. This is especially useful when
                ASLR is on, otherwise you don't get a chance to
                resolve the address to object/file/line after
                the app is no longer running

                The lookup is performed when you read
                trace,trace_pipe. Example:

        a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]

  trace_printk - Can disable trace_printk() from writing into the buffer.

  userstacktrace - This option changes the trace. It records a
                   stacktrace of the current userspace thread at each event.

  verbose - This deals with the trace file when the
            latency-format option is enabled.

    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
    (+0.000ms): simple_strtoul (strict_strtoul)




This has been quite an in depth look at how to use ftrace via the
debug file system. But it can be quite daunting to handle all these
different files. Luckily, there's a tool that can do most of this
work for you. It's called "trace-cmd".

Using trace-cmd
---------------

trace-cmd is a tool that interacts with the ftrace tracing facility.
It reads and writes to the same files that are described above as
well as reading the files that can transfer the binary data of
the kernel tracing buffers in an efficient manner to be read later.
The tool is very simple and easy to use.

There are several man pages for trace-cmd. First look at 

  man trace-cmd

to find out more information on the other commands. All of trace-cmd's
commands also have their own man pages in the format of:

  man trace-cmd-<command>

For example, the "record" command's man page is under trace-cmd-record.

This document will describe all the options for each command, but
instead will briefly discuss how to use trace-cmd and describe most of
its commands.



trace-cmd record and report
---------------------------

To use ftrace tracers and events you must first have to start tracing
by either echoing a name of a tracer into the "current_tracer" file
or by echoing "1" into one of the event "enable" files.

For trace-cmd, the record option starts the tracing and will also save
the traced data into a file. Let's start with an example:


># cd ~
># trace-cmd record -p function
  plugin 'function'
Hit Ctrl^C to stop recording
(^C)
Kernel buffer statistics:
  Note: "entries" are the entries left in the kernel ring buffer and are not
        recorded in the trace data. They should all be zero.

CPU: 0
entries: 0
overrun: 38650181
commit overrun: 0
bytes: 3060
oldest event ts: 15634.891771
now ts: 15634.953219
dropped events: 0

CPU: 1
entries: 0
overrun: 38523960
commit overrun: 0
bytes: 1368
oldest event ts: 15634.891771
now ts: 15634.953938
dropped events: 0

CPU: 2
entries: 0
overrun: 41461508
commit overrun: 0
bytes: 1872
oldest event ts: 15634.891773
now ts: 15634.954630
dropped events: 0

CPU: 3
entries: 0
overrun: 38246206
commit overrun: 0
bytes: 36
oldest event ts: 15634.891785
now ts: 15634.955263
dropped events: 0

CPU: 4
entries: 0
overrun: 32730902
commit overrun: 0
bytes: 432
oldest event ts: 15634.891716
now ts: 15634.955952
dropped events: 0

CPU: 5
entries: 0
overrun: 33264601
commit overrun: 0
bytes: 2952
oldest event ts: 15634.891769
now ts: 15634.956630
dropped events: 0

CPU: 6
entries: 0
overrun: 30974204
commit overrun: 0
bytes: 2484
oldest event ts: 15634.891772
now ts: 15634.957249
dropped events: 0

CPU: 7
entries: 0
overrun: 32374274
commit overrun: 0
bytes: 3564
oldest event ts: 15634.891652
now ts: 15634.957938
dropped events: 0

CPU0 data recorded at offset=0x302000
    146325504 bytes in size
CPU1 data recorded at offset=0x8e8e000
    148217856 bytes in size
CPU2 data recorded at offset=0x11be8000
    148066304 bytes in size
CPU3 data recorded at offset=0x1a91d000
    146219008 bytes in size
CPU4 data recorded at offset=0x2348f000
    145940480 bytes in size
CPU5 data recorded at offset=0x2bfbd000
    145403904 bytes in size
CPU6 data recorded at offset=0x34a68000
    141570048 bytes in size
CPU7 data recorded at offset=0x3d16b000
    147513344 bytes in size


The "-p" is for ftrace tracers (use to be known as 'plugins' and the name
is kept for historical reasons). In this case we started the
"function" tracer. Since we did not add a command to execute, by
default, trace-cmd will just start the tracing and record the data
and wait for the user to hit Ctrl^C to stop.

When the trace stops, it prints out status of each of the kernel's
per cpu trace buffers. The are:

  entries:  - Which is the number of entries still in the kernel buffer.
              Ideally this should be zero, as trace-cmd would consume them
            all and put them into the data file.

  overrun:  - As tracing can be much faster than the saving of data,
              events can be lost due to overwriting of the old events
            that were not consumed yet when the buffer filled up.
            This is the number of events that were lost.

            The "function" tracer can fill up the buffer extremely fast
            it is not uncommon to lose millions of events when
            tracing functions for any length of time.

  commit overrun: - This should always be zero, and if it is not, then
              the buffer size is way too small or something went wrong
            with the tracer.

  bytes: - The number of bytes consumed (not read as pages). This is
             more a status for developers of the tracing utitily.

  oldest event ts: - The timestamp for the oldest event still in the ring
             buffer. Unless it gets overwritten, it will be the timestamp
           of the next event read.

  now ts: The current timestamp used by the tracing facility.

  dropped events: - If the buffer has overwrite mode disabled (from the
            trace options), then this will show the number of events that
          were lost due to not being able to write to the buffer because
          it was full. This is similar to the overrun field except that
          those are events that made it into the buffer but were overwritten.



By default, the file used to record the trace is called "trace.dat".
You can override the output file with the -o option.

To read the trace.dat file, simply run the trace-cmd report command:

># trace-cmd report
version = 6
cpus=8
       trace-cmd-3735  [003] 15618.722889: function:             __hrtimer_start_range_ns
       trace-cmd-3734  [002] 15618.722889: function:             _mutex_unlock
          <idle>-0     [000] 15618.722889: function:             cpuidle_wrap_enter
       trace-cmd-3735  [003] 15618.722890: function:                lock_hrtimer_base.isra.25
       trace-cmd-3734  [002] 15618.722890: function:                rt_mutex_unlock
          <idle>-0     [000] 15618.722890: function:                ktime_get
       trace-cmd-3735  [003] 15618.722890: function:                   _raw_spin_lock_irqsave
       trace-cmd-3735  [003] 15618.722891: function:                      add_preempt_count
       trace-cmd-3734  [002] 15618.722891: function:             __fsnotify_parent
          <idle>-0     [000] 15618.722891: function:             intel_idle
       trace-cmd-3735  [003] 15618.722891: function:                idle_cpu
       trace-cmd-3734  [002] 15618.722891: function:             fsnotify
          <idle>-0     [000] 15618.722891: function:                leave_mm
       trace-cmd-3735  [003] 15618.722891: function:                ktime_get
       trace-cmd-3734  [002] 15618.722891: function:                __srcu_read_lock
          <idle>-0     [000] 15618.722891: function:                   __phys_addr
       trace-cmd-3734  [002] 15618.722891: function:                   add_preempt_count
       trace-cmd-3735  [003] 15618.722891: function:                enqueue_hrtimer
       trace-cmd-3735  [003] 15618.722892: function:                _raw_spin_unlock_irqrestore
       trace-cmd-3734  [002] 15618.722892: function:                   sub_preempt_count
       trace-cmd-3735  [003] 15618.722892: function:                   sub_preempt_count
       trace-cmd-3734  [002] 15618.722892: function:                __srcu_read_unlock
       trace-cmd-3735  [003] 15618.722892: function:             schedule
       trace-cmd-3734  [002] 15618.722892: function:                   add_preempt_count
       trace-cmd-3735  [003] 15618.722893: function:                __schedule
       trace-cmd-3734  [002] 15618.722893: function:                   sub_preempt_count
       trace-cmd-3735  [003] 15618.722893: function:                   add_preempt_count
       trace-cmd-3735  [003] 15618.722893: function:                   rcu_note_context_switch
       trace-cmd-3734  [002] 15618.722893: function:             __audit_syscall_exit
       trace-cmd-3735  [003] 15618.722893: function:                   _raw_spin_lock_irq
       trace-cmd-3735  [003] 15618.722894: function:                      add_preempt_count
       trace-cmd-3734  [002] 15618.722894: function:                path_put
       trace-cmd-3735  [003] 15618.722894: function:                   deactivate_task
       trace-cmd-3734  [002] 15618.722894: function:                   dput
       trace-cmd-3735  [003] 15618.722894: function:                      dequeue_task
       trace-cmd-3734  [002] 15618.722894: function:                   mntput
       trace-cmd-3735  [003] 15618.722894: function:                         update_rq_clock
       trace-cmd-3734  [002] 15618.722894: function:                unroll_tree_refs


To filter out a CPU, use the --cpu option.

># trace-cmd report --cpu 1
version = 6
cpus=8
          <idle>-0     [001] 15618.723287: function:             ktime_get
          <idle>-0     [001] 15618.723288: function:             smp_apic_timer_interrupt
          <idle>-0     [001] 15618.723289: function:                irq_enter
          <idle>-0     [001] 15618.723289: function:                   rcu_irq_enter
          <idle>-0     [001] 15618.723289: function:                      rcu_eqs_exit_common.isra.45
          <idle>-0     [001] 15618.723289: function:                   tick_check_idle
          <idle>-0     [001] 15618.723290: function:                      tick_check_oneshot_broadcast
          <idle>-0     [001] 15618.723290: function:                      ktime_get
          <idle>-0     [001] 15618.723290: function:                      tick_nohz_stop_idle
          <idle>-0     [001] 15618.723290: function:                         update_ts_time_stats
          <idle>-0     [001] 15618.723290: function:                            nr_iowait_cpu
          <idle>-0     [001] 15618.723291: function:             touch_softlockup_watchdog
          <idle>-0     [001] 15618.723291: function:                      tick_do_update_jiffies64
          <idle>-0     [001] 15618.723291: function:                      touch_softlockup_watchdog
          <idle>-0     [001] 15618.723291: function:                   irqtime_account_irq
          <idle>-0     [001] 15618.723292: function:                      in_serving_softirq
          <idle>-0     [001] 15618.723292: function:                   add_preempt_count
          <idle>-0     [001] 15618.723292: function:             exit_idle
          <idle>-0     [001] 15618.723292: function:                atomic_notifier_call_chain
          <idle>-0     [001] 15618.723293: function:                   __atomic_notifier_call_chain
          <idle>-0     [001] 15618.723293: function:                      __rcu_read_lock


Notice how the functions are indented similar to the function_graph
tracer.  This is because trace-cmd can post process the trace data
with more complex algorithms than are acceptable to implement in the
kernel. It uses the parent function to follow which function is called
by other functions and be able to deduce a call graph.

To disable the indentation, use the -O report option.

># trace-cmd report --cpu 1 -O indent=0
version = 6
cpus=8
          <idle>-0     [001] 15618.723287: function:             ktime_get
          <idle>-0     [001] 15618.723288: function:             smp_apic_timer_interrupt
          <idle>-0     [001] 15618.723289: function:             irq_enter
          <idle>-0     [001] 15618.723289: function:             rcu_irq_enter
          <idle>-0     [001] 15618.723289: function:             rcu_eqs_exit_common.isra.45
          <idle>-0     [001] 15618.723289: function:             tick_check_idle
          <idle>-0     [001] 15618.723290: function:             tick_check_oneshot_broadcast
          <idle>-0     [001] 15618.723290: function:             ktime_get
          <idle>-0     [001] 15618.723290: function:             tick_nohz_stop_idle
          <idle>-0     [001] 15618.723290: function:             update_ts_time_stats
          <idle>-0     [001] 15618.723290: function:             nr_iowait_cpu
          <idle>-0     [001] 15618.723291: function:             touch_softlockup_watchdog
          <idle>-0     [001] 15618.723291: function:             tick_do_update_jiffies64
          <idle>-0     [001] 15618.723291: function:             touch_softlockup_watchdog


To add back the parent:

># trace-cmd report --cpu 1 -O indent=0 -O parent=1
version = 6
cpus=8
          <idle>-0     [001] 15618.723287: function:             ktime_get <-- cpuidle_wrap_enter
          <idle>-0     [001] 15618.723288: function:             smp_apic_timer_interrupt <-- apic_timer_interrupt
          <idle>-0     [001] 15618.723289: function:             irq_enter <-- smp_apic_timer_interrupt
          <idle>-0     [001] 15618.723289: function:             rcu_irq_enter <-- irq_enter
          <idle>-0     [001] 15618.723289: function:             rcu_eqs_exit_common.isra.45 <-- rcu_irq_enter
          <idle>-0     [001] 15618.723289: function:             tick_check_idle <-- irq_enter
          <idle>-0     [001] 15618.723290: function:             tick_check_oneshot_broadcast <-- tick_check_idle
          <idle>-0     [001] 15618.723290: function:             ktime_get <-- tick_check_idle
          <idle>-0     [001] 15618.723290: function:             tick_nohz_stop_idle <-- tick_check_idle
          <idle>-0     [001] 15618.723290: function:             update_ts_time_stats <-- tick_nohz_stop_idle
          <idle>-0     [001] 15618.723290: function:             nr_iowait_cpu <-- update_ts_time_stats
          <idle>-0     [001] 15618.723291: function:             touch_softlockup_watchdog <-- sched_clock_idle_wakeup_event
          <idle>-0     [001] 15618.723291: function:             tick_do_update_jiffies64 <-- tick_check_idle
          <idle>-0     [001] 15618.723291: function:             touch_softlockup_watchdog <-- tick_check_idle
          <idle>-0     [001] 15618.723291: function:             irqtime_account_irq <-- irq_enter
          <idle>-0     [001] 15618.723292: function:             in_serving_softirq <-- irqtime_account_irq
          <idle>-0     [001] 15618.723292: function:             add_preempt_count <-- irq_enter
          <idle>-0     [001] 15618.723292: function:             exit_idle <-- smp_apic_timer_interrupt
          <idle>-0     [001] 15618.723292: function:             atomic_notifier_call_chain <-- exit_idle
          <idle>-0     [001] 15618.723293: function:             __atomic_notifier_call_chain <-- atomic_notifier_call_chain


Now the trace looks similar to the debug file system output.



Use the "-e" option to record events:

># trace-cmd record -e sched_switch
/sys/kernel/debug/tracing/events/sched_switch/filter
/sys/kernel/debug/tracing/events/*/sched_switch/filter
Hit Ctrl^C to stop recording
(^C)
[...]

># trace-cmd report
version = 6
cpus=8
          <idle>-0     [006] 21642.751755: sched_switch:         swapper/6:0 [120] R ==> trace-cmd:4876 [120]
          <idle>-0     [002] 21642.751776: sched_switch:         swapper/2:0 [120] R ==> sshd:1208 [120]
       trace-cmd-4875  [005] 21642.751782: sched_switch:         trace-cmd:4875 [120] D ==> swapper/5:0 [120]
       trace-cmd-4869  [001] 21642.751792: sched_switch:         trace-cmd:4869 [120] S ==> swapper/1:0 [120]
       trace-cmd-4873  [003] 21642.751819: sched_switch:         trace-cmd:4873 [120] S ==> swapper/3:0 [120]
          <idle>-0     [005] 21642.751835: sched_switch:         swapper/5:0 [120] R ==> trace-cmd:4875 [120]
       trace-cmd-4877  [007] 21642.751847: sched_switch:         trace-cmd:4877 [120] D ==> swapper/7:0 [120]
            sshd-1208  [002] 21642.751875: sched_switch:         sshd:1208 [120] S ==> swapper/2:0 [120]
          <idle>-0     [007] 21642.751880: sched_switch:         swapper/7:0 [120] R ==> trace-cmd:4877 [120]
       trace-cmd-4874  [004] 21642.751885: sched_switch:         trace-cmd:4874 [120] S ==> swapper/4:0 [120]
          <idle>-0     [001] 21642.751902: sched_switch:         swapper/1:0 [120] R ==> irq/43-em1:865 [49]
       trace-cmd-4876  [006] 21642.751903: sched_switch:         trace-cmd:4876 [120] D ==> swapper/6:0 [120]
          <idle>-0     [006] 21642.751926: sched_switch:         swapper/6:0 [120] R ==> trace-cmd:4876 [120]
      irq/43-em1-865   [001] 21642.751927: sched_switch:         irq/43-em1:865 [49] S ==> swapper/1:0 [120]
       trace-cmd-4875  [005] 21642.752029: sched_switch:         trace-cmd:4875 [120] S ==> swapper/5:0 [120]


Notice that only the "sched_switch" name was used. trace-cmd will
search for a match of "-e"'s option for trace event systems, or single
trace events themselves. To trace all interrupt events:

># trace-cmd record -e irq sleep 10
/sys/kernel/debug/tracing/events/irq/filter
/sys/kernel/debug/tracing/events/*/irq/filter
[...]

Notice that when a command is passed to trace-cmd, it will just run that
command and exit the trace when complete.

># trace-cmd report
version = 6
cpus=8
          <idle>-0     [002] 21767.342089: softirq_raise:        vec=9 [action=RCU]
           sleep-4917  [007] 21767.342089: softirq_raise:        vec=9 [action=RCU]
          <idle>-0     [006] 21767.342089: softirq_raise:        vec=9 [action=RCU]
     ksoftirqd/0-3     [000] 21767.342096: softirq_entry:        vec=1 [action=TIMER]
     ksoftirqd/4-33    [004] 21767.342096: softirq_entry:        vec=1 [action=TIMER]
     ksoftirqd/3-27    [003] 21767.342097: softirq_entry:        vec=1 [action=TIMER]
     ksoftirqd/7-51    [007] 21767.342097: softirq_entry:        vec=1 [action=TIMER]
     ksoftirqd/4-33    [004] 21767.342097: softirq_exit:         vec=1 [action=TIMER]



To get the status information of events similar to what the debug
file system provides, add the "-l" (think "latency") option to the report.


># trace-cmd report -l
version = 6
cpus=8
  <idle>-0       3d.h20 21767.341545: softirq_raise:        vec=8 [action=HRTIMER]
ksoftirq-27      3...11 21767.341552: softirq_entry:        vec=8 [action=HRTIMER]
ksoftirq-27      3...11 21767.341554: softirq_exit:         vec=8 [action=HRTIMER]
  <idle>-0       4d.h20 21767.342085: softirq_raise:        vec=7 [action=SCHED]
  <idle>-0       0d.h20 21767.342086: softirq_raise:        vec=7 [action=SCHED]
  <idle>-0       3d.h20 21767.342086: softirq_raise:        vec=7 [action=SCHED]
   sleep-4917    7d.h10 21767.342086: softirq_raise:        vec=7 [action=SCHED]
  <idle>-0       6d.h20 21767.342087: softirq_raise:        vec=7 [action=SCHED]
  <idle>-0       2d.h20 21767.342087: softirq_raise:        vec=1 [action=TIMER]
  <idle>-0       1d.h20 21767.342087: softirq_raise:        vec=1 [action=TIMER]



Tracing all events
------------------

As mentioned above, the "-e" option to trace-cmd record is to choose
what event should be traced. You can specify either an individual event,
or a trace system:

># trace-cmd record -e irq

The above enables all tracepoints within the "irq" system.

># trace-cmd record -e irq_handler_enter
># trace-cmd record -e irq:irq_handler_enter

The commands above are equivalent and will enable the tracepoint
event "irq_handler_enter".

But then there is the case where you want to trace all events.
To do this, use the keyword "all".

># trace-cmd record -e all

This will enable all events.


Tracing tracers and events
--------------------------

As events can be enabled within any tracer, it makes sense that trace-cmd
would allow this as well. This is indeed the case. You may use both
the "-p" and the "-e" options at the same time.

># trace-cmd record -p function_graph -e all
[...]
># trace-cmd report
version = 6
cpus=8
       trace-cmd-1698  [002]  2724.485397: funcgraph_entry:                   |                  kmem_cache_alloc() {
       trace-cmd-1699  [007]  2724.485397: funcgraph_entry:        0.073 us   |      find_vma();
       trace-cmd-1696  [000]  2724.485397: funcgraph_entry:                   |          lg_local_lock() {
       trace-cmd-1698  [002]  2724.485397: funcgraph_entry:        0.033 us   |                    add_preempt_count();
       trace-cmd-1696  [000]  2724.485397: funcgraph_entry:                   |            migrate_disable() {
       trace-cmd-1699  [007]  2724.485398: funcgraph_entry:                   |      handle_mm_fault() {
       trace-cmd-1696  [000]  2724.485398: funcgraph_entry:        0.027 us   |              add_preempt_count();
       trace-cmd-1698  [002]  2724.485398: funcgraph_entry:        0.034 us   |                    sub_preempt_count();
       trace-cmd-1699  [007]  2724.485398: funcgraph_entry:                   |        __mem_cgroup_count_vm_event() {
       trace-cmd-1696  [000]  2724.485398: funcgraph_entry:        0.031 us   |              pin_current_cpu();
       trace-cmd-1699  [007]  2724.485398: funcgraph_entry:        0.029 us   |          __rcu_read_lock();
       trace-cmd-1698  [002]  2724.485398: kmem_cache_alloc:     (return_to_handler+0x0) call_site=ffffffff81662345 ptr=0xffff880114e260f0 bytes_req=240 bytes_alloc=240 gfp_flags=G
FP_KERNEL
       trace-cmd-1696  [000]  2724.485398: funcgraph_entry:        0.034 us   |              sub_preempt_count();
       trace-cmd-1699  [007]  2724.485398: funcgraph_entry:        0.028 us   |          __rcu_read_unlock();
       trace-cmd-1698  [002]  2724.485398: funcgraph_exit:         0.758 us   |                  }
       trace-cmd-1698  [002]  2724.485398: funcgraph_entry:        0.029 us   |                  __rt_mutex_init();
       trace-cmd-1696  [000]  2724.485398: funcgraph_exit:         0.727 us   |            }
       trace-cmd-1699  [007]  2724.485398: funcgraph_exit:         0.466 us   |        }

Notice here that trace-cmd report does not disply the function graph
tracer any different than any other trace, like the "trace" file does.


Function filtering
------------------

The "set_ftrace_filter" and "set_ftrace_notrace" is very useful in
filtering out functions that you do not care about. These can be done
with trace-cmd as well.

The "-l" and "-n" are used the same as "set_ftrace_filter" and
"set_ftrace_notrace" respectively. Think of "limit functions" for
"-l" as the "-f" is used for event filtering.

To add more than one function to the list, either used the glob expressions
described previously, or use multiple "-l" or "-n" options.

># trace-cmd record -p function -l "sched*" -n "*stat*"

The above traces all functions that start with "sched" except those that
have "stat" in their names.



Event filtering
---------------

To filter events the same way as writing to the "filter" file inside
the "events" directory (see "Filtering events" above), use the "-f"
option. This option must follow the event that it will filter.

># trace-cmd record -e sched_switch -f "prev_prio < 100" \
   -e sched_wakeup -f 'comm == "bash"'


Graph a function
----------------

To perform a graph of a specific function using "function_graph" tracer,
trace-cmd provides the "-g" option.

># trace-cmd record -p function_graph -g sys_read ls /
[...]
># trace-cmd report
version = 6
CPU 3 is empty
CPU 4 is empty
CPU 5 is empty
cpus=8
       trace-cmd-2183  [006]  4689.643252: funcgraph_entry:                   |  sys_read() {
       trace-cmd-2183  [006]  4689.643253: funcgraph_entry:        0.147 us   |    fget_light();
       trace-cmd-2183  [006]  4689.643254: funcgraph_entry:                   |    vfs_read() {
       trace-cmd-2183  [006]  4689.643254: funcgraph_entry:                   |      rw_verify_area() {
       trace-cmd-2183  [006]  4689.643255: funcgraph_entry:                   |        security_file_permission() {
       trace-cmd-2183  [006]  4689.643255: funcgraph_entry:        0.068 us   |          cap_file_permission();
       trace-cmd-2183  [006]  4689.643256: funcgraph_entry:        0.064 us   |          __fsnotify_parent();
       trace-cmd-2183  [006]  4689.643256: funcgraph_entry:        0.095 us   |          fsnotify();
       trace-cmd-2183  [006]  4689.643257: funcgraph_exit:         1.792 us   |        }
       trace-cmd-2183  [006]  4689.643257: funcgraph_exit:         2.328 us   |      }
       trace-cmd-2183  [006]  4689.643257: funcgraph_entry:                   |      seq_read() {
       trace-cmd-2183  [006]  4689.643257: funcgraph_entry:                   |        _mutex_lock() {
       trace-cmd-2183  [006]  4689.643258: funcgraph_entry:        0.062 us   |          rt_mutex_lock();
       trace-cmd-2183  [006]  4689.643258: funcgraph_exit:         0.584 us   |        }
       trace-cmd-2183  [006]  4689.643259: funcgraph_entry:                   |        m_start() {
       trace-cmd-2183  [006]  4689.643259: funcgraph_entry:                   |          rt_down_read() {
       trace-cmd-2183  [006]  4689.643259: funcgraph_entry:                   |            rt_mutex_lock() {


Modify trace buffer size via trace-cmd
--------------------------------------

The trace-cmd record "-b" option lets you change the size of the
ftrace buffer before recording the trace. Note, currently trace-cmd
does not support per-cpu resize. The size is what is entered into
"buffer_size_kb" at the top level.


># trace-cmd record -b 10000 -p function


trace-cmd start, stop and extract
---------------------------------

The trace-cmd start command takes almost all the options as the trace-cmd
record command does. The difference between the two is that "start"
will only enable ftrace, it will not do any recording. It is equivalent
to enabling ftrace via the debug file system.

># trace-cmd start -p function -e all
># cat /sys/kernel/debug/tracing/trace
# tracer: function
#
# entries-in-buffer/entries-written: 1544167/2039168   #P:8
#
#                              _-------=> irqs-off          
#                            /  _------=> need-resched      
#                            |/  _-----=> need-resched_lazy 
#                            ||/  _----=> hardirq/softirq   
#                            |||/  _---=> preempt-depth     
#                            ||||/  _--=> preempt-lazy-depth
#                            ||||| / _-=> migrate-disable   
#                            |||||| /     delay
#           TASK-PID   CPU#  |||||||    TIMESTAMP  FUNCTION
#              | |       |   |||||||       |         |
       trace-cmd-2390  [003] .......  5946.816132: _mutex_unlock <-rb_simple_write
       trace-cmd-2390  [003] .......  5946.816133: rt_mutex_unlock <-_mutex_unlock
       trace-cmd-2390  [003] .......  5946.816134: __fsnotify_parent <-vfs_write
       trace-cmd-2390  [003] .......  5946.816134: fsnotify <-vfs_write
       trace-cmd-2390  [003] .......  5946.816135: __srcu_read_lock <-fsnotify
       trace-cmd-2390  [003] .......  5946.816135: add_preempt_count <-__srcu_read_lock
       trace-cmd-2390  [003] ....1..  5946.816135: sub_preempt_count <-__srcu_read_lock
       trace-cmd-2390  [003] .......  5946.816135: __srcu_read_unlock <-fsnotify
       trace-cmd-2390  [003] .......  5946.816136: add_preempt_count <-__srcu_read_unlock
       trace-cmd-2390  [003] ....1..  5946.816136: sub_preempt_count <-__srcu_read_unlock
       trace-cmd-2390  [003] .......  5946.816137: syscall_trace_leave <-int_check_syscall_exit_work
       trace-cmd-2390  [003] .......  5946.816137: __audit_syscall_exit <-syscall_trace_leave
       trace-cmd-2390  [003] .......  5946.816137: path_put <-__audit_syscall_exit
       trace-cmd-2390  [003] .......  5946.816137: dput <-path_put
       trace-cmd-2390  [003] .......  5946.816138: mntput <-path_put
       trace-cmd-2390  [003] .......  5946.816138: unroll_tree_refs <-__audit_syscall_exit
       trace-cmd-2390  [003] .......  5946.816138: kfree <-__audit_syscall_exit
       trace-cmd-2390  [003] ....1..  5946.816139: kfree: call_site=ffffffff810eaff0 ptr=          (null)
       trace-cmd-2390  [003] ....1..  5946.816139: sys_exit: NR 1 = 1
       trace-cmd-2390  [003] d......  5946.816140: sys_write -> 0x1
       trace-cmd-2390  [003] d......  5946.816151: do_page_fault <-page_fault
       trace-cmd-2390  [003] d......  5946.816151: __do_page_fault <-do_page_fault
       trace-cmd-2390  [003] .......  5946.816152: rt_down_read_trylock <-__do_page_fault
       trace-cmd-2390  [003] .......  5946.816152: rt_mutex_trylock <-rt_down_read_trylock


Running trace-cmd stop is exactly the same as echoing "0" into the
"tracing_on" file in the debug file system. This only stops writing to
the trace buffers, it does not stop all the tracing mechanisms inside
the kernel and still adds some overhead to the system.

># cat /sys/kernel/debug/tracing/tracing_on
1
># trace-cmd stop
># cat /sys/kernel/debug/tracing/tracing_on
0


Finally, if you want to create a "trace.dat" file from the ftrace
kernel buffers you use the "extract" command. The tracing could
have started with the "start" command or by manually modifying the
ftrace debug file system files. This is useful if you found a trace
and want to save it off where you can send it to other people, and
also have the full features of the trace-cmd "report" command.

># trace-cmd extract
># trace-cmd report
version = 6
cpus=8
CPU:6 [2544372 EVENTS DROPPED]
     ksoftirqd/6-45    [006]  6192.717580: function:             rcu_note_context_switch
     ksoftirqd/6-45    [006]  6192.717580: rcu_utilization:      ffffffff819e743b
     ksoftirqd/6-45    [006]  6192.717580: rcu_utilization:      ffffffff819e7450
     ksoftirqd/6-45    [006]  6192.717581: function:             add_preempt_count
     ksoftirqd/6-45    [006]  6192.717581: function:             kthread_should_stop
     ksoftirqd/6-45    [006]  6192.717581: function:             kthread_should_park
     ksoftirqd/6-45    [006]  6192.717581: function:             ksoftirqd_should_run
     ksoftirqd/6-45    [006]  6192.717582: function:             sub_preempt_count
     ksoftirqd/6-45    [006]  6192.717582: function:             schedule
     ksoftirqd/6-45    [006]  6192.717582: function:                __schedule
     ksoftirqd/6-45    [006]  6192.717582: function:                   add_preempt_count
     ksoftirqd/6-45    [006]  6192.717582: function:                   rcu_note_context_switch
     ksoftirqd/6-45    [006]  6192.717583: rcu_utilization:      ffffffff819e743b
     ksoftirqd/6-45    [006]  6192.717583: rcu_utilization:      ffffffff819e7450
     ksoftirqd/6-45    [006]  6192.717583: function:                   _raw_spin_lock_irq
     ksoftirqd/6-45    [006]  6192.717583: function:                      add_preempt_count
     ksoftirqd/6-45    [006]  6192.717584: function:                   deactivate_task
     ksoftirqd/6-45    [006]  6192.717584: function:                      dequeue_task
     ksoftirqd/6-45    [006]  6192.717584: function:                         update_rq_clock


The "extract" command takes a "-o" option to save the trace in a different
name like the "record" command does. By default it just saves it into
a file called "trace.dat".


Resetting the trace
-------------------

As mentioned, the "stop" command does not lower the overhead of ftrace.
It simply disables writing to the ftrace buffer. There's two ways of
resetting ftrace with trace-cmd.

The first way is with the "reset" command.

># trace-cmd reset

This disables practically everything in ftrace. It also sets the
"tracing_on" file to "0". It also erases everything inside the buffers,
so make sure to do your "extract" before running the "reset" command.

The "reset" command also takes a "-b" option that lets you resize the
buffer as well. This is useful to free the allocated buffers when you
are finished tracing.

># trace-cmd reset -b 0
># cat /sys/kernel/debug/tracing/buffer_total_size_kb
8


The problem with the "reset" command is that it may make it hard to
use the debug file system tracing files directly. It may disable various
parts of tracing that may give unexpected results when trying to use
the files directly. If you plan to use ftrace's files directly after
using trace-cmd, the trick is to start the "nop" tracer.

># trace-cmd start -p nop

This sets up ftrace to run the "nop" tracer, which does no tracing and
has no overhead when enabled, and disables all events, and clears out
the "trace" file. After running this command, the system should be
set up to use the ftrace files directly as they are expected.



Using trace-cmd over the network
--------------------------------

If the target system to trace is limited on disk space, or perhaps
the disk usage is what is being traced, it can be prudent to record
the trace via another median than to the hard drive. The "listen"
command sets up a way for trace-cmd to record over the network.

[Server]
>$ mkdir traces
>$ cd traces
>$ trace-cmd listen -p 55577


Notice that the prompt above is "$". This denotes that the listen command
does not need to be root if the listening port is not a privileged port.

[Target]
># trace-cmd record -e all -N Server:55577 ls /


[Server]
connected!
Connected with Target:50671
cpus=8
pagesize=4096
version = 6
CPU0 data recorded at offset=0x3a7000
    0 bytes in size
CPU1 data recorded at offset=0x3a7000
    8192 bytes in size
CPU2 data recorded at offset=0x3a9000
    8192 bytes in size
CPU3 data recorded at offset=0x3ab000
    8192 bytes in size
CPU4 data recorded at offset=0x3ad000
    8192 bytes in size
CPU5 data recorded at offset=0x3af000
    8192 bytes in size
CPU6 data recorded at offset=0x3b1000
    4096 bytes in size
CPU7 data recorded at offset=0x3b2000
    8192 bytes in size
connected!
(^C)

>$ ls
trace.Target:50671.dat
>$ trace-cmd report trace.Target:50671.dat
version = 6
CPU 0 is empty
cpus=8
           <...>-2976  [007]  8865.266143: mm_page_alloc:        page=0xffffea00007e8740 pfn=8292160 order=0 migratetype=0 gfp_flags=GFP_KERNEL|GFP_REPEAT|GFP_ZERO|GFP_NOTRACK
           <...>-2976  [007]  8865.266145: kmalloc:              (pte_lock_init+0x2c) call_site=ffffffff8116d78c ptr=0xffff880111e40d00 bytes_req=48 bytes_alloc=64 gfp_flags=GFP_KERNEL
           <...>-2976  [007]  8865.266152: mm_page_alloc:        page=0xffffea00034a50c0 pfn=55201984 order=0 migratetype=0 gfp_flags=GFP_KERNEL|GFP_REPEAT|GFP_ZERO|GFP_NOTRACK
           <...>-2976  [007]  8865.266153: kmalloc:              (pte_lock_init+0x2c) call_site=ffffffff8116d78c ptr=0xffff880111e40e40 bytes_req=48 bytes_alloc=64 gfp_flags=GFP_KERNEL
           <...>-2976  [007]  8865.266155: mm_page_alloc:        page=0xffffea000307d380 pfn=50844544 order=0 migratetype=2 gfp_flags=GFP_HIGHUSER_MOVABLE
           <...>-2976  [007]  8865.266167: mm_page_alloc:        page=0xffffea000323f900 pfn=52689152 order=0 migratetype=2 gfp_flags=GFP_HIGHUSER_MOVABLE
           <...>-2976  [007]  8865.266171: mm_page_alloc:        page=0xffffea00032cda80 pfn=53271168 order=0 migratetype=2 gfp_flags=GFP_HIGHUSER_MOVABLE
           <...>-2976  [007]  8865.266192: hrtimer_cancel:       hrtimer=0xffff88011ebccf40
          <idle>-0     [006]  8865.266193: hrtimer_cancel:       hrtimer=0xffff88011eb8cf40
           <...>-2976  [007]  8865.266193: hrtimer_expire_entry: hrtimer=0xffff88011ebccf40 now=8905356001470 function=tick_sched_timer/0x0
          <idle>-0     [006]  8865.266194: hrtimer_expire_entry: hrtimer=0xffff88011eb8cf40 now=8905356002620 function=tick_sched_timer/0x0
           <...>-2976  [007]  8865.266196: sched_stat_runtime:   comm=trace-cmd pid=2976 runtime=228684 [ns] vruntime=2941412131 [ns]
          <idle>-0     [006]  8865.266197: softirq_raise:        vec=1 [action=TIMER]
          <idle>-0     [006]  8865.266197: rcu_utilization:      ffffffff819e740d
           <...>-2976  [007]  8865.266198: softirq_raise:        vec=1 [action=TIMER]
          <idle>-0     [006]  8865.266198: softirq_raise:        vec=9 [action=RCU]
           <...>-2976  [007]  8865.266199: rcu_utilization:      ffffffff819e740d


By default, the data is transfered via UDP. This is very efficient but
it is possible to lose data and not know it. If you are worried about
a full connection, then use the TCP protocol. The "-t" option
on the "record" command forces trace-cmd to send the data over a TCP
connection instead of a UDP one.


Summary
-------

This document just highlighted the most common features of ftrace and
trace-cmd. For more in depth look at what trace-cmd can do, read
the man pages:

  trace-cmd
  trace-cmd-record
  trace-cmd-report
  trace-cmd-start
  trace-cmd-stop
  trace-cmd-extract
  trace-cmd-reset
  trace-cmd-listen
  trace-cmd-split
  trace-cmd-restore
  trace-cmd-list
  trace-cmd-stack