Kernel-3.10.0-957.el7_ftrace

    ftrace - Function Tracer
    ========================

Copyright 2008 Red Hat Inc.
Author: Steven Rostedt srostedt@redhat.com
License: The GNU Free Documentation License, Version 1.2
(dual licensed under the GPL v2)
Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
John Kacur, and David Teigland.
Written for: 2.6.28-rc2
Updated for: 3.10

Introduction

Ftrace is an internal tracer designed to help out developers and
designers of systems to find what is going on inside the kernel.
It can be used for debugging or analyzing latencies and
performance issues that take place outside of user-space.

Although ftrace is typically considered the function tracer, it
is really a frame work of several assorted tracing utilities.
There’s latency tracing to examine what occurs between interrupts
disabled and enabled, as well as for preemption and from a time
a task is woken to the task is actually scheduled in.

One of the most common uses of ftrace is the event tracing.
Through out the kernel is hundreds of static event points that
can be enabled via the debugfs file system to see what is
going on in certain parts of the kernel.

Implementation Details

See ftrace-design.txt for details for arch porters and such.

The File System

Ftrace uses the debugfs file system to hold the control files as
well as the files to display output.

When debugfs is configured into the kernel (which selecting any ftrace
option will do) the directory /sys/kernel/debug will be created. To mount
this directory, you can add to your /etc/fstab file:

debugfs /sys/kernel/debug debugfs defaults 0 0

Or you can mount it at run time with:

mount -t debugfs nodev /sys/kernel/debug

For quicker access to that directory you may want to make a soft link to
it:

ln -s /sys/kernel/debug /debug

Any selected ftrace option will also create a directory called tracing
within the debugfs. The rest of the document will assume that you are in
the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
on the files within that directory and not distract from the content with
the extended “/sys/kernel/debug/tracing” path name.

That’s it! (assuming that you have ftrace configured into your kernel)

After mounting debugfs, you can see a directory called
“tracing”. This directory contains the control and output files
of ftrace. Here is a list of some of the key files:

Note: all time values are in microseconds.

current_tracer:

This is used to set or display the current tracer
that is configured.

available_tracers:

This holds the different types of tracers that
have been compiled into the kernel. The
tracers listed here can be configured by
echoing their name into current_tracer.

tracing_on:

This sets or displays whether writing to the trace
ring buffer is enabled. Echo 0 into this file to disable
the tracer or 1 to enable it. Note, this only disables
writing to the ring buffer, the tracing overhead may
still be occurring.

trace:

This file holds the output of the trace in a human
readable format (described below).

trace_pipe:

The output is the same as the "trace" file but this
file is meant to be streamed with live tracing.
Reads from this file will block until new data is
retrieved.  Unlike the "trace" file, this file is a
consumer. This means reading from this file causes
sequential reads to display more current data. Once
data is read from this file, it is consumed, and
will not be read again with a sequential read. The
"trace" file is static, and if the tracer is not
adding more data,they will display the same
information every time they are read.

trace_options:

This file lets the user control the amount of data
that is displayed in one of the above output
files. Options also exist to modify how a tracer
or events work (stack traces, timestamps, etc).

options:

This is a directory that has a file for every available
trace option (also in trace_options). Options may also be set
or cleared by writing a "1" or "0" respectively into the
corresponding file with the option name.

tracing_max_latency:

Some of the tracers record the max latency.
For example, the time interrupts are disabled.
This time is saved in this file. The max trace
will also be stored, and displayed by "trace".
A new max trace will only be recorded if the
latency is greater than the value in this
file. (in microseconds)

tracing_thresh:

Some latency tracers will record a trace whenever the
latency is greater than the number in this file.
Only active when the file contains a number greater than 0.
(in microseconds)

buffer_size_kb:

This sets or displays the number of kilobytes each CPU
buffer holds. By default, the trace buffers are the same size
for each CPU. The displayed number is the size of the
CPU buffer and not total size of all buffers. The
trace buffers are allocated in pages (blocks of memory
that the kernel uses for allocation, usually 4 KB in size).
If the last page allocated has room for more bytes
than requested, the rest of the page will be used,
making the actual allocation bigger than requested.
( Note, the size may not be a multiple of the page size
  due to buffer management meta-data. )

buffer_total_size_kb:

This displays the total combined size of all the trace buffers.

free_buffer:

If a process is performing the tracing, and the ring buffer
should be shrunk "freed" when the process is finished, even
if it were to be killed by a signal, this file can be used
for that purpose. On close of this file, the ring buffer will
be resized to its minimum size. Having a process that is tracing
also open this file, when the process exits its file descriptor
for this file will be closed, and in doing so, the ring buffer
will be "freed".

It may also stop tracing if disable_on_free option is set.

tracing_cpumask:

This is a mask that lets the user only trace
on specified CPUs. The format is a hex string
representing the CPUs.

set_ftrace_filter:

When dynamic ftrace is configured in (see the
section below "dynamic ftrace"), the code is dynamically
modified (code text rewrite) to disable calling of the
function profiler (mcount). This lets tracing be configured
in with practically no overhead in performance.  This also
has a side effect of enabling or disabling specific functions
to be traced. Echoing names of functions into this file
will limit the trace to only those functions.

This interface also allows for commands to be used. See the
"Filter commands" section for more details.

set_ftrace_notrace:

This has an effect opposite to that of
set_ftrace_filter. Any function that is added here will not
be traced. If a function exists in both set_ftrace_filter
and set_ftrace_notrace,    the function will _not_ be traced.

set_ftrace_pid:

Have the function tracer only trace a single thread.

set_graph_function:

Set a "trigger" function where tracing should start
with the function graph tracer (See the section
"dynamic ftrace" for more details).

available_filter_functions:

This lists the functions that ftrace
has processed and can trace. These are the function
names that you can pass to "set_ftrace_filter" or
"set_ftrace_notrace". (See the section "dynamic ftrace"
below for more details.)

enabled_functions:

This file is more for debugging ftrace, but can also be useful
in seeing if any function has a callback attached to it.
Not only does the trace infrastructure use ftrace function
trace utility, but other subsystems might too. This file
displays all functions that have a callback attached to them
as well as the number of callbacks that have been attached.
Note, a callback may also call multiple functions which will
not be listed in this count.

If the callback registered to be traced by a function with
the "save regs" attribute (thus even more overhead), a 'R'
will be displayed on the same line as the function that
is returning registers.

If the callback registered to be traced by a function with
the "ip modify" attribute (thus the regs->ip can be changed),
an 'I' will be displayed on the same line as the function that
can be overridden.

function_profile_enabled:

When set it will enable all functions with either the function
tracer, or if enabled, the function graph tracer. It will
keep a histogram of the number of functions that were called
and if run with the function graph tracer, it will also keep
track of the time spent in those functions. The histogram
content can be displayed in the files:

trace_stats/function<cpu> ( function0, function1, etc).

trace_stats:

A directory that holds different tracing stats.

kprobe_events:

Enable dynamic trace points. See kprobetrace.txt.

kprobe_profile:

Dynamic trace points stats. See kprobetrace.txt.

max_graph_depth:

Used with the function graph tracer. This is the max depth
it will trace into a function. Setting this to a value of
one will show only the first kernel function that is called
from user space.

printk_formats:

This is for tools that read the raw format files. If an event in
the ring buffer references a string (currently only trace_printk()
does this), only a pointer to the string is recorded into the buffer
and not the string itself. This prevents tools from knowing what
that string was. This file displays the string and address for
the string allowing tools to map the pointers to what the
strings were.

saved_cmdlines:

Only the pid of the task is recorded in a trace event unless
the event specifically saves the task comm as well. Ftrace
makes a cache of pid mappings to comms to try to display
comms for events. If a pid for a comm is not listed, then
"<...>" is displayed in the output.

snapshot:

This displays the "snapshot" buffer and also lets the user
take a snapshot of the current running trace.
See the "Snapshot" section below for more details.

stack_max_size:

When the stack tracer is activated, this will display the
maximum stack size it has encountered.
See the "Stack Trace" section below.

stack_trace:

This displays the stack back trace of the largest stack
that was encountered when the stack tracer is activated.
See the "Stack Trace" section below.

stack_trace_filter:

This is similar to "set_ftrace_filter" but it limits what
functions the stack tracer will check.

trace_clock:

Whenever an event is recorded into the ring buffer, a
"timestamp" is added. This stamp comes from a specified
clock. By default, ftrace uses the "local" clock. This
clock is very fast and strictly per cpu, but on some
systems it may not be monotonic with respect to other
CPUs. In other words, the local clocks may not be in sync
with local clocks on other CPUs.

Usual clocks for tracing:

  # cat trace_clock
  [local] global counter x86-tsc

  local: Default clock, but may not be in sync across CPUs

  global: This clock is in sync with all CPUs but may
        be a bit slower than the local clock.

  counter: This is not a clock at all, but literally an atomic
         counter. It counts up one by one, but is in sync
       with all CPUs. This is useful when you need to
       know exactly the order events occurred with respect to
       each other on different CPUs.

  uptime: This uses the jiffies counter and the time stamp
        is relative to the time since boot up.

  perf: This makes ftrace use the same clock that perf uses.
      Eventually perf will be able to read ftrace buffers
    and this will help out in interleaving the data.

  x86-tsc: Architectures may define their own clocks. For
         example, x86 uses its own TSC cycle clock here.

To set a clock, simply echo the clock name into this file.

  echo global > trace_clock

trace_marker:

This is a very useful file for synchronizing user space
with events happening in the kernel. Writing strings into
this file will be written into the ftrace buffer.

It is useful in applications to open this file at the start
of the application and just reference the file descriptor
for the file.

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);
}

start:

    trace_fd = open("trace_marker", WR_ONLY);

uprobe_events:

Add dynamic tracepoints in programs.
See uprobetracer.txt

uprobe_profile:

Uprobe statistics. See uprobetrace.txt

instances:

This is a way to make multiple trace buffers where different
events can be recorded in different buffers.
See "Instances" section below.

events:

This is the trace event directory. It holds event tracepoints
(also known as static tracepoints) that have been compiled
into the kernel. It shows what event tracepoints exist
and how they are grouped by system. There are "enable"
files at various levels that can enable the tracepoints
when a "1" is written to them.

See events.txt for more information.

per_cpu:

This is a directory that contains the trace per_cpu information.

per_cpu/cpu0/buffer_size_kb:

The ftrace buffer is defined per_cpu. That is, there's a separate
buffer for each CPU to allow writes to be done atomically,
and free from cache bouncing. These buffers may have different
size buffers. This file is similar to the buffer_size_kb
file, but it only displays or sets the buffer size for the
specific CPU. (here cpu0).

per_cpu/cpu0/trace:

This is similar to the "trace" file, but it will only display
the data specific for the CPU. If written to, it only clears
the specific CPU buffer.

per_cpu/cpu0/trace_pipe

This is similar to the "trace_pipe" file, and is a consuming
read, but it will only display (and consume) the data specific
for the CPU.

per_cpu/cpu0/trace_pipe_raw

For tools that can parse the ftrace ring buffer binary format,
the trace_pipe_raw file can be used to extract the data
from the ring buffer directly. With the use of the splice()
system call, the buffer data can be quickly transferred to
a file or to the network where a server is collecting the
data.

Like trace_pipe, this is a consuming reader, where multiple
reads will always produce different data.

per_cpu/cpu0/snapshot:

This is similar to the main "snapshot" file, but will only
snapshot the current CPU (if supported). It only displays
the content of the snapshot for a given CPU, and if
written to, only clears this CPU buffer.

per_cpu/cpu0/snapshot_raw:

Similar to the trace_pipe_raw, but will read the binary format
from the snapshot buffer for the given CPU.

per_cpu/cpu0/stats:

This displays certain stats about the ring buffer:

 entries: The number of events that are still in the buffer.

 overrun: The number of lost events due to overwriting when
       the buffer was full.

 commit overrun: Should always be zero.
     This gets set if so many events happened within a nested
    event (ring buffer is re-entrant), that it fills the
    buffer and starts dropping events.

 bytes: Bytes actually read (not overwritten).

 oldest event ts: The oldest timestamp in the buffer

 now ts: The current timestamp

 dropped events: Events lost due to overwrite option being off.

 read events: The number of events read.

The Tracers

Here is the list of current tracers that may be configured.

“function”

Function call tracer to trace all kernel functions.

“function_graph”

Similar to the function tracer except that the
function tracer probes the functions on their entry
whereas the function graph tracer traces on both entry
and exit of the functions. It then provides the ability
to draw a graph of function calls similar to C code
source.

“irqsoff”

Traces the areas that disable interrupts and saves
the trace with the longest max latency.
See tracing_max_latency. When a new max is recorded,
it replaces the old trace. It is best to view this
trace with the latency-format option enabled.

“preemptoff”

Similar to irqsoff but traces and records the amount of
time for which preemption is disabled.

“preemptirqsoff”

Similar to irqsoff and preemptoff, but traces and
records the largest time for which irqs and/or preemption
is disabled.

“wakeup”

Traces and records the max latency that it takes for
the highest priority task to get scheduled after
it has been woken up.
    Traces all tasks as an average developer would expect.

“wakeup_rt”

    Traces and records the max latency that it takes for just
    RT tasks (as the current "wakeup" does). This is useful
    for those interested in wake up timings of RT tasks.

“nop”

This is the "trace nothing" tracer. To remove all
tracers from tracing simply echo "nop" into
current_tracer.

Examples of using the tracer

Here are typical examples of using the tracers when controlling
them only with the debugfs interface (without using any
user-land utilities).

Output format:

Here is an example of the output format of the file “trace”

                         --------

tracer: function

entries-in-buffer/entries-written: 140080/250280 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

        bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
        bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
        bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
        sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
        bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
        bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
        bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
        bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
        bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
        sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
                         --------

A header is printed with the tracer name that is represented by
the trace. In this case the tracer is “function”. Then it shows the
number of events in the buffer as well as the total number of entries
that were written. The difference is the number of entries that were
lost due to the buffer filling up (250280 - 140080 = 110200 events
lost).

The header explains the content of the events. Task name “bash”, the task
PID “1977”, the CPU that it was running on “000”, the latency format
(explained below), the timestamp in . format, the
function name that was traced “sys_close” and the parent function that
called this function “system_call_fastpath”. The timestamp is the time
at which the function was entered.

Latency trace format

When the latency-format option is enabled or when one of the latency
tracers is set, the trace file gives somewhat more information to see
why a latency happened. Here is a typical trace.

tracer: irqsoff

irqsoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: __lock_task_sighand

=> ended at: _raw_spin_unlock_irqrestore

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

  ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
  ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
  ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
  ps-6143    2d..1  306us : <stack trace>

=> trace_hardirqs_on_caller
=> trace_hardirqs_on
=> _raw_spin_unlock_irqrestore
=> do_task_stat
=> proc_tgid_stat
=> proc_single_show
=> seq_read
=> vfs_read
=> sys_read
=> system_call_fastpath

This shows that the current tracer is “irqsoff” tracing the time
for which interrupts were disabled. It gives the trace version (which
never changes) and the version of the kernel upon which this was executed on
(3.10). Then it displays the max latency in microseconds (259 us). The number
of trace entries displayed and the total number (both are four: #4/4).
VP, KP, SP, and HP are always zero and are reserved for later use.
#P is the number of online CPUs (#P:4).

The task is the process that was running when the latency
occurred. (ps pid: 6143).

The start and stop (the functions in which the interrupts were
disabled and enabled respectively) that caused the latencies:

__lock_task_sighand is where the interrupts were disabled.
_raw_spin_unlock_irqrestore is where they were enabled again.

The next lines after the header are the trace itself. The header
explains which is which.

cmd: The name of the process in the trace.

pid: The PID of that process.

CPU#: The CPU which the process was running on.

irqs-off: ‘d’ interrupts are disabled. ‘.’ otherwise.
Note: If the architecture does not support a way to
read the irq flags variable, an ‘X’ will always
be printed here.

need-resched: ‘N’ task need_resched is set, ‘.’ otherwise.

hardirq/softirq:
‘H’ - hard irq occurred inside a softirq.
‘h’ - hard irq is running
‘s’ - soft irq is running
‘.’ - normal context.

preempt-depth: The level of preempt_disabled

The above is mostly meaningful for kernel developers.

time: When the latency-format option is enabled, the trace file
output includes a timestamp relative to the start of the
trace. This differs from the output when latency-format
is disabled, which includes an absolute timestamp.

delay: This is just to help catch your eye a bit better. And
needs to be fixed to be only relative to the same CPU.
The marks are determined by the difference between this
current trace and the next trace.
‘!’ - greater than preempt_mark_thresh (default 100)
‘+’ - greater than 1 microsecond
‘ ‘ - less than or equal to 1 microsecond.

The rest is the same as the ‘trace’ file.

Note, the latency tracers will usually end with a back trace
to easily find where the latency occurred.

trace_options

The trace_options file (or the options directory) is used to control
what gets printed in the trace output, or manipulate the tracers.
To see what is available, simply cat the file:

cat trace_options
print-parent
nosym-offset
nosym-addr
noverbose
noraw
nohex
nobin
noblock
nostacktrace
trace_printk
noftrace_preempt
nobranch
annotate
nouserstacktrace
nosym-userobj
noprintk-msg-only
context-info
latency-format
sleep-time
graph-time
record-cmd
overwrite
nodisable_on_free
irq-info
markers
function-trace

To disable one of the options, echo in the option prepended with
“no”.

echo noprint-parent > trace_options

To enable an option, leave off the “no”.

echo sym-offset > trace_options

Here are the available options:

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

print-parent:
bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul

noprint-parent:
bash-4000 [01] 1477.606694: simple_strtoul

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-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0

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

sym-addr:
bash-4000 [01] 1477.606694: simple_strtoul

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 (kstrtoul)

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.

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

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

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

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.

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

branch - Enable branch tracing with the tracer.

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     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
      <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
      <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore

CPU 2 buffer started

      <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
      <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
      <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock

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

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[+0

x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]

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()).

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

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”.

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.

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.

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.

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)

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

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

tracer: function

entries-in-buffer/entries-written: 144405/9452052 #P:4

TASK-PID CPU# TIMESTAMP FUNCTION

| | | | |

      <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
      <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
      <idle>-0     [002]  23636.756055: enqueue_task <-activate_task

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

function-trace - The latency tracers will enable function tracing
if this option is enabled (default it is). When
it is disabled, the latency tracers do not trace
functions. This keeps the overhead of the tracer down
when performing latency tests.

Note: Some tracers have their own options. They only appear
when the tracer is active.

irqsoff

When interrupts are disabled, the CPU can not react to any other
external event (besides NMIs and SMIs). This prevents the timer
interrupt from triggering or the mouse interrupt from letting
the kernel know of a new mouse event. The result is a latency
with the reaction time.

The irqsoff tracer tracks the time for which interrupts are
disabled. When a new maximum latency is hit, the tracer saves
the trace leading up to that latency point so that every time a
new maximum is reached, the old saved trace is discarded and the
new trace is saved.

To reset the maximum, echo 0 into tracing_max_latency. Here is
an example:

echo 0 > options/function-trace

echo irqsoff > current_tracer

echo 1 > tracing_on

echo 0 > tracing_max_latency

ls -ltr

[…]

echo 0 > tracing_on

cat trace

tracer: irqsoff

irqsoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: run_timer_softirq

=> ended at: run_timer_softirq

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
-0 0dNs3 25us :
=> _raw_spin_unlock_irq
=> run_timer_softirq
=> __do_softirq
=> call_softirq
=> do_softirq
=> irq_exit
=> smp_apic_timer_interrupt
=> apic_timer_interrupt
=> rcu_idle_exit
=> cpu_idle
=> rest_init
=> start_kernel
=> x86_64_start_reservations
=> x86_64_start_kernel

Here we see that that we had a latency of 16 microseconds (which is
very good). The _raw_spin_lock_irq in run_timer_softirq disabled
interrupts. The difference between the 16 and the displayed
timestamp 25us occurred because the clock was incremented
between the time of recording the max latency and the time of
recording the function that had that latency.

Note the above example had function-trace not set. If we set
function-trace, we get a much larger output:

with echo 1 > options/function-trace

tracer: irqsoff

irqsoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: ata_scsi_queuecmd

=> ended at: ata_scsi_queuecmd

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat

[…]
bash-2042 3d..1 67us : delay_tsc <-__delay
bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
bash-2042 3d..1 120us :
=> _raw_spin_unlock_irqrestore
=> ata_scsi_queuecmd
=> scsi_dispatch_cmd
=> scsi_request_fn
=> __blk_run_queue_uncond
=> __blk_run_queue
=> blk_queue_bio
=> generic_make_request
=> submit_bio
=> submit_bh
=> __ext3_get_inode_loc
=> ext3_iget
=> ext3_lookup
=> lookup_real
=> __lookup_hash
=> walk_component
=> lookup_last
=> path_lookupat
=> filename_lookup
=> user_path_at_empty
=> user_path_at
=> vfs_fstatat
=> vfs_stat
=> sys_newstat
=> system_call_fastpath

Here we traced a 71 microsecond latency. But we also see all the
functions that were called during that time. Note that by
enabling function tracing, we incur an added overhead. This
overhead may extend the latency times. But nevertheless, this
trace has provided some very helpful debugging information.

preemptoff

When preemption is disabled, we may be able to receive
interrupts but the task cannot be preempted and a higher
priority task must wait for preemption to be enabled again
before it can preempt a lower priority task.

The preemptoff tracer traces the places that disable preemption.
Like the irqsoff tracer, it records the maximum latency for
which preemption was disabled. The control of preemptoff tracer
is much like the irqsoff tracer.

echo 0 > options/function-trace

echo preemptoff > current_tracer

echo 1 > tracing_on

echo 0 > tracing_max_latency

ls -ltr

[…]

echo 0 > tracing_on

cat trace

tracer: preemptoff

preemptoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: do_IRQ

=> ended at: do_IRQ

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
sshd-1991    1d..1   46us : irq_exit <-do_IRQ
sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
sshd-1991    1d..1   52us : <stack trace>

=> sub_preempt_count
=> irq_exit
=> do_IRQ
=> ret_from_intr

This has some more changes. Preemption was disabled when an
interrupt came in (notice the ‘h’), and was enabled on exit.
But we also see that interrupts have been disabled when entering
the preempt off section and leaving it (the ‘d’). We do not know if
interrupts were enabled in the mean time or shortly after this
was over.

tracer: preemptoff

preemptoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: wake_up_new_task

=> ended at: task_rq_unlock

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
bash-1994    1d..1    1us : source_load <-select_task_rq_fair
bash-1994    1d..1    1us : source_load <-select_task_rq_fair

[…]
bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
bash-1994 1d..1 13us : add_preempt_count <-irq_enter
bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
[…]
bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
bash-1994 1d..2 36us : do_softirq <-irq_exit
bash-1994 1d..2 36us : __do_softirq <-call_softirq
bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
[…]
bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
bash-1994 1dN.2 82us : idle_cpu <-irq_exit
bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
bash-1994 1.N.1 104us :
=> sub_preempt_count
=> _raw_spin_unlock_irqrestore
=> task_rq_unlock
=> wake_up_new_task
=> do_fork
=> sys_clone
=> stub_clone

The above is an example of the preemptoff trace with
function-trace set. Here we see that interrupts were not disabled
the entire time. The irq_enter code lets us know that we entered
an interrupt ‘h’. Before that, the functions being traced still
show that it is not in an interrupt, but we can see from the
functions themselves that this is not the case.

preemptirqsoff

Knowing the locations that have interrupts disabled or
preemption disabled for the longest times is helpful. But
sometimes we would like to know when either preemption and/or
interrupts are disabled.

Consider the following code:

local_irq_disable();
call_function_with_irqs_off();
preempt_disable();
call_function_with_irqs_and_preemption_off();
local_irq_enable();
call_function_with_preemption_off();
preempt_enable();

The irqsoff tracer will record the total length of
call_function_with_irqs_off() and
call_function_with_irqs_and_preemption_off().

The preemptoff tracer will record the total length of
call_function_with_irqs_and_preemption_off() and
call_function_with_preemption_off().

But neither will trace the time that interrupts and/or
preemption is disabled. This total time is the time that we can
not schedule. To record this time, use the preemptirqsoff
tracer.

Again, using this trace is much like the irqsoff and preemptoff
tracers.

echo 0 > options/function-trace

echo preemptirqsoff > current_tracer

echo 1 > tracing_on

echo 0 > tracing_max_latency

ls -ltr

[…]

echo 0 > tracing_on

cat trace

tracer: preemptirqsoff

preemptirqsoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: ata_scsi_queuecmd

=> ended at: ata_scsi_queuecmd

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

  ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
  ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
  ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
  ls-2230    3...1  111us : <stack trace>

=> sub_preempt_count
=> _raw_spin_unlock_irqrestore
=> ata_scsi_queuecmd
=> scsi_dispatch_cmd
=> scsi_request_fn
=> __blk_run_queue_uncond
=> __blk_run_queue
=> blk_queue_bio
=> generic_make_request
=> submit_bio
=> submit_bh
=> ext3_bread
=> ext3_dir_bread
=> htree_dirblock_to_tree
=> ext3_htree_fill_tree
=> ext3_readdir
=> vfs_readdir
=> sys_getdents
=> system_call_fastpath

The trace_hardirqs_off_thunk is called from assembly on x86 when
interrupts are disabled in the assembly code. Without the
function tracing, we do not know if interrupts were enabled
within the preemption points. We do see that it started with
preemption enabled.

Here is a trace with function-trace set:

tracer: preemptirqsoff

preemptirqsoff latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)

—————–

=> started at: schedule

=> ended at: mutex_unlock

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

kworker/-59 3…1 0us : __schedule <-schedule
kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
kworker/-59 3d..2 1us : deactivate_task <-__schedule
kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
ls-2269 3d..2 7us : finish_task_switch <-__schedule
ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
ls-2269 3d..2 8us : irq_enter <-do_IRQ
ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
ls-2269 3d..2 9us : add_preempt_count <-irq_enter
ls-2269 3d.h2 9us : exit_idle <-do_IRQ
[…]
ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
ls-2269 3d.h2 20us : irq_exit <-do_IRQ
ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
ls-2269 3d..3 21us : do_softirq <-irq_exit
ls-2269 3d..3 21us : __do_softirq <-call_softirq
ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
ls-2269 3d.s5 31us : irq_enter <-do_IRQ
ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
[…]
ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
ls-2269 3d.H5 32us : exit_idle <-do_IRQ
ls-2269 3d.H5 32us : handle_irq <-do_IRQ
ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
[…]
ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
ls-2269 3d..3 159us : idle_cpu <-irq_exit
ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
ls-2269 3d… 161us : __mutex_unlock_slowpath <-mutex_unlock
ls-2269 3d… 162us+: trace_hardirqs_on <-mutex_unlock
ls-2269 3d… 186us :
=> __mutex_unlock_slowpath
=> mutex_unlock
=> process_output
=> n_tty_write
=> tty_write
=> vfs_write
=> sys_write
=> system_call_fastpath

This is an interesting trace. It started with kworker running and
scheduling out and ls taking over. But as soon as ls released the
rq lock and enabled interrupts (but not preemption) an interrupt
triggered. When the interrupt finished, it started running softirqs.
But while the softirq was running, another interrupt triggered.
When an interrupt is running inside a softirq, the annotation is ‘H’.

wakeup

One common case that people are interested in tracing is the
time it takes for a task that is woken to actually wake up.
Now for non Real-Time tasks, this can be arbitrary. But tracing
it none the less can be interesting.

Without function tracing:

echo 0 > options/function-trace

echo wakeup > current_tracer

echo 1 > tracing_on

echo 0 > tracing_max_latency

chrt -f 5 sleep 1

echo 0 > tracing_on

cat trace

tracer: wakeup

wakeup latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)

—————–

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
-0 3d..3 15us : __schedule <-schedule
-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H

The tracer only traces the highest priority task in the system
to avoid tracing the normal circumstances. Here we see that
the kworker with a nice priority of -20 (not very nice), took
just 15 microseconds from the time it woke up, to the time it
ran.

Non Real-Time tasks are not that interesting. A more interesting
trace is to concentrate only on Real-Time tasks.

wakeup_rt

In a Real-Time environment it is very important to know the
wakeup time it takes for the highest priority task that is woken
up to the time that it executes. This is also known as “schedule
latency”. I stress the point that this is about RT tasks. It is
also important to know the scheduling latency of non-RT tasks,
but the average schedule latency is better for non-RT tasks.
Tools like LatencyTop are more appropriate for such
measurements.

Real-Time environments are interested in the worst case latency.
That is the longest latency it takes for something to happen,
and not the average. We can have a very fast scheduler that may
only have a large latency once in a while, but that would not
work well with Real-Time tasks. The wakeup_rt tracer was designed
to record the worst case wakeups of RT tasks. Non-RT tasks are
not recorded because the tracer only records one worst case and
tracing non-RT tasks that are unpredictable will overwrite the
worst case latency of RT tasks (just run the normal wakeup
tracer for a while to see that effect).

Since this tracer only deals with RT tasks, we will run this
slightly differently than we did with the previous tracers.
Instead of performing an ‘ls’, we will run ‘sleep 1’ under
‘chrt’ which changes the priority of the task.

echo 0 > options/function-trace

echo wakeup_rt > current_tracer

echo 1 > tracing_on

echo 0 > tracing_max_latency

chrt -f 5 sleep 1

echo 0 > tracing_on

cat trace

tracer: wakeup

tracer: wakeup_rt

wakeup_rt latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)

—————–

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
-0 3d..3 5us : __schedule <-schedule
-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep

Running this on an idle system, we see that it only took 5 microseconds
to perform the task switch. Note, since the trace point in the schedule
is before the actual “switch”, we stop the tracing when the recorded task
is about to schedule in. This may change if we add a new marker at the
end of the scheduler.

Notice that the recorded task is ‘sleep’ with the PID of 2389
and it has an rt_prio of 5. This priority is user-space priority
and not the internal kernel priority. The policy is 1 for
SCHED_FIFO and 2 for SCHED_RR.

Note, that the trace data shows the internal priority (99 - rtprio).

-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep

The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
and in the running state ‘R’. The sleep task was scheduled in with
2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
and it too is in the running state.

Doing the same with chrt -r 5 and function-trace set.

echo 1 > options/function-trace

tracer: wakeup_rt

wakeup_rt latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)

—————–

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
-0 3d.h3 3us : resched_curr <-check_preempt_curr
-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
-0 3dNh1 7us : clockevents_program_event <-tick_program_event
-0 3dNh1 8us : ktime_get <-clockevents_program_event
-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
-0 3dNh1 9us : sub_preempt_count <-irq_exit
-0 3dN.2 9us : idle_cpu <-irq_exit
-0 3dN.2 9us : rcu_irq_exit <-irq_exit
-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
-0 3dN.2 10us : sub_preempt_count <-irq_exit
-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit
-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz
-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz
-0 3dN.2 14us : sched_avg_update <-__update_cpu_load
-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz
-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit
-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
-0 3dN.2 18us : clockevents_program_event <-tick_program_event
-0 3dN.2 18us : ktime_get <-clockevents_program_event
-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
-0 3dN.2 23us : clockevents_program_event <-tick_program_event
-0 3dN.2 23us : ktime_get <-clockevents_program_event
-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
-0 3dN.1 24us : account_idle_time <-account_idle_ticks
-0 3.N.1 25us : sub_preempt_count <-cpu_idle
-0 3.N.. 25us : schedule <-cpu_idle
-0 3.N.. 25us : __schedule <-preempt_schedule
-0 3.N.. 26us : add_preempt_count <-__schedule
-0 3.N.1 26us : rcu_note_context_switch <-__schedule
-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
-0 3dN.2 28us : put_prev_task_idle <-__schedule
-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
-0 3d..3 29us : __schedule <-preempt_schedule
-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep

This isn’t that big of a trace, even with function tracing enabled,
so I included the entire trace.

The interrupt went off while when the system was idle. Somewhere
before task_woken_rt() was called, the NEED_RESCHED flag was set,
this is indicated by the first occurrence of the ‘N’ flag.

Latency tracing and events

As function tracing can induce a much larger latency, but without
seeing what happens within the latency it is hard to know what
caused it. There is a middle ground, and that is with enabling
events.

echo 0 > options/function-trace

echo wakeup_rt > current_tracer

echo 1 > events/enable

echo 1 > tracing_on

echo 0 > tracing_max_latency

chrt -f 5 sleep 1

echo 0 > tracing_on

cat trace

tracer: wakeup_rt

wakeup_rt latency trace v1.1.5 on 3.8.0-test+

——————————————————————–

latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)

—————–

| task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)

—————–

_——=> CPU#

/ _—–=> irqs-off

| / _—-=> need-resched

|| / _—=> hardirq/softirq

||| / _–=> preempt-depth

|||| / delay

cmd pid ||||| time | caller

\ / ||||| \ | /

-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
-0 2.N.2 2us : power_end: cpu_id=2
-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
-0 2.N.2 5us : rcu_utilization: Start context switch
-0 2.N.2 5us : rcu_utilization: End context switch
-0 2d..3 6us : __schedule <-schedule
-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep

function

This tracer is the function tracer. Enabling the function tracer
can be done from the debug file system. Make sure the
ftrace_enabled is set; otherwise this tracer is a nop.
See the “ftrace_enabled” section below.

sysctl kernel.ftrace_enabled=1

echo function > current_tracer

echo 1 > tracing_on

usleep 1

echo 0 > tracing_on

cat trace

tracer: function

entries-in-buffer/entries-written: 24799/24799 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

        bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
        bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
        bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
        bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
        bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
        bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
        bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
        bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify

[…]

Note: function tracer uses ring buffers to store the above
entries. The newest data may overwrite the oldest data.
Sometimes using echo to stop the trace is not sufficient because
the tracing could have overwritten the data that you wanted to
record. For this reason, it is sometimes better to disable
tracing directly from a program. This allows you to stop the
tracing at the point that you hit the part that you are
interested in. To disable the tracing directly from a C program,
something like following code snippet can be used:

int trace_fd;
[…]
int main(int argc, char *argv[]) {
[…]
trace_fd = open(tracing_file(“tracing_on”), O_WRONLY);
[…]
if (condition_hit()) {
write(trace_fd, “0”, 1);
}
[…]
}

Single thread tracing

By writing into set_ftrace_pid you can trace a
single thread. For example:

cat set_ftrace_pid

no pid

echo 3111 > set_ftrace_pid

cat set_ftrace_pid

3111

echo function > current_tracer

cat trace | head

tracer: function

TASK-PID CPU# TIMESTAMP FUNCTION

| | | | |

 yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
 yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
 yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
 yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
 yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
 yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll

echo -1 > set_ftrace_pid

cat trace |head

tracer: function

TASK-PID CPU# TIMESTAMP FUNCTION

| | | | |

CPU 3 buffer started

 yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
 yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
 yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
 yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
 yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit

If you want to trace a function when executing, you could use
something like this simple program:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define _STR(x) #x
#define STR(x) _STR(x)
#define MAX_PATH 256

const char *find_debugfs(void)
{
static char debugfs[MAX_PATH+1];
static int debugfs_found;
char type[100];
FILE *fp;

   if (debugfs_found)
           return debugfs;

   if ((fp = fopen("/proc/mounts","r")) == NULL) {
           perror("/proc/mounts");
           return NULL;
   }

   while (fscanf(fp, "%*s %"
                 STR(MAX_PATH)
                 "s %99s %*s %*d %*d\n",
                 debugfs, type) == 2) {
           if (strcmp(type, "debugfs") == 0)
                   break;
   }
   fclose(fp);

   if (strcmp(type, "debugfs") != 0) {
           fprintf(stderr, "debugfs not mounted");
           return NULL;
   }

   strcat(debugfs, "/tracing/");
   debugfs_found = 1;

   return debugfs;

}

const char *tracing_file(const char *file_name)
{
static char trace_file[MAX_PATH+1];
snprintf(trace_file, MAX_PATH, “%s/%s”, find_debugfs(), file_name);
return trace_file;
}

int main (int argc, char **argv)
{
if (argc < 1)
exit(-1);

    if (fork() > 0) {
            int fd, ffd;
            char line[64];
            int s;

            ffd = open(tracing_file("current_tracer"), O_WRONLY);
            if (ffd < 0)
                    exit(-1);
            write(ffd, "nop", 3);

            fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
            s = sprintf(line, "%d\n", getpid());
            write(fd, line, s);

            write(ffd, "function", 8);

            close(fd);
            close(ffd);

            execvp(argv[1], argv+1);
    }

    return 0;

}

Or this simple script!

---

#!/bin/bash

debugfs=sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts
echo nop > $debugfs/tracing/current_tracer
echo 0 > $debugfs/tracing/tracing_on
echo $$ > $debugfs/tracing/set_ftrace_pid
echo function > $debugfs/tracing/current_tracer
echo 1 > $debugfs/tracing/tracing_on
exec “$@”

---

function graph tracer

This tracer is similar to the function tracer except that it
probes a function on its entry and its exit. This is done by
using a dynamically allocated stack of return addresses in each
task_struct. On function entry the tracer overwrites the return
address of each function traced to set a custom probe. Thus the
original return address is stored on the stack of return address
in the task_struct.

Probing on both ends of a function leads to special features
such as:

• measure of a function’s time execution
• having a reliable call stack to draw function calls graph

This tracer is useful in several situations:

• you want to find the reason of a strange kernel behavior and
  need to see what happens in detail on any areas (or specific
  ones).

• you are experiencing weird latencies but it’s difficult to
  find its origin.

• you want to find quickly which path is taken by a specific
  function

• you just want to peek inside a working kernel and want to see
  what happens there.

tracer: function_graph

CPU DURATION FUNCTION CALLS

| | | | | | |

0.           |  sys_open() {
   

1.           |    do_sys_open() {
   

2.           |      getname() {
   

3.           |        kmem_cache_alloc() {
   

4. 1.382 us | __might_sleep();
5. 2.478 us | }

6.           |        strncpy_from_user() {
   

7.           |          might_fault() {
   

8. 1.389 us | __might_sleep();
9. 2.553 us | }
10. 3.807 us | }
11. 7.876 us | }

12.           |      alloc_fd() {
    

13. 0.668 us | _spin_lock();
14. 0.570 us | expand_files();
15. 0.586 us | _spin_unlock();

There are several columns that can be dynamically
enabled/disabled. You can use every combination of options you
want, depending on your needs.

• The cpu number on which the function executed is default
  enabled. It is sometimes better to only trace one cpu (see
  tracing_cpu_mask file) or you might sometimes see unordered
  function calls while cpu tracing switch.

  hide: echo nofuncgraph-cpu > trace_options
  show: echo funcgraph-cpu > trace_options

• The duration (function’s time of execution) is displayed on
  the closing bracket line of a function or on the same line
  than the current function in case of a leaf one. It is default
  enabled.

  hide: echo nofuncgraph-duration > trace_options
  show: echo funcgraph-duration > trace_options

• The overhead field precedes the duration field in case of
  reached duration thresholds.

  hide: echo nofuncgraph-overhead > trace_options
  show: echo funcgraph-overhead > trace_options
  depends on: funcgraph-duration

  ie:

  0.           |    up_write() {
     

  1. 0.646 us | _spin_lock_irqsave();
  2. 0.684 us | _spin_unlock_irqrestore();
  3. 3.123 us | }
  4. 0.548 us | fput();
  5. • 58.628 us | }

  […]

  0.           |      putname() {
     

  1.           |        kmem_cache_free() {
     

  2. 0.518 us | __phys_addr();
  3. 1.757 us | }
  4. 2.861 us | }
  5. ! 115.305 us | }
  6. ! 116.402 us | }
  • means that the function exceeded 10 usecs.
    ! means that the function exceeded 100 usecs.

• The task/pid field displays the thread cmdline and pid which
  executed the function. It is default disabled.

  hide: echo nofuncgraph-proc > trace_options
  show: echo funcgraph-proc > trace_options

  ie:

  tracer: function_graph

  CPU TASK/PID DURATION FUNCTION CALLS

  | | | | | | | | |

  0. sh-4802 | | d_free() {
  1. sh-4802 | | call_rcu() {
  2. sh-4802 | | __call_rcu() {
  3. sh-4802 | 0.616 us | rcu_process_gp_end();
  4. sh-4802 | 0.586 us | check_for_new_grace_period();
  5. sh-4802 | 2.899 us | }
  6. sh-4802 | 4.040 us | }
  7. sh-4802 | 5.151 us | }
  8. sh-4802 | + 49.370 us | }

• The absolute time field is an absolute timestamp given by the
  system clock since it started. A snapshot of this time is
  given on each entry/exit of functions

  hide: echo nofuncgraph-abstime > trace_options
  show: echo funcgraph-abstime > trace_options

  ie:

  TIME CPU DURATION FUNCTION CALLS

  | | | | | | | |

  360.774522 | 1) 0.541 us | }
  360.774522 | 1) 4.663 us | }
  360.774523 | 1) 0.541 us | __wake_up_bit();
  360.774524 | 1) 6.796 us | }
  360.774524 | 1) 7.952 us | }
  360.774525 | 1) 9.063 us | }
  360.774525 | 1) 0.615 us | journal_mark_dirty();
  360.774527 | 1) 0.578 us | __brelse();
  360.774528 | 1) | reiserfs_prepare_for_journal() {
  360.774528 | 1) | unlock_buffer() {
  360.774529 | 1) | wake_up_bit() {
  360.774529 | 1) | bit_waitqueue() {
  360.774530 | 1) 0.594 us | __phys_addr();

You can put some comments on specific functions by using
trace_printk() For example, if you want to put a comment inside
the __might_sleep() function, you just have to include
<linux/ftrace.h> and call trace_printk() inside __might_sleep()

trace_printk(“I’m a comment!\n”)

will produce:

1.           |             __might_sleep() {
   

2.           |                /* I'm a comment! */
   

3. 1.449 us | }

You might find other useful features for this tracer in the
following “dynamic ftrace” section such as tracing only specific
functions or tasks.

dynamic ftrace

If CONFIG_DYNAMIC_FTRACE is set, the system will run with
virtually no overhead when function tracing is disabled. The way
this works is the mcount function call (placed at the start of
every kernel function, produced by the -pg switch in gcc),
starts of pointing to a simple return. (Enabling FTRACE will
include the -pg switch in the compiling of the kernel.)

At compile time every C file object is run through the
recordmcount program (located in the scripts directory). This
program will parse the ELF headers in the C object to find all
the locations in the .text section that call mcount. (Note, only
white listed .text sections are processed, since processing other
sections like .init.text may cause races due to those sections
being freed unexpectedly).

A new section called “__mcount_loc” is created that holds
references to all the mcount call sites in the .text section.
The recordmcount program re-links this section back into the
original object. The final linking stage of the kernel will add all these
references into a single table.

On boot up, before SMP is initialized, the dynamic ftrace code
scans this table and updates all the locations into nops. It
also records the locations, which are added to the
available_filter_functions list. Modules are processed as they
are loaded and before they are executed. When a module is
unloaded, it also removes its functions from the ftrace function
list. This is automatic in the module unload code, and the
module author does not need to worry about it.

When tracing is enabled, the process of modifying the function
tracepoints is dependent on architecture. The old method is to use
kstop_machine to prevent races with the CPUs executing code being
modified (which can cause the CPU to do undesirable things, especially
if the modified code crosses cache (or page) boundaries), and the nops are
patched back to calls. But this time, they do not call mcount
(which is just a function stub). They now call into the ftrace
infrastructure.

The new method of modifying the function tracepoints is to place
a breakpoint at the location to be modified, sync all CPUs, modify
the rest of the instruction not covered by the breakpoint. Sync
all CPUs again, and then remove the breakpoint with the finished
version to the ftrace call site.

Some archs do not even need to monkey around with the synchronization,
and can just slap the new code on top of the old without any
problems with other CPUs executing it at the same time.

One special side-effect to the recording of the functions being
traced is that we can now selectively choose which functions we
wish to trace and which ones we want the mcount calls to remain
as nops.

Two files are used, one for enabling and one for disabling the
tracing of specified functions. They are:

set_ftrace_filter

and

set_ftrace_notrace

A list of available functions that you can add to these files is
listed in:

available_filter_functions

cat available_filter_functions

put_prev_task_idle
kmem_cache_create
pick_next_task_rt
get_online_cpus
pick_next_task_fair
mutex_lock
[…]

If I am only interested in sys_nanosleep and hrtimer_interrupt:

echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter

echo function > current_tracer

echo 1 > tracing_on

usleep 1

echo 0 > tracing_on

cat trace

tracer: function

entries-in-buffer/entries-written: 5/5 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

      usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
      <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
      usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
      <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
      <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt

To see which functions are being traced, you can cat the file:

cat set_ftrace_filter

hrtimer_interrupt
sys_nanosleep

Perhaps this is not enough. The filters also allow simple wild
cards. Only the following are currently available

* - will match functions that begin with
* - will match functions that end with
- will match functions that have in it

These are the only wild cards which are supported.

* will not work.

Note: It is better to use quotes to enclose the wild cards,
otherwise the shell may expand the parameters into names
of files in the local directory.

echo ‘hrtimer_*’ > set_ftrace_filter

Produces:

tracer: function

entries-in-buffer/entries-written: 897/897 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

      <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
      <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
      <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
      <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
      <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
      <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
      <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
      <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem

Notice that we lost the sys_nanosleep.

cat set_ftrace_filter

hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper

This is because the ‘>’ and ‘>>’ act just like they do in bash.
To rewrite the filters, use ‘>’
To append to the filters, use ‘>>’

To clear out a filter so that all functions will be recorded
again:

echo > set_ftrace_filter

cat set_ftrace_filter

Again, now we want to append.

echo sys_nanosleep > set_ftrace_filter

cat set_ftrace_filter

sys_nanosleep

echo ‘hrtimer_*’ >> set_ftrace_filter

cat set_ftrace_filter

hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
sys_nanosleep
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper

The set_ftrace_notrace prevents those functions from being
traced.

echo ‘preempt‘ ‘lock‘ > set_ftrace_notrace

Produces:

tracer: function

entries-in-buffer/entries-written: 39608/39608 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

        bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
        bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
        bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
        bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
        bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
        bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
        bash-1994  [000] ....  4342.324899: do_truncate <-do_last
        bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
        bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
        bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
        bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
        bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time

We can see that there’s no more lock or preempt tracing.

Dynamic ftrace with the function graph tracer

Although what has been explained above concerns both the
function tracer and the function-graph-tracer, there are some
special features only available in the function-graph tracer.

If you want to trace only one function and all of its children,
you just have to echo its name into set_graph_function:

echo __do_fault > set_graph_function

will produce the following “expanded” trace of the __do_fault()
function:

0.           |  __do_fault() {
   

1.           |    filemap_fault() {
   

2.           |      find_lock_page() {
   

3. 0.804 us | find_get_page();

4.           |        __might_sleep() {
   

5. 1.329 us | }
6. 3.904 us | }
7. 4.979 us | }
8. 0.653 us | _spin_lock();
9. 0.578 us | page_add_file_rmap();
10. 0.525 us | native_set_pte_at();
11. 0.585 us | _spin_unlock();

12.           |    unlock_page() {
    

13. 0.541 us | page_waitqueue();
14. 0.639 us | __wake_up_bit();
15. 2.786 us | }
16. • 14.237 us | }

17.           |  __do_fault() {
    

18.           |    filemap_fault() {
    

19.           |      find_lock_page() {
    

20. 0.698 us | find_get_page();

21.           |        __might_sleep() {
    

22. 1.412 us | }
23. 3.950 us | }
24. 5.098 us | }
25. 0.631 us | _spin_lock();
26. 0.571 us | page_add_file_rmap();
27. 0.526 us | native_set_pte_at();
28. 0.586 us | _spin_unlock();

29.           |    unlock_page() {
    

30. 0.533 us | page_waitqueue();
31. 0.638 us | __wake_up_bit();
32. 2.793 us | }
33. • 14.012 us | }

You can also expand several functions at once:

echo sys_open > set_graph_function
echo sys_close >> set_graph_function

Now if you want to go back to trace all functions you can clear
this special filter via:

echo > set_graph_function

ftrace_enabled

Note, the proc sysctl ftrace_enable is a big on/off switch for the
function tracer. By default it is enabled (when function tracing is
enabled in the kernel). If it is disabled, all function tracing is
disabled. This includes not only the function tracers for ftrace, but
also for any other uses (perf, kprobes, stack tracing, profiling, etc).

Please disable this with care.

This can be disable (and enabled) with:

sysctl kernel.ftrace_enabled=0
sysctl kernel.ftrace_enabled=1

or

echo 0 > /proc/sys/kernel/ftrace_enabled
echo 1 > /proc/sys/kernel/ftrace_enabled

Filter commands

A few commands are supported by the set_ftrace_filter interface.
Trace commands have the following format:

::

The following commands are supported:

• mod
  This command enables function filtering per module. The
  parameter defines the module. For example, if only the write*
  functions in the ext3 module are desired, run:

  echo ‘write*:mod:ext3’ > set_ftrace_filter

  This command interacts with the filter in the same way as
  filtering based on function names. Thus, adding more functions
  in a different module is accomplished by appending (>>) to the
  filter file. Remove specific module functions by prepending
  ‘!’:

  echo ‘!writeback*:mod:ext3’ >> set_ftrace_filter

• traceon/traceoff
  These commands turn tracing on and off when the specified
  functions are hit. The parameter determines how many times the
  tracing system is turned on and off. If unspecified, there is
  no limit. For example, to disable tracing when a schedule bug
  is hit the first 5 times, run:

  echo ‘__schedule_bug:traceoff:5’ > set_ftrace_filter

  To always disable tracing when __schedule_bug is hit:

  echo ‘__schedule_bug:traceoff’ > set_ftrace_filter

  These commands are cumulative whether or not they are appended
  to set_ftrace_filter. To remove a command, prepend it by ‘!’
  and drop the parameter:

  echo ‘!__schedule_bug:traceoff:0’ > set_ftrace_filter

  The above removes the traceoff command for __schedule_bug
  that have a counter. To remove commands without counters:

  echo ‘!__schedule_bug:traceoff’ > set_ftrace_filter

• snapshot
  Will cause a snapshot to be triggered when the function is hit.

  echo ‘native_flush_tlb_others:snapshot’ > set_ftrace_filter

  To only snapshot once:

  echo ‘native_flush_tlb_others:snapshot:1’ > set_ftrace_filter

  To remove the above commands:

  echo ‘!native_flush_tlb_others:snapshot’ > set_ftrace_filter
  echo ‘!native_flush_tlb_others:snapshot:0’ > set_ftrace_filter

• enable_event/disable_event
  These commands can enable or disable a trace event. Note, because
  function tracing callbacks are very sensitive, when these commands
  are registered, the trace point is activated, but disabled in
  a “soft” mode. That is, the tracepoint will be called, but
  just will not be traced. The event tracepoint stays in this mode
  as long as there’s a command that triggers it.

  echo ‘try_to_wake_up:enable_event:sched:sched_switch:2’ > \

    set_ftrace_filter
  

  The format is:

  :enable_event::[:count]
  :disable_event::[:count]

  To remove the events commands:

echo ‘!try_to_wake_up:enable_event:sched:sched_switch:0’ >
set_ftrace_filter
echo ‘!schedule:disable_event:sched:sched_switch’ >
set_ftrace_filter

• dump
  When the function is hit, it will dump the contents of the ftrace
  ring buffer to the console. This is useful if you need to debug
  something, and want to dump the trace when a certain function
  is hit. Perhaps its a function that is called before a tripple
  fault happens and does not allow you to get a regular dump.

• cpudump
  When the function is hit, it will dump the contents of the ftrace
  ring buffer for the current CPU to the console. Unlike the “dump”
  command, it only prints out the contents of the ring buffer for the
  CPU that executed the function that triggered the dump.

trace_pipe

The trace_pipe outputs the same content as the trace file, but
the effect on the tracing is different. Every read from
trace_pipe is consumed. This means that subsequent reads will be
different. The trace is live.

echo function > current_tracer

cat trace_pipe > /tmp/trace.out &

[1] 4153

echo 1 > tracing_on

usleep 1

echo 0 > tracing_on

cat trace

tracer: function

entries-in-buffer/entries-written: 0/0 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

cat /tmp/trace.out

        bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
        bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
        bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
        bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
        bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
        bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
        bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
        bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
        bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath

Note, reading the trace_pipe file will block until more input is
added.

trace entries

Having too much or not enough data can be troublesome in
diagnosing an issue in the kernel. The file buffer_size_kb is
used to modify the size of the internal trace buffers. The
number listed is the number of entries that can be recorded per
CPU. To know the full size, multiply the number of possible CPUs
with the number of entries.

cat buffer_size_kb

1408 (units kilobytes)

Or simply read buffer_total_size_kb

cat buffer_total_size_kb

5632

To modify the buffer, simple echo in a number (in 1024 byte segments).

echo 10000 > buffer_size_kb

cat buffer_size_kb

10000 (units kilobytes)

It will try to allocate as much as possible. If you allocate too
much, it can cause Out-Of-Memory to trigger.

echo 1000000000000 > buffer_size_kb

-bash: echo: write error: Cannot allocate memory

cat buffer_size_kb

85

The per_cpu buffers can be changed individually as well:

echo 10000 > per_cpu/cpu0/buffer_size_kb

echo 100 > per_cpu/cpu1/buffer_size_kb

When the per_cpu buffers are not the same, the buffer_size_kb
at the top level will just show an X

cat buffer_size_kb

X

This is where the buffer_total_size_kb is useful:

cat buffer_total_size_kb

12916

Writing to the top level buffer_size_kb will reset all the buffers
to be the same again.

Snapshot

CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
available to all non latency tracers. (Latency tracers which
record max latency, such as “irqsoff” or “wakeup”, can’t use
this feature, since those are already using the snapshot
mechanism internally.)

Snapshot preserves a current trace buffer at a particular point
in time without stopping tracing. Ftrace swaps the current
buffer with a spare buffer, and tracing continues in the new
current (=previous spare) buffer.

The following debugfs files in “tracing” are related to this
feature:

snapshot:

This is used to take a snapshot and to read the output
of the snapshot. Echo 1 into this file to allocate a
spare buffer and to take a snapshot (swap), then read
the snapshot from this file in the same format as
"trace" (described above in the section "The File
System"). Both reads snapshot and tracing are executable
in parallel. When the spare buffer is allocated, echoing
0 frees it, and echoing else (positive) values clear the
snapshot contents.
More details are shown in the table below.

status\input  |     0      |     1      |    else    |
--------------+------------+------------+------------+
not allocated |(do nothing)| alloc+swap |(do nothing)|
--------------+------------+------------+------------+
allocated     |    free    |    swap    |   clear    |
--------------+------------+------------+------------+

Here is an example of using the snapshot feature.

echo 1 > events/sched/enable

echo 1 > snapshot

cat snapshot

tracer: nop

entries-in-buffer/entries-written: 71/71 #P:8

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

      <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
       sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120

[…]
-0 [002] d… 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120

cat trace

tracer: nop

entries-in-buffer/entries-written: 77/77 #P:8

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

      <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120

snapshot-test-2-2229 [002] d… 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
[…]

If you try to use this snapshot feature when current tracer is
one of the latency tracers, you will get the following results.

echo wakeup > current_tracer

echo 1 > snapshot

bash: echo: write error: Device or resource busy

cat snapshot

cat: snapshot: Device or resource busy

Instances

In the debugfs tracing directory is a directory called “instances”.
This directory can have new directories created inside of it using
mkdir, and removing directories with rmdir. The directory created
with mkdir in this directory will already contain files and other
directories after it is created.

mkdir instances/foo

ls instances/foo

buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
set_event snapshot trace trace_clock trace_marker trace_options
trace_pipe tracing_on

As you can see, the new directory looks similar to the tracing directory
itself. In fact, it is very similar, except that the buffer and
events are agnostic from the main director, or from any other
instances that are created.

The files in the new directory work just like the files with the
same name in the tracing directory except the buffer that is used
is a separate and new buffer. The files affect that buffer but do not
affect the main buffer with the exception of trace_options. Currently,
the trace_options affect all instances and the top level buffer
the same, but this may change in future releases. That is, options
may become specific to the instance they reside in.

Notice that none of the function tracer files are there, nor is
current_tracer and available_tracers. This is because the buffers
can currently only have events enabled for them.

mkdir instances/foo

mkdir instances/bar

mkdir instances/zoot

echo 100000 > buffer_size_kb

echo 1000 > instances/foo/buffer_size_kb

echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb

echo function > current_trace

echo 1 > instances/foo/events/sched/sched_wakeup/enable

echo 1 > instances/foo/events/sched/sched_wakeup_new/enable

echo 1 > instances/foo/events/sched/sched_switch/enable

echo 1 > instances/bar/events/irq/enable

echo 1 > instances/zoot/events/syscalls/enable

cat trace_pipe

CPU:2 [LOST 11745 EVENTS]
bash-2044 [002] …. 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
bash-2044 [002] d… 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
bash-2044 [002] d… 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
bash-2044 [002] d… 10594.481034: __mod_zone_page_state <-get_page_from_freelist
bash-2044 [002] d… 10594.481034: zone_statistics <-get_page_from_freelist
bash-2044 [002] d… 10594.481034: __inc_zone_state <-zone_statistics
bash-2044 [002] d… 10594.481034: __inc_zone_state <-zone_statistics
bash-2044 [002] …. 10594.481035: arch_dup_task_struct <-copy_process
[…]

cat instances/foo/trace_pipe

        bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
        bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
      <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
      <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
 rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
        bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
        bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
        bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
 kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
 kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120

[…]

cat instances/bar/trace_pipe

 migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
      <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
        bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
        bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
        bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
        bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
        bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
        bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
        sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
        sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
        sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
        sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled

[…]

cat instances/zoot/trace

tracer: nop

entries-in-buffer/entries-written: 18996/18996 #P:4

_—–=> irqs-off

/ _—-=> need-resched

| / _—=> hardirq/softirq

|| / _–=> preempt-depth

||| / delay

TASK-PID CPU# |||| TIMESTAMP FUNCTION

| | | |||| | |

        bash-1998  [000] d...   140.733501: sys_write -> 0x2
        bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
        bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
        bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
        bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
        bash-1998  [000] d...   140.733510: sys_close(fd: a)
        bash-1998  [000] d...   140.733510: sys_close -> 0x0
        bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
        bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
        bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
        bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0

You can see that the trace of the top most trace buffer shows only
the function tracing. The foo instance displays wakeups and task
switches.

To remove the instances, simply delete their directories:

rmdir instances/foo

rmdir instances/bar

rmdir instances/zoot

Note, if a process has a trace file open in one of the instance
directories, the rmdir will fail with EBUSY.

Stack trace

Since the kernel has a fixed sized stack, it is important not to
waste it in functions. A kernel developer must be conscience of
what they allocate on the stack. If they add too much, the system
can be in danger of a stack overflow, and corruption will occur,
usually leading to a system panic.

There are some tools that check this, usually with interrupts
periodically checking usage. But if you can perform a check
at every function call that will become very useful. As ftrace provides
a function tracer, it makes it convenient to check the stack size
at every function call. This is enabled via the stack tracer.

CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
To enable it, write a ‘1’ into /proc/sys/kernel/stack_tracer_enabled.

echo 1 > /proc/sys/kernel/stack_tracer_enabled

You can also enable it from the kernel command line to trace
the stack size of the kernel during boot up, by adding “stacktrace”
to the kernel command line parameter.

After running it for a few minutes, the output looks like:

cat stack_max_size

2928

cat stack_trace

    Depth    Size   Location    (18 entries)
    -----    ----   --------

0. 2928     224   update_sd_lb_stats+0xbc/0x4ac
   

1. 2704     160   find_busiest_group+0x31/0x1f1
   

2. 2544     256   load_balance+0xd9/0x662
   

3. 2288      80   idle_balance+0xbb/0x130
   

4. 2208     128   __schedule+0x26e/0x5b9
   

5. 2080      16   schedule+0x64/0x66
   

6. 2064     128   schedule_timeout+0x34/0xe0
   

7. 1936     112   wait_for_common+0x97/0xf1
   

8. 1824      16   wait_for_completion+0x1d/0x1f
   

9. 1808     128   flush_work+0xfe/0x119
   

10. 1680      16   tty_flush_to_ldisc+0x1e/0x20
    

11. 1664      48   input_available_p+0x1d/0x5c
    

12. 1616      48   n_tty_poll+0x6d/0x134
    

13. 1568      64   tty_poll+0x64/0x7f
    

14. 1504     880   do_select+0x31e/0x511
    

15.  624     400   core_sys_select+0x177/0x216
    

16.  224      96   sys_select+0x91/0xb9
    

17.  128     128   system_call_fastpath+0x16/0x1b
    

Note, if -mfentry is being used by gcc, functions get traced before
they set up the stack frame. This means that leaf level functions
are not tested by the stack tracer when -mfentry is used.

Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.

---

More details can be found in the source code, in the
kernel/trace/*.c files.