--- /dev/null
+ 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 <secs>.<usecs> 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' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
+ 'n' only TIF_NEED_RESCHED is set,
+ 'p' only PREEMPT_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 1 second
+ '#' - greater than 1000 microsecond
+ '!' - greater than 100 microsecond
+ '+' - greater than 10 microsecond
+ ' ' - less than or equal to 10 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 <c0339346>
+
+ 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
+# \ / ||||| \ | /
+ <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
+ <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
+ <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
+ <idle>-0 0dNs3 25us : <stack trace>
+ => _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 : <stack trace>
+ => _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 : <stack trace>
+ => 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 : <stack trace>
+ => __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
+# \ / ||||| \ | /
+ <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
+ <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d..3 15us : __schedule <-schedule
+ <idle>-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
+# \ / ||||| \ | /
+ <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
+ <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d..3 5us : __schedule <-schedule
+ <idle>-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).
+
+ <idle>-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
+# \ / ||||| \ | /
+ <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
+ <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
+ <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
+ <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
+ <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
+ <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
+ <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
+ <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
+ <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
+ <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
+ <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
+ <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
+ <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
+ <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
+ <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
+ <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
+ <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
+ <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
+ <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
+ <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
+ <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
+ <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz
+ <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
+ <idle>-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz
+ <idle>-0 3dN.2 14us : sched_avg_update <-__update_cpu_load
+ <idle>-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz
+ <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
+ <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
+ <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+ <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
+ <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
+ <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
+ <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
+ <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
+ <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
+ <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
+ <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
+ <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
+ <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
+ <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
+ <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+ <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
+ <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
+ <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
+ <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
+ <idle>-0 3.N.. 25us : schedule <-cpu_idle
+ <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
+ <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
+ <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
+ <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
+ <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
+ <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
+ <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
+ <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
+ <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
+ <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
+ <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
+ <idle>-0 3d..3 29us : __schedule <-preempt_schedule
+ <idle>-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
+# \ / ||||| \ | /
+ <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
+ <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
+ <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
+ <idle>-0 2.N.2 2us : power_end: cpu_id=2
+ <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
+ <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
+ <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
+ <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
+ <idle>-0 2.N.2 5us : rcu_utilization: End context switch
+ <idle>-0 2d..3 6us : __schedule <-schedule
+ <idle>-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 > 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() {
+ 0) | do_sys_open() {
+ 0) | getname() {
+ 0) | kmem_cache_alloc() {
+ 0) 1.382 us | __might_sleep();
+ 0) 2.478 us | }
+ 0) | strncpy_from_user() {
+ 0) | might_fault() {
+ 0) 1.389 us | __might_sleep();
+ 0) 2.553 us | }
+ 0) 3.807 us | }
+ 0) 7.876 us | }
+ 0) | alloc_fd() {
+ 0) 0.668 us | _spin_lock();
+ 0) 0.570 us | expand_files();
+ 0) 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() {
+ 0) 0.646 us | _spin_lock_irqsave();
+ 0) 0.684 us | _spin_unlock_irqrestore();
+ 0) 3.123 us | }
+ 0) 0.548 us | fput();
+ 0) + 58.628 us | }
+
+ [...]
+
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | }
+ 0) 2.861 us | }
+ 0) ! 115.305 us | }
+ 0) ! 116.402 us | }
+
+ + means that the function exceeded 10 usecs.
+ ! means that the function exceeded 100 usecs.
+ # means that the function exceeded 1000 usecs.
+ $ means that the function exceeded 1 sec.
+
+
+- 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() {
+ 0) sh-4802 | | call_rcu() {
+ 0) sh-4802 | | __call_rcu() {
+ 0) sh-4802 | 0.616 us | rcu_process_gp_end();
+ 0) sh-4802 | 0.586 us | check_for_new_grace_period();
+ 0) sh-4802 | 2.899 us | }
+ 0) sh-4802 | 4.040 us | }
+ 0) sh-4802 | 5.151 us | }
+ 0) 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();
+
+
+The function name is always displayed after the closing bracket
+for a function if the start of that function is not in the
+trace buffer.
+
+Display of the function name after the closing bracket may be
+enabled for functions whose start is in the trace buffer,
+allowing easier searching with grep for function durations.
+It is default disabled.
+
+ hide: echo nofuncgraph-tail > trace_options
+ show: echo funcgraph-tail > trace_options
+
+ Example with nofuncgraph-tail (default):
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | }
+ 0) 2.861 us | }
+
+ Example with funcgraph-tail:
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | } /* kmem_cache_free() */
+ 0) 2.861 us | } /* putname() */
+
+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() {
+ 1) | /* I'm a comment! */
+ 1) 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
+
+ <match>* - will match functions that begin with <match>
+ *<match> - will match functions that end with <match>
+ *<match>* - will match functions that have <match> in it
+
+These are the only wild cards which are supported.
+
+ <match>*<match> 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() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.804 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.329 us | }
+ 0) 3.904 us | }
+ 0) 4.979 us | }
+ 0) 0.653 us | _spin_lock();
+ 0) 0.578 us | page_add_file_rmap();
+ 0) 0.525 us | native_set_pte_at();
+ 0) 0.585 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.541 us | page_waitqueue();
+ 0) 0.639 us | __wake_up_bit();
+ 0) 2.786 us | }
+ 0) + 14.237 us | }
+ 0) | __do_fault() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.698 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.412 us | }
+ 0) 3.950 us | }
+ 0) 5.098 us | }
+ 0) 0.631 us | _spin_lock();
+ 0) 0.571 us | page_add_file_rmap();
+ 0) 0.526 us | native_set_pte_at();
+ 0) 0.586 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.533 us | page_waitqueue();
+ 0) 0.638 us | __wake_up_bit();
+ 0) 2.793 us | }
+ 0) + 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:
+
+<function>:<command>:<parameter>
+
+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:
+
+ <function>:enable_event:<system>:<event>[:count]
+ <function>:disable_event:<system>:<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
+[...]
+ <idle>-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.
Code Review / kvmfornfv.git / blobdiff