| ======================== |
| 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) |
| :Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, |
| John Kacur, and David Teigland. |
| |
| - Written for: 2.6.28-rc2 |
| - Updated for: 3.10 |
| - Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt |
| - Converted to rst format - Changbin Du <changbin.du@intel.com> |
| |
| 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 framework 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. |
| Throughout the kernel is hundreds of static event points that |
| can be enabled via the tracefs file system to see what is |
| going on in certain parts of the kernel. |
| |
| See events.rst for more information. |
| |
| |
| Implementation Details |
| ---------------------- |
| |
| See Documentation/trace/ftrace-design.rst for details for arch porters and such. |
| |
| |
| The File System |
| --------------- |
| |
| Ftrace uses the tracefs file system to hold the control files as |
| well as the files to display output. |
| |
| When tracefs is configured into the kernel (which selecting any ftrace |
| option will do) the directory /sys/kernel/tracing will be created. To mount |
| this directory, you can add to your /etc/fstab file:: |
| |
| tracefs /sys/kernel/tracing tracefs defaults 0 0 |
| |
| Or you can mount it at run time with:: |
| |
| mount -t tracefs nodev /sys/kernel/tracing |
| |
| For quicker access to that directory you may want to make a soft link to |
| it:: |
| |
| ln -s /sys/kernel/tracing /tracing |
| |
| .. attention:: |
| |
| Before 4.1, all ftrace tracing control files were within the debugfs |
| file system, which is typically located at /sys/kernel/debug/tracing. |
| For backward compatibility, when mounting the debugfs file system, |
| the tracefs file system will be automatically mounted at: |
| |
| /sys/kernel/debug/tracing |
| |
| All files located in the tracefs file system will be located in that |
| debugfs file system directory as well. |
| |
| .. attention:: |
| |
| Any selected ftrace option will also create the tracefs file system. |
| The rest of the document will assume that you are in the ftrace directory |
| (cd /sys/kernel/tracing) and will only concentrate on the files within that |
| directory and not distract from the content with the extended |
| "/sys/kernel/tracing" path name. |
| |
| That's it! (assuming that you have ftrace configured into your kernel) |
| |
| After mounting tracefs you will have access to 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. Changing the current tracer clears |
| the ring buffer content as well as the "snapshot" buffer. |
| |
| 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. |
| |
| The kernel function tracing_off() can be used within the |
| kernel to disable writing to the ring buffer, which will |
| set this file to "0". User space can re-enable tracing by |
| echoing "1" into the file. |
| |
| Note, the function and event trigger "traceoff" will also |
| set this file to zero and stop tracing. Which can also |
| be re-enabled by user space using this file. |
| |
| trace: |
| |
| This file holds the output of the trace in a human |
| readable format (described below). Opening this file for |
| writing with the O_TRUNC flag clears the ring buffer content. |
| Note, this file is not a consumer. If tracing is off |
| (no tracer running, or tracing_on is zero), it will produce |
| the same output each time it is read. When tracing is on, |
| it may produce inconsistent results as it tries to read |
| the entire buffer without consuming it. |
| |
| 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, it will display the same |
| information every time it is 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 maximum time that interrupts are disabled. |
| The maximum 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). |
| |
| By echoing in a time into this file, no latency will be recorded |
| unless it is greater than the time in this file. |
| |
| 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). |
| A few extra pages may be allocated to accommodate buffer management |
| meta-data. 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 or shown. |
| ( Note, the size may not be a multiple of the page size |
| due to buffer management meta-data. ) |
| |
| Buffer sizes for individual CPUs may vary |
| (see "per_cpu/cpu0/buffer_size_kb" below), and if they do |
| this file will show "X". |
| |
| buffer_total_size_kb: |
| |
| This displays the total combined size of all the trace buffers. |
| |
| free_buffer: |
| |
| If a process is performing 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 influences the tracers "function" and "function_graph" |
| and thus also function profiling (see "function_profile_enabled"). |
| |
| The functions listed in "available_filter_functions" are what |
| can be written into this file. |
| |
| This interface also allows for commands to be used. See the |
| "Filter commands" section for more details. |
| |
| As a speed up, since processing strings can be quite expensive |
| and requires a check of all functions registered to tracing, instead |
| an index can be written into this file. A number (starting with "1") |
| written will instead select the same corresponding at the line position |
| of the "available_filter_functions" file. |
| |
| 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 the threads whose PID are |
| listed in this file. |
| |
| If the "function-fork" option is set, then when a task whose |
| PID is listed in this file forks, the child's PID will |
| automatically be added to this file, and the child will be |
| traced by the function tracer as well. This option will also |
| cause PIDs of tasks that exit to be removed from the file. |
| |
| set_ftrace_notrace_pid: |
| |
| Have the function tracer ignore threads whose PID are listed in |
| this file. |
| |
| If the "function-fork" option is set, then when a task whose |
| PID is listed in this file forks, the child's PID will |
| automatically be added to this file, and the child will not be |
| traced by the function tracer as well. This option will also |
| cause PIDs of tasks that exit to be removed from the file. |
| |
| If a PID is in both this file and "set_ftrace_pid", then this |
| file takes precedence, and the thread will not be traced. |
| |
| set_event_pid: |
| |
| Have the events only trace a task with a PID listed in this file. |
| Note, sched_switch and sched_wake_up will also trace events |
| listed in this file. |
| |
| To have the PIDs of children of tasks with their PID in this file |
| added on fork, enable the "event-fork" option. That option will also |
| cause the PIDs of tasks to be removed from this file when the task |
| exits. |
| |
| set_event_notrace_pid: |
| |
| Have the events not trace a task with a PID listed in this file. |
| Note, sched_switch and sched_wakeup will trace threads not listed |
| in this file, even if a thread's PID is in the file if the |
| sched_switch or sched_wakeup events also trace a thread that should |
| be traced. |
| |
| To have the PIDs of children of tasks with their PID in this file |
| added on fork, enable the "event-fork" option. That option will also |
| cause the PIDs of tasks to be removed from this file when the task |
| exits. |
| |
| set_graph_function: |
| |
| Functions listed in this file will cause the function graph |
| tracer to only trace these functions and the functions that |
| they call. (See the section "dynamic ftrace" for more details). |
| Note, set_ftrace_filter and set_ftrace_notrace still affects |
| what functions are being traced. |
| |
| set_graph_notrace: |
| |
| Similar to set_graph_function, but will disable function graph |
| tracing when the function is hit until it exits the function. |
| This makes it possible to ignore tracing functions that are called |
| by a specific function. |
| |
| 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", "set_ftrace_notrace", |
| "set_graph_function", or "set_graph_notrace". |
| (See the section "dynamic ftrace" below for more details.) |
| |
| dyn_ftrace_total_info: |
| |
| This file is for debugging purposes. The number of functions that |
| have been converted to nops and are available to be traced. |
| |
| 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. |
| |
| If the architecture supports it, it will also show what callback |
| is being directly called by the function. If the count is greater |
| than 1 it most likely will be ftrace_ops_list_func(). |
| |
| If the callback of a function jumps to a trampoline that is |
| specific to the callback and which is not the standard trampoline, |
| its address will be printed as well as the function that the |
| trampoline calls. |
| |
| function_profile_enabled: |
| |
| When set it will enable all functions with either the function |
| tracer, or if configured, the function graph tracer. It will |
| keep a histogram of the number of functions that were called |
| and if the function graph tracer was configured, it will also keep |
| track of the time spent in those functions. The histogram |
| content can be displayed in the files: |
| |
| trace_stat/function<cpu> ( function0, function1, etc). |
| |
| trace_stat: |
| |
| A directory that holds different tracing stats. |
| |
| kprobe_events: |
| |
| Enable dynamic trace points. See kprobetrace.rst. |
| |
| kprobe_profile: |
| |
| Dynamic trace points stats. See kprobetrace.rst. |
| |
| 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, 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. |
| |
| If the option "record-cmd" is set to "0", then comms of tasks |
| will not be saved during recording. By default, it is enabled. |
| |
| saved_cmdlines_size: |
| |
| By default, 128 comms are saved (see "saved_cmdlines" above). To |
| increase or decrease the amount of comms that are cached, echo |
| the number of comms to cache into this file. |
| |
| saved_tgids: |
| |
| If the option "record-tgid" is set, on each scheduling context switch |
| the Task Group ID of a task is saved in a table mapping the PID of |
| the thread to its TGID. By default, the "record-tgid" option is |
| disabled. |
| |
| 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 |
| |
| The clock with the square brackets around it is the one in effect. |
| |
| 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. |
| |
| ppc-tb: |
| This uses the powerpc timebase register value. |
| This is in sync across CPUs and can also be used |
| to correlate events across hypervisor/guest if |
| tb_offset is known. |
| |
| mono: |
| This uses the fast monotonic clock (CLOCK_MONOTONIC) |
| which is monotonic and is subject to NTP rate adjustments. |
| |
| mono_raw: |
| This is the raw monotonic clock (CLOCK_MONOTONIC_RAW) |
| which is monotonic but is not subject to any rate adjustments |
| and ticks at the same rate as the hardware clocksource. |
| |
| boot: |
| This is the boot clock (CLOCK_BOOTTIME) and is based on the |
| fast monotonic clock, but also accounts for time spent in |
| suspend. Since the clock access is designed for use in |
| tracing in the suspend path, some side effects are possible |
| if clock is accessed after the suspend time is accounted before |
| the fast mono clock is updated. In this case, the clock update |
| appears to happen slightly sooner than it normally would have. |
| Also on 32-bit systems, it's possible that the 64-bit boot offset |
| sees a partial update. These effects are rare and post |
| processing should be able to handle them. See comments in the |
| ktime_get_boot_fast_ns() function for more information. |
| |
| To set a clock, simply echo the clock name into this file:: |
| |
| # echo global > trace_clock |
| |
| Setting a clock clears the ring buffer content as well as the |
| "snapshot" buffer. |
| |
| 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); |
| |
| Note: Writing into the trace_marker file can also initiate triggers |
| that are written into /sys/kernel/tracing/events/ftrace/print/trigger |
| See "Event triggers" in Documentation/trace/events.rst and an |
| example in Documentation/trace/histogram.rst (Section 3.) |
| |
| trace_marker_raw: |
| |
| This is similar to trace_marker above, but is meant for binary data |
| to be written to it, where a tool can be used to parse the data |
| from trace_pipe_raw. |
| |
| uprobe_events: |
| |
| Add dynamic tracepoints in programs. |
| See uprobetracer.rst |
| |
| 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.rst for more information. |
| |
| set_event: |
| |
| By echoing in the event into this file, will enable that event. |
| |
| See events.rst for more information. |
| |
| available_events: |
| |
| A list of events that can be enabled in tracing. |
| |
| See events.rst for more information. |
| |
| timestamp_mode: |
| |
| Certain tracers may change the timestamp mode used when |
| logging trace events into the event buffer. Events with |
| different modes can coexist within a buffer but the mode in |
| effect when an event is logged determines which timestamp mode |
| is used for that event. The default timestamp mode is |
| 'delta'. |
| |
| Usual timestamp modes for tracing: |
| |
| # cat timestamp_mode |
| [delta] absolute |
| |
| The timestamp mode with the square brackets around it is the |
| one in effect. |
| |
| delta: Default timestamp mode - timestamp is a delta against |
| a per-buffer timestamp. |
| |
| absolute: The timestamp is a full timestamp, not a delta |
| against some other value. As such it takes up more |
| space and is less efficient. |
| |
| hwlat_detector: |
| |
| Directory for the Hardware Latency Detector. |
| See "Hardware Latency Detector" section below. |
| |
| 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. |
| |
| "blk" |
| |
| The block tracer. The tracer used by the blktrace user |
| application. |
| |
| "hwlat" |
| |
| The Hardware Latency tracer is used to detect if the hardware |
| produces any latency. See "Hardware Latency Detector" section |
| below. |
| |
| "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, which |
| happens automatically when the tracer is selected. |
| |
| "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. |
| |
| "wakeup_dl" |
| |
| Traces and records the max latency that it takes for |
| a SCHED_DEADLINE task to be woken (as the "wakeup" and |
| "wakeup_rt" does). |
| |
| "mmiotrace" |
| |
| A special tracer that is used to trace binary module. |
| It will trace all the calls that a module makes to the |
| hardware. Everything it writes and reads from the I/O |
| as well. |
| |
| "branch" |
| |
| This tracer can be configured when tracing likely/unlikely |
| calls within the kernel. It will trace when a likely and |
| unlikely branch is hit and if it was correct in its prediction |
| of being correct. |
| |
| "nop" |
| |
| This is the "trace nothing" tracer. To remove all |
| tracers from tracing simply echo "nop" into |
| current_tracer. |
| |
| Error conditions |
| ---------------- |
| |
| For most ftrace commands, failure modes are obvious and communicated |
| using standard return codes. |
| |
| For other more involved commands, extended error information may be |
| available via the tracing/error_log file. For the commands that |
| support it, reading the tracing/error_log file after an error will |
| display more detailed information about what went wrong, if |
| information is available. The tracing/error_log file is a circular |
| error log displaying a small number (currently, 8) of ftrace errors |
| for the last (8) failed commands. |
| |
| The extended error information and usage takes the form shown in |
| this example:: |
| |
| # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger |
| echo: write error: Invalid argument |
| |
| # cat /sys/kernel/debug/tracing/error_log |
| [ 5348.887237] location: error: Couldn't yyy: zzz |
| Command: xxx |
| ^ |
| [ 7517.023364] location: error: Bad rrr: sss |
| Command: ppp qqq |
| ^ |
| |
| To clear the error log, echo the empty string into it:: |
| |
| # echo > /sys/kernel/debug/tracing/error_log |
| |
| Examples of using the tracer |
| ---------------------------- |
| |
| Here are typical examples of using the tracers when controlling |
| them only with the tracefs 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 |
| (3.8). 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. |
| .. caution:: 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: |
| - 'Z' - NMI occurred inside a hardirq |
| - 'z' - NMI is running |
| - '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 100 millisecond |
| - '*' - greater than 10 millisecond |
| - '#' - 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 |
| trace_printk |
| annotate |
| nouserstacktrace |
| nosym-userobj |
| noprintk-msg-only |
| context-info |
| nolatency-format |
| record-cmd |
| norecord-tgid |
| overwrite |
| nodisable_on_free |
| irq-info |
| markers |
| noevent-fork |
| function-trace |
| nofunction-fork |
| nodisplay-graph |
| nostacktrace |
| nobranch |
| |
| 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. |
| |
| trace_printk |
| Can disable trace_printk() from writing into the buffer. |
| |
| 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 user space thread after |
| each trace event. |
| |
| 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 output. When it is enabled, |
| the trace displays additional information about the |
| latency, as described in "Latency trace format". |
| |
| pause-on-trace |
| When set, opening the trace file for read, will pause |
| writing to the ring buffer (as if tracing_on was set to zero). |
| This simulates the original behavior of the trace file. |
| When the file is closed, tracing will be enabled again. |
| |
| hash-ptr |
| When set, "%p" in the event printk format displays the |
| hashed pointer value instead of real address. |
| This will be useful if you want to find out which hashed |
| value is corresponding to the real value in trace log. |
| |
| 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. See "saved_cmdlines". |
| |
| record-tgid |
| When any event or tracer is enabled, a hook is enabled |
| in the sched_switch trace point to fill the cache of |
| mapped Thread Group IDs (TGID) mapping to pids. See |
| "saved_tgids". |
| |
| 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. |
| |
| event-fork |
| When set, tasks with PIDs listed in set_event_pid will have |
| the PIDs of their children added to set_event_pid when those |
| tasks fork. Also, when tasks with PIDs in set_event_pid exit, |
| their PIDs will be removed from the file. |
| |
| This affects PIDs listed in set_event_notrace_pid as well. |
| |
| 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. |
| |
| function-fork |
| When set, tasks with PIDs listed in set_ftrace_pid will |
| have the PIDs of their children added to set_ftrace_pid |
| when those tasks fork. Also, when tasks with PIDs in |
| set_ftrace_pid exit, their PIDs will be removed from the |
| file. |
| |
| This affects PIDs in set_ftrace_notrace_pid as well. |
| |
| display-graph |
| When set, the latency tracers (irqsoff, wakeup, etc) will |
| use function graph tracing instead of function tracing. |
| |
| stacktrace |
| When set, a stack trace is recorded after any trace event |
| is recorded. |
| |
| branch |
| Enable branch tracing with the tracer. This enables branch |
| tracer along with the currently set tracer. Enabling this |
| with the "nop" tracer is the same as just enabling the |
| "branch" tracer. |
| |
| .. tip:: Some tracers have their own options. They only appear in this |
| file when the tracer is active. They always appear in the |
| options directory. |
| |
| |
| Here are the per tracer options: |
| |
| Options for function tracer: |
| |
| func_stack_trace |
| When set, a stack trace is recorded after every |
| function that is recorded. NOTE! Limit the functions |
| that are recorded before enabling this, with |
| "set_ftrace_filter" otherwise the system performance |
| will be critically degraded. Remember to disable |
| this option before clearing the function filter. |
| |
| Options for function_graph tracer: |
| |
| Since the function_graph tracer has a slightly different output |
| it has its own options to control what is displayed. |
| |
| funcgraph-overrun |
| When set, the "overrun" of the graph stack is |
| displayed after each function traced. The |
| overrun, is when the stack depth of the calls |
| is greater than what is reserved for each task. |
| Each task has a fixed array of functions to |
| trace in the call graph. If the depth of the |
| calls exceeds that, the function is not traced. |
| The overrun is the number of functions missed |
| due to exceeding this array. |
| |
| funcgraph-cpu |
| When set, the CPU number of the CPU where the trace |
| occurred is displayed. |
| |
| funcgraph-overhead |
| When set, if the function takes longer than |
| A certain amount, then a delay marker is |
| displayed. See "delay" above, under the |
| header description. |
| |
| funcgraph-proc |
| Unlike other tracers, the process' command line |
| is not displayed by default, but instead only |
| when a task is traced in and out during a context |
| switch. Enabling this options has the command |
| of each process displayed at every line. |
| |
| funcgraph-duration |
| At the end of each function (the return) |
| the duration of the amount of time in the |
| function is displayed in microseconds. |
| |
| funcgraph-abstime |
| When set, the timestamp is displayed at each line. |
| |
| funcgraph-irqs |
| When disabled, functions that happen inside an |
| interrupt will not be traced. |
| |
| funcgraph-tail |
| When set, the return event will include the function |
| that it represents. By default this is off, and |
| only a closing curly bracket "}" is displayed for |
| the return of a function. |
| |
| 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 profiler with 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. |
| |
| Options for blk tracer: |
| |
| blk_classic |
| Shows a more minimalistic output. |
| |
| |
| 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 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 |
| => submit_bio_noacct |
| => 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. |
| |
| If we prefer function graph output instead of function, we can set |
| display-graph option:: |
| |
| with echo 1 > options/display-graph |
| |
| # tracer: irqsoff |
| # |
| # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ |
| # -------------------------------------------------------------------- |
| # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) |
| # ----------------- |
| # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) |
| # ----------------- |
| # => started at: free_debug_processing |
| # => ended at: return_to_handler |
| # |
| # |
| # _-----=> irqs-off |
| # / _----=> need-resched |
| # | / _---=> hardirq/softirq |
| # || / _--=> preempt-depth |
| # ||| / |
| # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS |
| # | | | | |||| | | | | | | |
| 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); |
| 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); |
| 1 us | 0) bash-1507 | d..2 | | set_track() { |
| 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { |
| 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { |
| 3 us | 0) bash-1507 | d..2 | | __unwind_start() { |
| 3 us | 0) bash-1507 | d..2 | | get_stack_info() { |
| 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); |
| 4 us | 0) bash-1507 | d..2 | 1.107 us | } |
| [...] |
| 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); |
| 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); |
| 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); |
| bash-1507 0d..1 3792us : <stack trace> |
| => free_debug_processing |
| => __slab_free |
| => kmem_cache_free |
| => vm_area_free |
| => remove_vma |
| => exit_mmap |
| => mmput |
| => begin_new_exec |
| => load_elf_binary |
| => search_binary_handler |
| => __do_execve_file.isra.32 |
| => __x64_sys_execve |
| => do_syscall_64 |
| => entry_SYSCALL_64_after_hwframe |
| |
| 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 |
| => submit_bio_noacct |
| => 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 - 120) |
| 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 : cpu_load_update_nohz <-tick_nohz_idle_exit |
| <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz |
| <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock |
| <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz |
| <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update |
| <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz |
| <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock |
| <idle>-0 3dN.1 15us : calc_load_nohz_stop <-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 |
| |
| |
| Hardware Latency Detector |
| ------------------------- |
| |
| The hardware latency detector is executed by enabling the "hwlat" tracer. |
| |
| NOTE, this tracer will affect the performance of the system as it will |
| periodically make a CPU constantly busy with interrupts disabled. |
| :: |
| |
| # echo hwlat > current_tracer |
| # sleep 100 |
| # cat trace |
| # tracer: hwlat |
| # |
| # entries-in-buffer/entries-written: 13/13 #P:8 |
| # |
| # _-----=> irqs-off |
| # / _----=> need-resched |
| # | / _---=> hardirq/softirq |
| # || / _--=> preempt-depth |
| # ||| / delay |
| # TASK-PID CPU# |||| TIMESTAMP FUNCTION |
| # | | | |||| | | |
| <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6 |
| <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1 |
| <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5 |
| <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1 |
| <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1 |
| <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2 |
| <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1 |
| <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1 |
| <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1 |
| <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1 |
| <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1 |
| <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1 |
| |
| |
| The above output is somewhat the same in the header. All events will have |
| interrupts disabled 'd'. Under the FUNCTION title there is: |
| |
| #1 |
| This is the count of events recorded that were greater than the |
| tracing_threshold (See below). |
| |
| inner/outer(us): 11/11 |
| |
| This shows two numbers as "inner latency" and "outer latency". The test |
| runs in a loop checking a timestamp twice. The latency detected within |
| the two timestamps is the "inner latency" and the latency detected |
| after the previous timestamp and the next timestamp in the loop is |
| the "outer latency". |
| |
| ts:1581527483.343962693 |
| |
| The absolute timestamp that the first latency was recorded in the window. |
| |
| count:6 |
| |
| The number of times a latency was detected during the window. |
| |
| nmi-total:7 nmi-count:1 |
| |
| On architectures that support it, if an NMI comes in during the |
| test, the time spent in NMI is reported in "nmi-total" (in |
| microseconds). |
| |
| All architectures that have NMIs will show the "nmi-count" if an |
| NMI comes in during the test. |
| |
| hwlat files: |
| |
| tracing_threshold |
| This gets automatically set to "10" to represent 10 |
| microseconds. This is the threshold of latency that |
| needs to be detected before the trace will be recorded. |
| |
| Note, when hwlat tracer is finished (another tracer is |
| written into "current_tracer"), the original value for |
| tracing_threshold is placed back into this file. |
| |
| hwlat_detector/width |
| The length of time the test runs with interrupts disabled. |
| |
| hwlat_detector/window |
| The length of time of the window which the test |
| runs. That is, the test will run for "width" |
| microseconds per "window" microseconds |
| |
| tracing_cpumask |
| When the test is started. A kernel thread is created that |
| runs the test. This thread will alternate between CPUs |
| listed in the tracing_cpumask between each period |
| (one "window"). To limit the test to specific CPUs |
| set the mask in this file to only the CPUs that the test |
| should run on. |
| |
| 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_tracefs(void) |
| { |
| static char tracefs[MAX_PATH+1]; |
| static int tracefs_found; |
| char type[100]; |
| FILE *fp; |
| |
| if (tracefs_found) |
| return tracefs; |
| |
| 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", |
| tracefs, type) == 2) { |
| if (strcmp(type, "tracefs") == 0) |
| break; |
| } |
| fclose(fp); |
| |
| if (strcmp(type, "tracefs") != 0) { |
| fprintf(stderr, "tracefs not mounted"); |
| return NULL; |
| } |
| |
| strcat(tracefs, "/tracing/"); |
| tracefs_found = 1; |
| |
| return tracefs; |
| } |
| |
| const char *tracing_file(const char *file_name) |
| { |
| static char trace_file[MAX_PATH+1]; |
| snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), 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 |
| |
| tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` |
| echo nop > $tracefs/tracing/current_tracer |
| echo 0 > $tracefs/tracing/tracing_on |
| echo $$ > $tracefs/tracing/set_ftrace_pid |
| echo function > $tracefs/tracing/current_tracer |
| echo 1 > $tracefs/tracing/tracing_on |
| exec "$@" |
| |
| |
| 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:: |
| |
| 3) # 1837.709 us | } /* __switch_to */ |
| 3) | finish_task_switch() { |
| 3) 0.313 us | _raw_spin_unlock_irq(); |
| 3) 3.177 us | } |
| 3) # 1889.063 us | } /* __schedule */ |
| 3) ! 140.417 us | } /* __schedule */ |
| 3) # 2034.948 us | } /* schedule */ |
| 3) * 33998.59 us | } /* schedule_preempt_disabled */ |
| |
| [...] |
| |
| 1) 0.260 us | msecs_to_jiffies(); |
| 1) 0.313 us | __rcu_read_unlock(); |
| 1) + 61.770 us | } |
| 1) + 64.479 us | } |
| 1) 0.313 us | rcu_bh_qs(); |
| 1) 0.313 us | __local_bh_enable(); |
| 1) ! 217.240 us | } |
| 1) 0.365 us | idle_cpu(); |
| 1) | rcu_irq_exit() { |
| 1) 0.417 us | rcu_eqs_enter_common.isra.47(); |
| 1) 3.125 us | } |
| 1) ! 227.812 us | } |
| 1) ! 457.395 us | } |
| 1) @ 119760.2 us | } |
| |
| [...] |
| |
| 2) | handle_IPI() { |
| 1) 6.979 us | } |
| 2) 0.417 us | scheduler_ipi(); |
| 1) 9.791 us | } |
| 1) + 12.917 us | } |
| 2) 3.490 us | } |
| 1) + 15.729 us | } |
| 1) + 18.542 us | } |
| 2) $ 3594274 us | } |
| |
| Flags:: |
| |
| + 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 10 msecs. |
| @ means that the function exceeded 100 msecs. |
| $ 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. Starting |
| with gcc version 4.6, the -mfentry has been added for x86, which |
| calls "__fentry__" instead of "mcount". Which is called before |
| the creation of the stack frame. |
| |
| Note, not all sections are traced. They may be prevented by either |
| a notrace, or blocked another way and all inline functions are not |
| traced. Check the "available_filter_functions" file to see what functions |
| can be traced. |
| |
| A section called "__mcount_loc" is created that holds |
| references to all the mcount/fentry 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 glob(7) matching. |
| |
| ``<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 |
| ``<match1>*<match2>`` |
| will match functions that begin with <match1> and end with <match2> |
| |
| .. 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. |
| |
| Selecting function filters via index |
| ------------------------------------ |
| |
| Because processing of strings is expensive (the address of the function |
| needs to be looked up before comparing to the string being passed in), |
| an index can be used as well to enable functions. This is useful in the |
| case of setting thousands of specific functions at a time. By passing |
| in a list of numbers, no string processing will occur. Instead, the function |
| at the specific location in the internal array (which corresponds to the |
| functions in the "available_filter_functions" file), is selected. |
| |
| :: |
| |
| # echo 1 > set_ftrace_filter |
| |
| Will select the first function listed in "available_filter_functions" |
| |
| :: |
| |
| # head -1 available_filter_functions |
| trace_initcall_finish_cb |
| |
| # cat set_ftrace_filter |
| trace_initcall_finish_cb |
| |
| # head -50 available_filter_functions | tail -1 |
| x86_pmu_commit_txn |
| |
| # echo 1 50 > set_ftrace_filter |
| # cat set_ftrace_filter |
| trace_initcall_finish_cb |
| x86_pmu_commit_txn |
| |
| 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). It |
| cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set |
| registered. |
| |
| 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 |
| |
| Mod command supports module globbing. Disable tracing for all |
| functions except a specific module:: |
| |
| echo '!*:mod:!ext3' >> set_ftrace_filter |
| |
| Disable tracing for all modules, but still trace kernel:: |
| |
| echo '!*:mod:*' >> set_ftrace_filter |
| |
| Enable filter only for kernel:: |
| |
| echo '*write*:mod:!*' >> set_ftrace_filter |
| |
| Enable filter for module globbing:: |
| |
| echo '*write*:mod:*snd*' >> 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 it's a function that is called before a triple |
| 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. |
| |
| - stacktrace: |
| When the function is hit, a stack trace is recorded. |
| |
| 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. This is contrary to the trace file. If any process opened |
| the trace file for reading, it will actually disable tracing and |
| prevent new entries from being added. The trace_pipe file does |
| not have this limitation. |
| |
| 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 tracefs 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 tracefs 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 directory, 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 |
| ---- |
| More details can be found in the source code, in the `kernel/trace/*.c` files. |