1901 lines
68 KiB
Plaintext
1901 lines
68 KiB
Plaintext
ftrace - Function Tracer
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========================
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Copyright 2008 Red Hat Inc.
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Author: Steven Rostedt <srostedt@redhat.com>
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License: The GNU Free Documentation License, Version 1.2
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(dual licensed under the GPL v2)
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Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
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John Kacur, and David Teigland.
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Written for: 2.6.28-rc2
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Introduction
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------------
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Ftrace is an internal tracer designed to help out developers and
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designers of systems to find what is going on inside the kernel.
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It can be used for debugging or analyzing latencies and
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performance issues that take place outside of user-space.
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Although ftrace is the function tracer, it also includes an
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infrastructure that allows for other types of tracing. Some of
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the tracers that are currently in ftrace include a tracer to
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trace context switches, the time it takes for a high priority
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task to run after it was woken up, the time interrupts are
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disabled, and more (ftrace allows for tracer plugins, which
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means that the list of tracers can always grow).
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Implementation Details
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----------------------
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See ftrace-design.txt for details for arch porters and such.
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The File System
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---------------
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Ftrace uses the debugfs file system to hold the control files as
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well as the files to display output.
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When debugfs is configured into the kernel (which selecting any ftrace
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option will do) the directory /sys/kernel/debug will be created. To mount
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this directory, you can add to your /etc/fstab file:
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debugfs /sys/kernel/debug debugfs defaults 0 0
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Or you can mount it at run time with:
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mount -t debugfs nodev /sys/kernel/debug
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For quicker access to that directory you may want to make a soft link to
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it:
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ln -s /sys/kernel/debug /debug
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Any selected ftrace option will also create a directory called tracing
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within the debugfs. The rest of the document will assume that you are in
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the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
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on the files within that directory and not distract from the content with
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the extended "/sys/kernel/debug/tracing" path name.
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That's it! (assuming that you have ftrace configured into your kernel)
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After mounting the debugfs, you can see a directory called
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"tracing". This directory contains the control and output files
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of ftrace. Here is a list of some of the key files:
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Note: all time values are in microseconds.
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current_tracer:
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This is used to set or display the current tracer
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that is configured.
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available_tracers:
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This holds the different types of tracers that
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have been compiled into the kernel. The
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tracers listed here can be configured by
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echoing their name into current_tracer.
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tracing_enabled:
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This sets or displays whether the current_tracer
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is activated and tracing or not. Echo 0 into this
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file to disable the tracer or 1 to enable it.
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trace:
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This file holds the output of the trace in a human
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readable format (described below).
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trace_pipe:
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The output is the same as the "trace" file but this
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file is meant to be streamed with live tracing.
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Reads from this file will block until new data is
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retrieved. Unlike the "trace" file, this file is a
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consumer. This means reading from this file causes
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sequential reads to display more current data. Once
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data is read from this file, it is consumed, and
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will not be read again with a sequential read. The
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"trace" file is static, and if the tracer is not
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adding more data,they will display the same
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information every time they are read.
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trace_options:
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This file lets the user control the amount of data
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that is displayed in one of the above output
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files.
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tracing_max_latency:
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Some of the tracers record the max latency.
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For example, the time interrupts are disabled.
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This time is saved in this file. The max trace
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will also be stored, and displayed by "trace".
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A new max trace will only be recorded if the
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latency is greater than the value in this
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file. (in microseconds)
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buffer_size_kb:
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This sets or displays the number of kilobytes each CPU
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buffer can hold. The tracer buffers are the same size
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for each CPU. The displayed number is the size of the
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CPU buffer and not total size of all buffers. The
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trace buffers are allocated in pages (blocks of memory
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that the kernel uses for allocation, usually 4 KB in size).
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If the last page allocated has room for more bytes
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than requested, the rest of the page will be used,
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making the actual allocation bigger than requested.
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( Note, the size may not be a multiple of the page size
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due to buffer management overhead. )
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This can only be updated when the current_tracer
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is set to "nop".
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tracing_cpumask:
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This is a mask that lets the user only trace
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on specified CPUS. The format is a hex string
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representing the CPUS.
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set_ftrace_filter:
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When dynamic ftrace is configured in (see the
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section below "dynamic ftrace"), the code is dynamically
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modified (code text rewrite) to disable calling of the
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function profiler (mcount). This lets tracing be configured
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in with practically no overhead in performance. This also
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has a side effect of enabling or disabling specific functions
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to be traced. Echoing names of functions into this file
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will limit the trace to only those functions.
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set_ftrace_notrace:
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This has an effect opposite to that of
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set_ftrace_filter. Any function that is added here will not
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be traced. If a function exists in both set_ftrace_filter
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and set_ftrace_notrace, the function will _not_ be traced.
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set_ftrace_pid:
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Have the function tracer only trace a single thread.
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set_graph_function:
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Set a "trigger" function where tracing should start
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with the function graph tracer (See the section
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"dynamic ftrace" for more details).
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available_filter_functions:
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This lists the functions that ftrace
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has processed and can trace. These are the function
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names that you can pass to "set_ftrace_filter" or
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"set_ftrace_notrace". (See the section "dynamic ftrace"
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below for more details.)
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The Tracers
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-----------
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Here is the list of current tracers that may be configured.
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"function"
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Function call tracer to trace all kernel functions.
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"function_graph"
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Similar to the function tracer except that the
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function tracer probes the functions on their entry
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whereas the function graph tracer traces on both entry
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and exit of the functions. It then provides the ability
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to draw a graph of function calls similar to C code
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source.
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"sched_switch"
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Traces the context switches and wakeups between tasks.
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"irqsoff"
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Traces the areas that disable interrupts and saves
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the trace with the longest max latency.
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See tracing_max_latency. When a new max is recorded,
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it replaces the old trace. It is best to view this
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trace with the latency-format option enabled.
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"preemptoff"
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Similar to irqsoff but traces and records the amount of
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time for which preemption is disabled.
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"preemptirqsoff"
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Similar to irqsoff and preemptoff, but traces and
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records the largest time for which irqs and/or preemption
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is disabled.
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"wakeup"
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Traces and records the max latency that it takes for
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the highest priority task to get scheduled after
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it has been woken up.
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"hw-branch-tracer"
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Uses the BTS CPU feature on x86 CPUs to traces all
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branches executed.
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"nop"
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This is the "trace nothing" tracer. To remove all
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tracers from tracing simply echo "nop" into
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current_tracer.
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Examples of using the tracer
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----------------------------
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Here are typical examples of using the tracers when controlling
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them only with the debugfs interface (without using any
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user-land utilities).
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Output format:
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--------------
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Here is an example of the output format of the file "trace"
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--------
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# tracer: function
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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bash-4251 [01] 10152.583854: path_put <-path_walk
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bash-4251 [01] 10152.583855: dput <-path_put
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bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput
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--------
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A header is printed with the tracer name that is represented by
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the trace. In this case the tracer is "function". Then a header
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showing the format. Task name "bash", the task PID "4251", the
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CPU that it was running on "01", the timestamp in <secs>.<usecs>
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format, the function name that was traced "path_put" and the
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parent function that called this function "path_walk". The
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timestamp is the time at which the function was entered.
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The sched_switch tracer also includes tracing of task wakeups
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and context switches.
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ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S
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ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S
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ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R
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events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R
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kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R
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ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R
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Wake ups are represented by a "+" and the context switches are
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shown as "==>". The format is:
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Context switches:
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Previous task Next Task
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<pid>:<prio>:<state> ==> <pid>:<prio>:<state>
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Wake ups:
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Current task Task waking up
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<pid>:<prio>:<state> + <pid>:<prio>:<state>
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The prio is the internal kernel priority, which is the inverse
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of the priority that is usually displayed by user-space tools.
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Zero represents the highest priority (99). Prio 100 starts the
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"nice" priorities with 100 being equal to nice -20 and 139 being
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nice 19. The prio "140" is reserved for the idle task which is
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the lowest priority thread (pid 0).
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Latency trace format
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--------------------
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When the latency-format option is enabled, the trace file gives
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somewhat more information to see why a latency happened.
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Here is a typical trace.
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# tracer: irqsoff
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#
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irqsoff latency trace v1.1.5 on 2.6.26-rc8
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--------------------------------------------------------------------
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latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
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-----------------
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| task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
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-----------------
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=> started at: apic_timer_interrupt
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=> ended at: do_softirq
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# _------=> CPU#
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# / _-----=> irqs-off
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# | / _----=> need-resched
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# || / _---=> hardirq/softirq
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# ||| / _--=> preempt-depth
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# |||| /
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# ||||| delay
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# cmd pid ||||| time | caller
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# \ / ||||| \ | /
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<idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
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<idle>-0 0d.s. 97us : __do_softirq (do_softirq)
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<idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq)
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This shows that the current tracer is "irqsoff" tracing the time
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for which interrupts were disabled. It gives the trace version
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and the version of the kernel upon which this was executed on
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(2.6.26-rc8). Then it displays the max latency in microsecs (97
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us). The number of trace entries displayed and the total number
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recorded (both are three: #3/3). The type of preemption that was
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used (PREEMPT). VP, KP, SP, and HP are always zero and are
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reserved for later use. #P is the number of online CPUS (#P:2).
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The task is the process that was running when the latency
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occurred. (swapper pid: 0).
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The start and stop (the functions in which the interrupts were
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disabled and enabled respectively) that caused the latencies:
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apic_timer_interrupt is where the interrupts were disabled.
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do_softirq is where they were enabled again.
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The next lines after the header are the trace itself. The header
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explains which is which.
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cmd: The name of the process in the trace.
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pid: The PID of that process.
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CPU#: The CPU which the process was running on.
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irqs-off: 'd' interrupts are disabled. '.' otherwise.
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Note: If the architecture does not support a way to
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read the irq flags variable, an 'X' will always
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be printed here.
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need-resched: 'N' task need_resched is set, '.' otherwise.
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hardirq/softirq:
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'H' - hard irq occurred inside a softirq.
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'h' - hard irq is running
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's' - soft irq is running
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'.' - normal context.
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preempt-depth: The level of preempt_disabled
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The above is mostly meaningful for kernel developers.
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time: When the latency-format option is enabled, the trace file
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output includes a timestamp relative to the start of the
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trace. This differs from the output when latency-format
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is disabled, which includes an absolute timestamp.
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delay: This is just to help catch your eye a bit better. And
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needs to be fixed to be only relative to the same CPU.
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The marks are determined by the difference between this
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current trace and the next trace.
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'!' - greater than preempt_mark_thresh (default 100)
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'+' - greater than 1 microsecond
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' ' - less than or equal to 1 microsecond.
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The rest is the same as the 'trace' file.
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trace_options
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-------------
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The trace_options file is used to control what gets printed in
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the trace output. To see what is available, simply cat the file:
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cat trace_options
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print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
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noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj
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To disable one of the options, echo in the option prepended with
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"no".
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echo noprint-parent > trace_options
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To enable an option, leave off the "no".
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echo sym-offset > trace_options
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Here are the available options:
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print-parent - On function traces, display the calling (parent)
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function as well as the function being traced.
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print-parent:
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bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
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noprint-parent:
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bash-4000 [01] 1477.606694: simple_strtoul
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sym-offset - Display not only the function name, but also the
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offset in the function. For example, instead of
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seeing just "ktime_get", you will see
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"ktime_get+0xb/0x20".
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sym-offset:
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bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
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sym-addr - this will also display the function address as well
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as the function name.
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sym-addr:
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bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
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verbose - This deals with the trace file when the
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latency-format option is enabled.
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bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
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(+0.000ms): simple_strtoul (strict_strtoul)
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raw - This will display raw numbers. This option is best for
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use with user applications that can translate the raw
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numbers better than having it done in the kernel.
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hex - Similar to raw, but the numbers will be in a hexadecimal
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format.
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bin - This will print out the formats in raw binary.
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block - TBD (needs update)
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stacktrace - This is one of the options that changes the trace
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itself. When a trace is recorded, so is the stack
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of functions. This allows for back traces of
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trace sites.
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userstacktrace - This option changes the trace. It records a
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stacktrace of the current userspace thread.
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sym-userobj - when user stacktrace are enabled, look up which
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object the address belongs to, and print a
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relative address. This is especially useful when
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ASLR is on, otherwise you don't get a chance to
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resolve the address to object/file/line after
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the app is no longer running
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The lookup is performed when you read
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trace,trace_pipe. Example:
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a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
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x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
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sched-tree - trace all tasks that are on the runqueue, at
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every scheduling event. Will add overhead if
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there's a lot of tasks running at once.
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latency-format - This option changes the trace. When
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it is enabled, the trace displays
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additional information about the
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latencies, as described in "Latency
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trace format".
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sched_switch
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------------
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This tracer simply records schedule switches. Here is an example
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of how to use it.
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# echo sched_switch > current_tracer
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# echo 1 > tracing_enabled
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# sleep 1
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# echo 0 > tracing_enabled
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# cat trace
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# tracer: sched_switch
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R
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bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R
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sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R
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bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S
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bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R
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sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R
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bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D
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bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R
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<idle>-0 [00] 240.132589: 0:140:R + 4:115:S
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<idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R
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ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R
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<idle>-0 [00] 240.132598: 0:140:R + 4:115:S
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<idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R
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ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R
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sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R
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[...]
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|
As we have discussed previously about this format, the header
|
|
shows the name of the trace and points to the options. The
|
|
"FUNCTION" is a misnomer since here it represents the wake ups
|
|
and context switches.
|
|
|
|
The sched_switch file only lists the wake ups (represented with
|
|
'+') and context switches ('==>') with the previous task or
|
|
current task first followed by the next task or task waking up.
|
|
The format for both of these is PID:KERNEL-PRIO:TASK-STATE.
|
|
Remember that the KERNEL-PRIO is the inverse of the actual
|
|
priority with zero (0) being the highest priority and the nice
|
|
values starting at 100 (nice -20). Below is a quick chart to map
|
|
the kernel priority to user land priorities.
|
|
|
|
Kernel Space User Space
|
|
===============================================================
|
|
0(high) to 98(low) user RT priority 99(high) to 1(low)
|
|
with SCHED_RR or SCHED_FIFO
|
|
---------------------------------------------------------------
|
|
99 sched_priority is not used in scheduling
|
|
decisions(it must be specified as 0)
|
|
---------------------------------------------------------------
|
|
100(high) to 139(low) user nice -20(high) to 19(low)
|
|
---------------------------------------------------------------
|
|
140 idle task priority
|
|
---------------------------------------------------------------
|
|
|
|
The task states are:
|
|
|
|
R - running : wants to run, may not actually be running
|
|
S - sleep : process is waiting to be woken up (handles signals)
|
|
D - disk sleep (uninterruptible sleep) : process must be woken up
|
|
(ignores signals)
|
|
T - stopped : process suspended
|
|
t - traced : process is being traced (with something like gdb)
|
|
Z - zombie : process waiting to be cleaned up
|
|
X - unknown
|
|
|
|
|
|
ftrace_enabled
|
|
--------------
|
|
|
|
The following tracers (listed below) give different output
|
|
depending on whether or not the sysctl ftrace_enabled is set. To
|
|
set ftrace_enabled, one can either use the sysctl function or
|
|
set it via the proc file system interface.
|
|
|
|
sysctl kernel.ftrace_enabled=1
|
|
|
|
or
|
|
|
|
echo 1 > /proc/sys/kernel/ftrace_enabled
|
|
|
|
To disable ftrace_enabled simply replace the '1' with '0' in the
|
|
above commands.
|
|
|
|
When ftrace_enabled is set the tracers will also record the
|
|
functions that are within the trace. The descriptions of the
|
|
tracers will also show an example with ftrace enabled.
|
|
|
|
|
|
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 irqsoff > current_tracer
|
|
# echo latency-format > trace_options
|
|
# echo 0 > tracing_max_latency
|
|
# echo 1 > tracing_enabled
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: irqsoff
|
|
#
|
|
irqsoff latency trace v1.1.5 on 2.6.26
|
|
--------------------------------------------------------------------
|
|
latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: sys_setpgid
|
|
=> ended at: sys_setpgid
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
bash-3730 1d... 0us : _write_lock_irq (sys_setpgid)
|
|
bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid)
|
|
bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid)
|
|
|
|
|
|
Here we see that that we had a latency of 12 microsecs (which is
|
|
very good). The _write_lock_irq in sys_setpgid disabled
|
|
interrupts. The difference between the 12 and the displayed
|
|
timestamp 14us 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 ftrace_enabled not set. If we set the
|
|
ftrace_enabled, we get a much larger output:
|
|
|
|
# tracer: irqsoff
|
|
#
|
|
irqsoff latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: __alloc_pages_internal
|
|
=> ended at: __alloc_pages_internal
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal)
|
|
ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist)
|
|
ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk)
|
|
ls-4339 0d..1 4us : add_preempt_count (_spin_lock)
|
|
ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk)
|
|
ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue)
|
|
ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest)
|
|
ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk)
|
|
ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue)
|
|
ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest)
|
|
ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk)
|
|
ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue)
|
|
[...]
|
|
ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue)
|
|
ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest)
|
|
ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk)
|
|
ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue)
|
|
ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest)
|
|
ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk)
|
|
ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock)
|
|
ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal)
|
|
ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal)
|
|
|
|
|
|
|
|
Here we traced a 50 microsecond latency. But we also see all the
|
|
functions that were called during that time. Note that by
|
|
enabling function tracing, we incur an added overhead. This
|
|
overhead may extend the latency times. But nevertheless, this
|
|
trace has provided some very helpful debugging information.
|
|
|
|
|
|
preemptoff
|
|
----------
|
|
|
|
When preemption is disabled, we may be able to receive
|
|
interrupts but the task cannot be preempted and a higher
|
|
priority task must wait for preemption to be enabled again
|
|
before it can preempt a lower priority task.
|
|
|
|
The preemptoff tracer traces the places that disable preemption.
|
|
Like the irqsoff tracer, it records the maximum latency for
|
|
which preemption was disabled. The control of preemptoff tracer
|
|
is much like the irqsoff tracer.
|
|
|
|
# echo preemptoff > current_tracer
|
|
# echo latency-format > trace_options
|
|
# echo 0 > tracing_max_latency
|
|
# echo 1 > tracing_enabled
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: preemptoff
|
|
#
|
|
preemptoff latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: do_IRQ
|
|
=> ended at: __do_softirq
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
sshd-4261 0d.h. 0us+: irq_enter (do_IRQ)
|
|
sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq)
|
|
sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq)
|
|
|
|
|
|
This has some more changes. Preemption was disabled when an
|
|
interrupt came in (notice the 'h'), and was enabled while doing
|
|
a softirq. (notice the 's'). 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.
|
|
|
|
# tracer: preemptoff
|
|
#
|
|
preemptoff latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: remove_wait_queue
|
|
=> ended at: __do_softirq
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue)
|
|
sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue)
|
|
sshd-4261 0d..1 2us : do_IRQ (common_interrupt)
|
|
sshd-4261 0d..1 2us : irq_enter (do_IRQ)
|
|
sshd-4261 0d..1 2us : idle_cpu (irq_enter)
|
|
sshd-4261 0d..1 3us : add_preempt_count (irq_enter)
|
|
sshd-4261 0d.h1 3us : idle_cpu (irq_enter)
|
|
sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ)
|
|
[...]
|
|
sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock)
|
|
sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
|
|
sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq)
|
|
sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq)
|
|
sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock)
|
|
sshd-4261 0d.h1 14us : irq_exit (do_IRQ)
|
|
sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit)
|
|
sshd-4261 0d..2 15us : do_softirq (irq_exit)
|
|
sshd-4261 0d... 15us : __do_softirq (do_softirq)
|
|
sshd-4261 0d... 16us : __local_bh_disable (__do_softirq)
|
|
sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable)
|
|
sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable)
|
|
[...]
|
|
sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable)
|
|
sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable)
|
|
sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip)
|
|
sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip)
|
|
sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable)
|
|
[...]
|
|
sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq)
|
|
sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq)
|
|
|
|
|
|
The above is an example of the preemptoff trace with
|
|
ftrace_enabled set. Here we see that interrupts were 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.
|
|
|
|
Notice that __do_softirq when called does not have a
|
|
preempt_count. It may seem that we missed a preempt enabling.
|
|
What really happened is that the preempt count is held on the
|
|
thread's stack and we switched to the softirq stack (4K stacks
|
|
in effect). The code does not copy the preempt count, but
|
|
because interrupts are disabled, we do not need to worry about
|
|
it. Having a tracer like this is good for letting people know
|
|
what really happens inside the kernel.
|
|
|
|
|
|
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 preemptirqsoff > current_tracer
|
|
# echo latency-format > trace_options
|
|
# echo 0 > tracing_max_latency
|
|
# echo 1 > tracing_enabled
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: preemptirqsoff
|
|
#
|
|
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: apic_timer_interrupt
|
|
=> ended at: __do_softirq
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
|
|
ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq)
|
|
ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq)
|
|
|
|
|
|
|
|
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 ftrace_enabled set:
|
|
|
|
|
|
# tracer: preemptirqsoff
|
|
#
|
|
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
|
|
-----------------
|
|
=> started at: write_chan
|
|
=> ended at: __do_softirq
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
ls-4473 0.N.. 0us : preempt_schedule (write_chan)
|
|
ls-4473 0dN.1 1us : _spin_lock (schedule)
|
|
ls-4473 0dN.1 2us : add_preempt_count (_spin_lock)
|
|
ls-4473 0d..2 2us : put_prev_task_fair (schedule)
|
|
[...]
|
|
ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts)
|
|
ls-4473 0d..2 13us : __switch_to (schedule)
|
|
sshd-4261 0d..2 14us : finish_task_switch (schedule)
|
|
sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch)
|
|
sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave)
|
|
sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set)
|
|
sshd-4261 0d..2 16us : do_IRQ (common_interrupt)
|
|
sshd-4261 0d..2 17us : irq_enter (do_IRQ)
|
|
sshd-4261 0d..2 17us : idle_cpu (irq_enter)
|
|
sshd-4261 0d..2 18us : add_preempt_count (irq_enter)
|
|
sshd-4261 0d.h2 18us : idle_cpu (irq_enter)
|
|
sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ)
|
|
sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq)
|
|
sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock)
|
|
sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq)
|
|
sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock)
|
|
[...]
|
|
sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq)
|
|
sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock)
|
|
sshd-4261 0d.h2 29us : irq_exit (do_IRQ)
|
|
sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit)
|
|
sshd-4261 0d..3 30us : do_softirq (irq_exit)
|
|
sshd-4261 0d... 30us : __do_softirq (do_softirq)
|
|
sshd-4261 0d... 31us : __local_bh_disable (__do_softirq)
|
|
sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable)
|
|
sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable)
|
|
[...]
|
|
sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip)
|
|
sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip)
|
|
sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt)
|
|
sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt)
|
|
sshd-4261 0d.s3 45us : idle_cpu (irq_enter)
|
|
sshd-4261 0d.s3 46us : add_preempt_count (irq_enter)
|
|
sshd-4261 0d.H3 46us : idle_cpu (irq_enter)
|
|
sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt)
|
|
sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt)
|
|
[...]
|
|
sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt)
|
|
sshd-4261 0d.H3 82us : ktime_get (tick_program_event)
|
|
sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get)
|
|
sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts)
|
|
sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts)
|
|
sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event)
|
|
sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event)
|
|
sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt)
|
|
sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit)
|
|
sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit)
|
|
sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable)
|
|
[...]
|
|
sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action)
|
|
sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq)
|
|
sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq)
|
|
sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq)
|
|
sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable)
|
|
sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq)
|
|
sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq)
|
|
|
|
|
|
This is a very interesting trace. It started with the preemption
|
|
of the ls task. We see that the task had the "need_resched" bit
|
|
set via the 'N' in the trace. Interrupts were disabled before
|
|
the spin_lock at the beginning of the trace. We see that a
|
|
schedule took place to run sshd. When the interrupts were
|
|
enabled, we took an interrupt. On return from the interrupt
|
|
handler, the softirq ran. We took another interrupt while
|
|
running the softirq as we see from the capital 'H'.
|
|
|
|
|
|
wakeup
|
|
------
|
|
|
|
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 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.
|
|
|
|
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 wakeup > current_tracer
|
|
# echo latency-format > trace_options
|
|
# echo 0 > tracing_max_latency
|
|
# echo 1 > tracing_enabled
|
|
# chrt -f 5 sleep 1
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: wakeup
|
|
#
|
|
wakeup latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: sleep-4901 (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 1d.h4 0us+: try_to_wake_up (wake_up_process)
|
|
<idle>-0 1d..4 4us : schedule (cpu_idle)
|
|
|
|
|
|
Running this on an idle system, we see that it only took 4
|
|
microseconds to perform the task switch. Note, since the trace
|
|
marker 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 4901
|
|
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.
|
|
|
|
Doing the same with chrt -r 5 and ftrace_enabled set.
|
|
|
|
# tracer: wakeup
|
|
#
|
|
wakeup latency trace v1.1.5 on 2.6.26-rc8
|
|
--------------------------------------------------------------------
|
|
latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
|
|
-----------------
|
|
| task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
|
|
-----------------
|
|
|
|
# _------=> CPU#
|
|
# / _-----=> irqs-off
|
|
# | / _----=> need-resched
|
|
# || / _---=> hardirq/softirq
|
|
# ||| / _--=> preempt-depth
|
|
# |||| /
|
|
# ||||| delay
|
|
# cmd pid ||||| time | caller
|
|
# \ / ||||| \ | /
|
|
ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process)
|
|
ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb)
|
|
ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up)
|
|
ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup)
|
|
ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr)
|
|
ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup)
|
|
ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up)
|
|
ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up)
|
|
[...]
|
|
ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt)
|
|
ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit)
|
|
ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit)
|
|
ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq)
|
|
[...]
|
|
ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks)
|
|
ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq)
|
|
ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable)
|
|
ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd)
|
|
ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd)
|
|
ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched)
|
|
ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched)
|
|
ksoftirq-7 1.N.2 33us : schedule (__cond_resched)
|
|
ksoftirq-7 1.N.2 33us : add_preempt_count (schedule)
|
|
ksoftirq-7 1.N.3 34us : hrtick_clear (schedule)
|
|
ksoftirq-7 1dN.3 35us : _spin_lock (schedule)
|
|
ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock)
|
|
ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule)
|
|
ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair)
|
|
[...]
|
|
ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline)
|
|
ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock)
|
|
ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline)
|
|
ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock)
|
|
ksoftirq-7 1d..4 50us : schedule (__cond_resched)
|
|
|
|
The interrupt went off while running ksoftirqd. This task runs
|
|
at SCHED_OTHER. Why did not we see the 'N' set early? This may
|
|
be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K
|
|
stacks configured, the interrupt and softirq run with their own
|
|
stack. Some information is held on the top of the task's stack
|
|
(need_resched and preempt_count are both stored there). The
|
|
setting of the NEED_RESCHED bit is done directly to the task's
|
|
stack, but the reading of the NEED_RESCHED is done by looking at
|
|
the current stack, which in this case is the stack for the hard
|
|
interrupt. This hides the fact that NEED_RESCHED has been set.
|
|
We do not see the 'N' until we switch back to the task's
|
|
assigned stack.
|
|
|
|
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.
|
|
|
|
# sysctl kernel.ftrace_enabled=1
|
|
# echo function > current_tracer
|
|
# echo 1 > tracing_enabled
|
|
# usleep 1
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
bash-4003 [00] 123.638713: finish_task_switch <-schedule
|
|
bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch
|
|
bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq
|
|
bash-4003 [00] 123.638715: hrtick_set <-schedule
|
|
bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set
|
|
bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave
|
|
bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set
|
|
bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore
|
|
bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set
|
|
bash-4003 [00] 123.638718: sub_preempt_count <-schedule
|
|
bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule
|
|
bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run
|
|
bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion
|
|
bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common
|
|
bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq
|
|
[...]
|
|
|
|
|
|
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_enabled"), O_WRONLY);
|
|
[...]
|
|
if (condition_hit()) {
|
|
write(trace_fd, "0", 1);
|
|
}
|
|
[...]
|
|
}
|
|
|
|
|
|
Single thread tracing
|
|
---------------------
|
|
|
|
By writing into set_ftrace_pid you can trace a
|
|
single thread. For example:
|
|
|
|
# cat set_ftrace_pid
|
|
no pid
|
|
# echo 3111 > set_ftrace_pid
|
|
# cat set_ftrace_pid
|
|
3111
|
|
# echo function > current_tracer
|
|
# cat trace | head
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
|
|
yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
|
|
yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
|
|
yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
|
|
yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
|
|
yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
|
|
# echo -1 > set_ftrace_pid
|
|
# cat trace |head
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
##### CPU 3 buffer started ####
|
|
yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
|
|
yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
|
|
yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
|
|
yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
|
|
yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
|
|
|
|
If you want to trace a function when executing, you could use
|
|
something like this simple program:
|
|
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <sys/types.h>
|
|
#include <sys/stat.h>
|
|
#include <fcntl.h>
|
|
#include <unistd.h>
|
|
#include <string.h>
|
|
|
|
#define _STR(x) #x
|
|
#define STR(x) _STR(x)
|
|
#define MAX_PATH 256
|
|
|
|
const char *find_debugfs(void)
|
|
{
|
|
static char debugfs[MAX_PATH+1];
|
|
static int debugfs_found;
|
|
char type[100];
|
|
FILE *fp;
|
|
|
|
if (debugfs_found)
|
|
return debugfs;
|
|
|
|
if ((fp = fopen("/proc/mounts","r")) == NULL) {
|
|
perror("/proc/mounts");
|
|
return NULL;
|
|
}
|
|
|
|
while (fscanf(fp, "%*s %"
|
|
STR(MAX_PATH)
|
|
"s %99s %*s %*d %*d\n",
|
|
debugfs, type) == 2) {
|
|
if (strcmp(type, "debugfs") == 0)
|
|
break;
|
|
}
|
|
fclose(fp);
|
|
|
|
if (strcmp(type, "debugfs") != 0) {
|
|
fprintf(stderr, "debugfs not mounted");
|
|
return NULL;
|
|
}
|
|
|
|
strcat(debugfs, "/tracing/");
|
|
debugfs_found = 1;
|
|
|
|
return debugfs;
|
|
}
|
|
|
|
const char *tracing_file(const char *file_name)
|
|
{
|
|
static char trace_file[MAX_PATH+1];
|
|
snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
|
|
return trace_file;
|
|
}
|
|
|
|
int main (int argc, char **argv)
|
|
{
|
|
if (argc < 1)
|
|
exit(-1);
|
|
|
|
if (fork() > 0) {
|
|
int fd, ffd;
|
|
char line[64];
|
|
int s;
|
|
|
|
ffd = open(tracing_file("current_tracer"), O_WRONLY);
|
|
if (ffd < 0)
|
|
exit(-1);
|
|
write(ffd, "nop", 3);
|
|
|
|
fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
|
|
s = sprintf(line, "%d\n", getpid());
|
|
write(fd, line, s);
|
|
|
|
write(ffd, "function", 8);
|
|
|
|
close(fd);
|
|
close(ffd);
|
|
|
|
execvp(argv[1], argv+1);
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
|
|
hw-branch-tracer (x86 only)
|
|
---------------------------
|
|
|
|
This tracer uses the x86 last branch tracing hardware feature to
|
|
collect a branch trace on all cpus with relatively low overhead.
|
|
|
|
The tracer uses a fixed-size circular buffer per cpu and only
|
|
traces ring 0 branches. The trace file dumps that buffer in the
|
|
following format:
|
|
|
|
# tracer: hw-branch-tracer
|
|
#
|
|
# CPU# TO <- FROM
|
|
0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6
|
|
2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a
|
|
0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf
|
|
0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf
|
|
2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a
|
|
0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf
|
|
|
|
|
|
The tracer may be used to dump the trace for the oops'ing cpu on
|
|
a kernel oops into the system log. To enable this,
|
|
ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one
|
|
can either use the sysctl function or set it via the proc system
|
|
interface.
|
|
|
|
sysctl kernel.ftrace_dump_on_oops=1
|
|
|
|
or
|
|
|
|
echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
|
|
|
|
|
|
Here's an example of such a dump after a null pointer
|
|
dereference in a kernel module:
|
|
|
|
[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
|
|
[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
|
|
[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
|
|
[57848.106019] Oops: 0002 [#1] SMP
|
|
[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
|
|
[57848.106019] Dumping ftrace buffer:
|
|
[57848.106019] ---------------------------------
|
|
[...]
|
|
[57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24
|
|
[57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165
|
|
[57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165
|
|
[57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165
|
|
[57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165
|
|
[57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops]
|
|
[57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30
|
|
[57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b
|
|
[57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31
|
|
[57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1
|
|
[57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30
|
|
[...]
|
|
[57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2
|
|
[57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881
|
|
[57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881
|
|
[57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96
|
|
[...]
|
|
[57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3
|
|
[57848.106019] ---------------------------------
|
|
[57848.106019] CPU 0
|
|
[57848.106019] Modules linked in: oops
|
|
[57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23
|
|
[57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops]
|
|
[57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246
|
|
[...]
|
|
|
|
|
|
function graph tracer
|
|
---------------------------
|
|
|
|
This tracer is similar to the function tracer except that it
|
|
probes a function on its entry and its exit. This is done by
|
|
using a dynamically allocated stack of return addresses in each
|
|
task_struct. On function entry the tracer overwrites the return
|
|
address of each function traced to set a custom probe. Thus the
|
|
original return address is stored on the stack of return address
|
|
in the task_struct.
|
|
|
|
Probing on both ends of a function leads to special features
|
|
such as:
|
|
|
|
- measure of a function's time execution
|
|
- having a reliable call stack to draw function calls graph
|
|
|
|
This tracer is useful in several situations:
|
|
|
|
- you want to find the reason of a strange kernel behavior and
|
|
need to see what happens in detail on any areas (or specific
|
|
ones).
|
|
|
|
- you are experiencing weird latencies but it's difficult to
|
|
find its origin.
|
|
|
|
- you want to find quickly which path is taken by a specific
|
|
function
|
|
|
|
- you just want to peek inside a working kernel and want to see
|
|
what happens there.
|
|
|
|
# tracer: function_graph
|
|
#
|
|
# CPU DURATION FUNCTION CALLS
|
|
# | | | | | | |
|
|
|
|
0) | sys_open() {
|
|
0) | do_sys_open() {
|
|
0) | getname() {
|
|
0) | kmem_cache_alloc() {
|
|
0) 1.382 us | __might_sleep();
|
|
0) 2.478 us | }
|
|
0) | strncpy_from_user() {
|
|
0) | might_fault() {
|
|
0) 1.389 us | __might_sleep();
|
|
0) 2.553 us | }
|
|
0) 3.807 us | }
|
|
0) 7.876 us | }
|
|
0) | alloc_fd() {
|
|
0) 0.668 us | _spin_lock();
|
|
0) 0.570 us | expand_files();
|
|
0) 0.586 us | _spin_unlock();
|
|
|
|
|
|
There are several columns that can be dynamically
|
|
enabled/disabled. You can use every combination of options you
|
|
want, depending on your needs.
|
|
|
|
- The cpu number on which the function executed is default
|
|
enabled. It is sometimes better to only trace one cpu (see
|
|
tracing_cpu_mask file) or you might sometimes see unordered
|
|
function calls while cpu tracing switch.
|
|
|
|
hide: echo nofuncgraph-cpu > trace_options
|
|
show: echo funcgraph-cpu > trace_options
|
|
|
|
- The duration (function's time of execution) is displayed on
|
|
the closing bracket line of a function or on the same line
|
|
than the current function in case of a leaf one. It is default
|
|
enabled.
|
|
|
|
hide: echo nofuncgraph-duration > trace_options
|
|
show: echo funcgraph-duration > trace_options
|
|
|
|
- The overhead field precedes the duration field in case of
|
|
reached duration thresholds.
|
|
|
|
hide: echo nofuncgraph-overhead > trace_options
|
|
show: echo funcgraph-overhead > trace_options
|
|
depends on: funcgraph-duration
|
|
|
|
ie:
|
|
|
|
0) | up_write() {
|
|
0) 0.646 us | _spin_lock_irqsave();
|
|
0) 0.684 us | _spin_unlock_irqrestore();
|
|
0) 3.123 us | }
|
|
0) 0.548 us | fput();
|
|
0) + 58.628 us | }
|
|
|
|
[...]
|
|
|
|
0) | putname() {
|
|
0) | kmem_cache_free() {
|
|
0) 0.518 us | __phys_addr();
|
|
0) 1.757 us | }
|
|
0) 2.861 us | }
|
|
0) ! 115.305 us | }
|
|
0) ! 116.402 us | }
|
|
|
|
+ means that the function exceeded 10 usecs.
|
|
! means that the function exceeded 100 usecs.
|
|
|
|
|
|
- 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();
|
|
|
|
|
|
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.pl script (located in the scripts directory). This
|
|
script will process the C object using objdump to find all the
|
|
locations in the .text section that call mcount. (Note, only the
|
|
.text section is processed, since processing other sections like
|
|
.init.text may cause races due to those sections being freed).
|
|
|
|
A new section called "__mcount_loc" is created that holds
|
|
references to all the mcount call sites in the .text section.
|
|
This section is compiled back into the original object. The
|
|
final linker 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, kstop_machine is called to prevent
|
|
races with the CPUS executing code being modified (which can
|
|
cause the CPU to do undesirable things), 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.
|
|
|
|
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_enabled
|
|
# usleep 1
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: ftrace
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
|
|
usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call
|
|
<idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
|
|
|
|
To see which functions are being traced, you can cat the file:
|
|
|
|
# cat set_ftrace_filter
|
|
hrtimer_interrupt
|
|
sys_nanosleep
|
|
|
|
|
|
Perhaps this is not enough. The filters also allow simple wild
|
|
cards. Only the following are currently available
|
|
|
|
<match>* - will match functions that begin with <match>
|
|
*<match> - will match functions that end with <match>
|
|
*<match>* - will match functions that have <match> in it
|
|
|
|
These are the only wild cards which are supported.
|
|
|
|
<match>*<match> will not work.
|
|
|
|
Note: It is better to use quotes to enclose the wild cards,
|
|
otherwise the shell may expand the parameters into names
|
|
of files in the local directory.
|
|
|
|
# echo 'hrtimer_*' > set_ftrace_filter
|
|
|
|
Produces:
|
|
|
|
# tracer: ftrace
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
bash-4003 [00] 1480.611794: hrtimer_init <-copy_process
|
|
bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set
|
|
bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear
|
|
bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
|
|
<idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
|
|
<idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
|
|
<idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
|
|
<idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
|
|
<idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
|
|
|
|
|
|
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: ftrace
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
bash-4043 [01] 115.281644: finish_task_switch <-schedule
|
|
bash-4043 [01] 115.281645: hrtick_set <-schedule
|
|
bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set
|
|
bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run
|
|
bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion
|
|
bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run
|
|
bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop
|
|
bash-4043 [01] 115.281648: wake_up_process <-kthread_stop
|
|
bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process
|
|
|
|
We can see that there's no more lock or preempt tracing.
|
|
|
|
|
|
Dynamic ftrace with the function graph tracer
|
|
---------------------------------------------
|
|
|
|
Although what has been explained above concerns both the
|
|
function tracer and the function-graph-tracer, there are some
|
|
special features only available in the function-graph tracer.
|
|
|
|
If you want to trace only one function and all of its children,
|
|
you just have to echo its name into set_graph_function:
|
|
|
|
echo __do_fault > set_graph_function
|
|
|
|
will produce the following "expanded" trace of the __do_fault()
|
|
function:
|
|
|
|
0) | __do_fault() {
|
|
0) | filemap_fault() {
|
|
0) | find_lock_page() {
|
|
0) 0.804 us | find_get_page();
|
|
0) | __might_sleep() {
|
|
0) 1.329 us | }
|
|
0) 3.904 us | }
|
|
0) 4.979 us | }
|
|
0) 0.653 us | _spin_lock();
|
|
0) 0.578 us | page_add_file_rmap();
|
|
0) 0.525 us | native_set_pte_at();
|
|
0) 0.585 us | _spin_unlock();
|
|
0) | unlock_page() {
|
|
0) 0.541 us | page_waitqueue();
|
|
0) 0.639 us | __wake_up_bit();
|
|
0) 2.786 us | }
|
|
0) + 14.237 us | }
|
|
0) | __do_fault() {
|
|
0) | filemap_fault() {
|
|
0) | find_lock_page() {
|
|
0) 0.698 us | find_get_page();
|
|
0) | __might_sleep() {
|
|
0) 1.412 us | }
|
|
0) 3.950 us | }
|
|
0) 5.098 us | }
|
|
0) 0.631 us | _spin_lock();
|
|
0) 0.571 us | page_add_file_rmap();
|
|
0) 0.526 us | native_set_pte_at();
|
|
0) 0.586 us | _spin_unlock();
|
|
0) | unlock_page() {
|
|
0) 0.533 us | page_waitqueue();
|
|
0) 0.638 us | __wake_up_bit();
|
|
0) 2.793 us | }
|
|
0) + 14.012 us | }
|
|
|
|
You can also expand several functions at once:
|
|
|
|
echo sys_open > set_graph_function
|
|
echo sys_close >> set_graph_function
|
|
|
|
Now if you want to go back to trace all functions you can clear
|
|
this special filter via:
|
|
|
|
echo > set_graph_function
|
|
|
|
|
|
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_enabled
|
|
# usleep 1
|
|
# echo 0 > tracing_enabled
|
|
# cat trace
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
|
|
#
|
|
# cat /tmp/trace.out
|
|
bash-4043 [00] 41.267106: finish_task_switch <-schedule
|
|
bash-4043 [00] 41.267106: hrtick_set <-schedule
|
|
bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set
|
|
bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run
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|
bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion
|
|
bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run
|
|
bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop
|
|
bash-4043 [00] 41.267110: wake_up_process <-kthread_stop
|
|
bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process
|
|
bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up
|
|
|
|
|
|
Note, reading the trace_pipe file will block until more input is
|
|
added. By changing the tracer, trace_pipe will issue an EOF. We
|
|
needed to set the function tracer _before_ we "cat" the
|
|
trace_pipe file.
|
|
|
|
|
|
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)
|
|
|
|
Note, to modify this, you must have tracing completely disabled.
|
|
To do that, echo "nop" into the current_tracer. If the
|
|
current_tracer is not set to "nop", an EINVAL error will be
|
|
returned.
|
|
|
|
# echo nop > current_tracer
|
|
# echo 10000 > buffer_size_kb
|
|
# cat buffer_size_kb
|
|
10000 (units kilobytes)
|
|
|
|
The number of pages which will be allocated is limited to a
|
|
percentage of available memory. Allocating too much will produce
|
|
an error.
|
|
|
|
# echo 1000000000000 > buffer_size_kb
|
|
-bash: echo: write error: Cannot allocate memory
|
|
# cat buffer_size_kb
|
|
85
|
|
|
|
-----------
|
|
|
|
More details can be found in the source code, in the
|
|
kernel/trace/*.c files.
|