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750912fa36
Add an "overwrite" trace_option for ftrace to control whether the buffer should be overwritten on overflow or not. The default remains to overwrite old events when the buffer is full. This patch adds the option to instead discard newest events when the buffer is full. This is useful to get a snapshot of traces just after enabling traces. Dropping the current event is also a simpler code path. Signed-off-by: David Sharp <dhsharp@google.com> LKML-Reference: <1291844807-15481-1-git-send-email-dhsharp@google.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
1842 lines
65 KiB
Plaintext
1842 lines
65 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_on:
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This sets or displays whether writing to the trace
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ring buffer is enabled. Echo 0 into this file to disable
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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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This interface also allows for commands to be used. See the
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"Filter commands" section for more details.
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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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"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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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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overwrite - This controls what happens when the trace buffer is
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full. If "1" (default), the oldest events are
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discarded and overwritten. If "0", then the newest
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events are discarded.
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ftrace_enabled
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--------------
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The following tracers (listed below) give different output
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depending on whether or not the sysctl ftrace_enabled is set. To
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set ftrace_enabled, one can either use the sysctl function or
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set it via the proc file system interface.
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sysctl kernel.ftrace_enabled=1
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or
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echo 1 > /proc/sys/kernel/ftrace_enabled
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To disable ftrace_enabled simply replace the '1' with '0' in the
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above commands.
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When ftrace_enabled is set the tracers will also record the
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functions that are within the trace. The descriptions of the
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tracers will also show an example with ftrace enabled.
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irqsoff
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-------
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When interrupts are disabled, the CPU can not react to any other
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external event (besides NMIs and SMIs). This prevents the timer
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interrupt from triggering or the mouse interrupt from letting
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the kernel know of a new mouse event. The result is a latency
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with the reaction time.
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The irqsoff tracer tracks the time for which interrupts are
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disabled. When a new maximum latency is hit, the tracer saves
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the trace leading up to that latency point so that every time a
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new maximum is reached, the old saved trace is discarded and the
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new trace is saved.
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To reset the maximum, echo 0 into tracing_max_latency. Here is
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an example:
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# echo irqsoff > current_tracer
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# echo latency-format > trace_options
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# echo 0 > tracing_max_latency
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# echo 1 > tracing_on
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# ls -ltr
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[...]
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# echo 0 > tracing_on
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# cat 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
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--------------------------------------------------------------------
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latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
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-----------------
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| task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
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-----------------
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=> started at: sys_setpgid
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=> ended at: sys_setpgid
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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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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_on
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > tracing_on
|
|
# 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_on
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > tracing_on
|
|
# 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_on
|
|
# chrt -f 5 sleep 1
|
|
# echo 0 > tracing_on
|
|
# 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_on
|
|
# usleep 1
|
|
# echo 0 > tracing_on
|
|
# 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_on"), O_WRONLY);
|
|
[...]
|
|
if (condition_hit()) {
|
|
write(trace_fd, "0", 1);
|
|
}
|
|
[...]
|
|
}
|
|
|
|
|
|
Single thread tracing
|
|
---------------------
|
|
|
|
By writing into set_ftrace_pid you can trace a
|
|
single thread. For example:
|
|
|
|
# cat set_ftrace_pid
|
|
no pid
|
|
# echo 3111 > set_ftrace_pid
|
|
# cat set_ftrace_pid
|
|
3111
|
|
# echo function > current_tracer
|
|
# cat trace | head
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
|
|
yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
|
|
yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
|
|
yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
|
|
yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
|
|
yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
|
|
# echo -1 > set_ftrace_pid
|
|
# cat trace |head
|
|
# tracer: function
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
##### CPU 3 buffer started ####
|
|
yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
|
|
yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
|
|
yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
|
|
yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
|
|
yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
|
|
|
|
If you want to trace a function when executing, you could use
|
|
something like this simple program:
|
|
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <sys/types.h>
|
|
#include <sys/stat.h>
|
|
#include <fcntl.h>
|
|
#include <unistd.h>
|
|
#include <string.h>
|
|
|
|
#define _STR(x) #x
|
|
#define STR(x) _STR(x)
|
|
#define MAX_PATH 256
|
|
|
|
const char *find_debugfs(void)
|
|
{
|
|
static char debugfs[MAX_PATH+1];
|
|
static int debugfs_found;
|
|
char type[100];
|
|
FILE *fp;
|
|
|
|
if (debugfs_found)
|
|
return debugfs;
|
|
|
|
if ((fp = fopen("/proc/mounts","r")) == NULL) {
|
|
perror("/proc/mounts");
|
|
return NULL;
|
|
}
|
|
|
|
while (fscanf(fp, "%*s %"
|
|
STR(MAX_PATH)
|
|
"s %99s %*s %*d %*d\n",
|
|
debugfs, type) == 2) {
|
|
if (strcmp(type, "debugfs") == 0)
|
|
break;
|
|
}
|
|
fclose(fp);
|
|
|
|
if (strcmp(type, "debugfs") != 0) {
|
|
fprintf(stderr, "debugfs not mounted");
|
|
return NULL;
|
|
}
|
|
|
|
strcat(debugfs, "/tracing/");
|
|
debugfs_found = 1;
|
|
|
|
return debugfs;
|
|
}
|
|
|
|
const char *tracing_file(const char *file_name)
|
|
{
|
|
static char trace_file[MAX_PATH+1];
|
|
snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
|
|
return trace_file;
|
|
}
|
|
|
|
int main (int argc, char **argv)
|
|
{
|
|
if (argc < 1)
|
|
exit(-1);
|
|
|
|
if (fork() > 0) {
|
|
int fd, ffd;
|
|
char line[64];
|
|
int s;
|
|
|
|
ffd = open(tracing_file("current_tracer"), O_WRONLY);
|
|
if (ffd < 0)
|
|
exit(-1);
|
|
write(ffd, "nop", 3);
|
|
|
|
fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
|
|
s = sprintf(line, "%d\n", getpid());
|
|
write(fd, line, s);
|
|
|
|
write(ffd, "function", 8);
|
|
|
|
close(fd);
|
|
close(ffd);
|
|
|
|
execvp(argv[1], argv+1);
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
|
|
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=n
|
|
|
|
or
|
|
|
|
echo n > /proc/sys/kernel/ftrace_dump_on_oops
|
|
|
|
If n = 1, ftrace will dump buffers of all CPUs, if n = 2 ftrace will
|
|
only dump the buffer of the CPU that triggered the 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_on
|
|
# usleep 1
|
|
# echo 0 > tracing_on
|
|
# 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
|
|
|
|
|
|
Filter commands
|
|
---------------
|
|
|
|
A few commands are supported by the set_ftrace_filter interface.
|
|
Trace commands have the following format:
|
|
|
|
<function>:<command>:<parameter>
|
|
|
|
The following commands are supported:
|
|
|
|
- mod
|
|
This command enables function filtering per module. The
|
|
parameter defines the module. For example, if only the write*
|
|
functions in the ext3 module are desired, run:
|
|
|
|
echo 'write*:mod:ext3' > set_ftrace_filter
|
|
|
|
This command interacts with the filter in the same way as
|
|
filtering based on function names. Thus, adding more functions
|
|
in a different module is accomplished by appending (>>) to the
|
|
filter file. Remove specific module functions by prepending
|
|
'!':
|
|
|
|
echo '!writeback*:mod:ext3' >> set_ftrace_filter
|
|
|
|
- traceon/traceoff
|
|
These commands turn tracing on and off when the specified
|
|
functions are hit. The parameter determines how many times the
|
|
tracing system is turned on and off. If unspecified, there is
|
|
no limit. For example, to disable tracing when a schedule bug
|
|
is hit the first 5 times, run:
|
|
|
|
echo '__schedule_bug:traceoff:5' > set_ftrace_filter
|
|
|
|
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' > set_ftrace_filter
|
|
|
|
|
|
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
|
|
#
|
|
# 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
|
|
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.
|