42ec632e7b
Impact: Documentation update only Update the version that the ftrace document was written for. Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
1340 lines
51 KiB
Plaintext
1340 lines
51 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 performance
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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 the
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tracers that are currently in ftrace include a tracer to trace
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context switches, the time it takes for a high priority task to
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run after it was woken up, the time interrupts are disabled, and
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more (ftrace allows for tracer plugins, which means that the list of
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tracers can always grow).
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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 well
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as the files to display output.
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To mount the debugfs system:
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# mkdir /debug
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# mount -t debugfs nodev /debug
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(Note: it is more common to mount at /sys/kernel/debug, but for simplicity
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this document will use /debug)
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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: This is used to set or display the current tracer
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that is configured.
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available_tracers: This holds the different types of tracers that
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have been compiled into the kernel. The tracers
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listed here can be configured by echoing their name
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into current_tracer.
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tracing_enabled: 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: This file holds the output of the trace in a human readable
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format (described below).
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latency_trace: This file shows the same trace but the information
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is organized more to display possible latencies
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in the system (described below).
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trace_pipe: 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
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is retrieved. Unlike the "trace" and "latency_trace"
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files, this file is a consumer. This means reading
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from this file causes sequential reads to display
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more current data. Once data is read from this
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file, it is consumed, and will not be read
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again with a sequential read. The "trace" and
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"latency_trace" files are static, and if the
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tracer is not adding more data, they will display
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the same information every time they are read.
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iter_ctrl: 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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trace_max_latency: 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 either
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"trace" or "latency_trace". A new max trace will
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only be recorded if the latency is greater than
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the value in this file. (in microseconds)
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trace_entries: This sets or displays the number of bytes 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 due
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to buffer managment 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: 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: 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: 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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available_filter_functions: 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 - function tracer that uses mcount to trace all functions.
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sched_switch - traces the context switches between tasks.
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irqsoff - 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 via the latency_trace file.
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preemptoff - 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 - 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 - 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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nop - This is not a tracer. To remove all tracers from tracing
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simply echo "nop" into 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 them only
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with the debugfs interface (without using any 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 the trace.
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In this case the tracer is "function". Then a header showing the format. Task
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name "bash", the task PID "4251", the CPU that it was running on
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"01", the timestamp in <secs>.<usecs> format, the function name that was
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traced "path_put" and the parent function that called this function
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"path_walk". The timestamp is the time at which the function was
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entered.
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The sched_switch tracer also includes tracing of task wakeups and
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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 shown as
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"==>". 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 of the
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priority that is usually displayed by user-space tools. Zero represents
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the highest priority (99). Prio 100 starts the "nice" priorities with
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100 being equal to nice -20 and 139 being nice 19. The prio "140" is
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reserved for the idle task which is the lowest priority thread (pid 0).
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Latency trace format
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--------------------
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For traces that display latency times, the latency_trace file gives
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somewhat more information to see why a latency happened. Here is a typical
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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 for which
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interrupts were disabled. It gives the trace version and the version
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of the kernel upon which this was executed on (2.6.26-rc8). Then it displays
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the max latency in microsecs (97 us). The number of trace entries displayed
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and the total number recorded (both are three: #3/3). The type of
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preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
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and are 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 occurred.
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(swapper pid: 0).
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The start and stop (the functions in which the interrupts were disabled and
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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: This differs from the trace file output. The trace file output
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includes an absolute timestamp. The timestamp used by the
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latency_trace file is relative to the start of the trace.
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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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iter_ctrl
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---------
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The iter_ctrl file is used to control what gets printed in the trace
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output. To see what is available, simply cat the file:
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cat /debug/tracing/iter_ctrl
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print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
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noblock nostacktrace nosched-tree
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To disable one of the options, echo in the option prepended with "no".
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echo noprint-parent > /debug/tracing/iter_ctrl
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To enable an option, leave off the "no".
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echo sym-offset > /debug/tracing/iter_ctrl
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Here are the available options:
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print-parent - On function traces, display the calling function
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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 offset
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in the function. For example, instead of seeing just
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"ktime_get", you will see "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 as
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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 latency_trace file.
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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 use with
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user applications that can translate the raw numbers better than
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having it done in the kernel.
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hex - Similar to raw, but the numbers will be in a hexadecimal 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 itself.
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When a trace is recorded, so is the stack of functions.
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This allows for back traces of trace sites.
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sched-tree - TBD (any users??)
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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 > /debug/tracing/current_tracer
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# echo 1 > /debug/tracing/tracing_enabled
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# sleep 1
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# echo 0 > /debug/tracing/tracing_enabled
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# cat /debug/tracing/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
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the name of the trace and points to the options. The "FUNCTION"
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is a misnomer since here it represents the wake ups and context
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switches.
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The sched_switch file only lists the wake ups (represented with '+')
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and context switches ('==>') with the previous task or current task
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first followed by the next task or task waking up. The format for both
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of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
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is the inverse of the actual priority with zero (0) being the highest
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priority and the nice values starting at 100 (nice -20). Below is
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a quick chart to map the kernel priority to user land priorities.
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Kernel priority: 0 to 99 ==> user RT priority 99 to 0
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Kernel priority: 100 to 139 ==> user nice -20 to 19
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Kernel priority: 140 ==> idle task priority
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The task states are:
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R - running : wants to run, may not actually be running
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S - sleep : process is waiting to be woken up (handles signals)
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D - disk sleep (uninterruptible sleep) : process must be woken up
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(ignores signals)
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T - stopped : process suspended
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t - traced : process is being traced (with something like gdb)
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Z - zombie : process waiting to be cleaned up
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X - unknown
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ftrace_enabled
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--------------
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The following tracers (listed below) give different output depending
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on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
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one can either use the sysctl function or set it via the proc
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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
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the above commands.
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When ftrace_enabled is set the tracers will also record the functions
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that are within the trace. The descriptions of the tracers
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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 the
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kernel know of a new mouse event. The result is a latency with the
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reaction time.
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The irqsoff tracer tracks the time for which interrupts are disabled.
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When a new maximum latency is hit, the tracer saves the trace leading up
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to that latency point so that every time a new maximum is reached, the old
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saved trace is discarded and the new trace is saved.
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To reset the maximum, echo 0 into tracing_max_latency. Here is an
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example:
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# echo irqsoff > /debug/tracing/current_tracer
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# echo 0 > /debug/tracing/tracing_max_latency
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# echo 1 > /debug/tracing/tracing_enabled
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# ls -ltr
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[...]
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# echo 0 > /debug/tracing/tracing_enabled
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# cat /debug/tracing/latency_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 > /debug/tracing/current_tracer
|
|
# echo 0 > /debug/tracing/tracing_max_latency
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/latency_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 > /debug/tracing/current_tracer
|
|
# echo 0 > /debug/tracing/tracing_max_latency
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# ls -ltr
|
|
[...]
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/latency_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 > /debug/tracing/current_tracer
|
|
# echo 0 > /debug/tracing/tracing_max_latency
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# chrt -f 5 sleep 1
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/latency_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 > /debug/tracing/current_tracer
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# usleep 1
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/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("/debug/tracing/tracing_enabled", O_WRONLY);
|
|
[...]
|
|
if (condition_hit()) {
|
|
write(trace_fd, "0", 1);
|
|
}
|
|
[...]
|
|
}
|
|
|
|
Note: Here we hard coded the path name. The debugfs mount is not
|
|
guaranteed to be at /debug (and is more commonly at /sys/kernel/debug).
|
|
For simple one time traces, the above is sufficent. For anything else,
|
|
a search through /proc/mounts may be needed to find where the debugfs
|
|
file-system is mounted.
|
|
|
|
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 undesireable 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 /debug/tracing/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 \
|
|
> /debug/tracing/set_ftrace_filter
|
|
# echo ftrace > /debug/tracing/current_tracer
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# usleep 1
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/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 /debug/tracing/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.
|
|
|
|
# echo hrtimer_* > /debug/tracing/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 /debug/tracing/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 > /debug/tracing/set_ftrace_filter
|
|
# cat /debug/tracing/set_ftrace_filter
|
|
#
|
|
|
|
Again, now we want to append.
|
|
|
|
# echo sys_nanosleep > /debug/tracing/set_ftrace_filter
|
|
# cat /debug/tracing/set_ftrace_filter
|
|
sys_nanosleep
|
|
# echo hrtimer_* >> /debug/tracing/set_ftrace_filter
|
|
# cat /debug/tracing/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*' > /debug/tracing/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.
|
|
|
|
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 > /debug/tracing/current_tracer
|
|
# cat /debug/tracing/trace_pipe > /tmp/trace.out &
|
|
[1] 4153
|
|
# echo 1 > /debug/tracing/tracing_enabled
|
|
# usleep 1
|
|
# echo 0 > /debug/tracing/tracing_enabled
|
|
# cat /debug/tracing/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 trace_entries 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 /debug/tracing/trace_entries
|
|
65620
|
|
|
|
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 > /debug/tracing/current_tracer
|
|
# echo 100000 > /debug/tracing/trace_entries
|
|
# cat /debug/tracing/trace_entries
|
|
100045
|
|
|
|
|
|
Notice that we echoed in 100,000 but the size is 100,045. The entries
|
|
are held in individual pages. It allocates the number of pages it takes
|
|
to fulfill the request. If more entries may fit on the last page
|
|
then they will be added.
|
|
|
|
# echo 1 > /debug/tracing/trace_entries
|
|
# cat /debug/tracing/trace_entries
|
|
85
|
|
|
|
This shows us that 85 entries can fit in a single page.
|
|
|
|
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 > /debug/tracing/trace_entries
|
|
-bash: echo: write error: Cannot allocate memory
|
|
# cat /debug/tracing/trace_entries
|
|
85
|
|
|