1.. SPDX-License-Identifier: GPL-2.0
2
3=================
4Boot-time tracing
5=================
6
7:Author: Masami Hiramatsu <mhiramat@kernel.org>
8
9Overview
10========
11
12Boot-time tracing allows users to trace boot-time process including
13device initialization with full features of ftrace including per-event
14filter and actions, histograms, kprobe-events and synthetic-events,
15and trace instances.
16Since kernel command line is not enough to control these complex features,
17this uses bootconfig file to describe tracing feature programming.
18
19Options in the Boot Config
20==========================
21
22Here is the list of available options list for boot time tracing in
23boot config file [1]_. All options are under "ftrace." or "kernel."
24prefix. See kernel parameters for the options which starts
25with "kernel." prefix [2]_.
26
27.. [1] See :ref:`Documentation/admin-guide/bootconfig.rst <bootconfig>`
28.. [2] See :ref:`Documentation/admin-guide/kernel-parameters.rst <kernelparameters>`
29
30Ftrace Global Options
31---------------------
32
33Ftrace global options have "kernel." prefix in boot config, which means
34these options are passed as a part of kernel legacy command line.
35
36kernel.tp_printk
37   Output trace-event data on printk buffer too.
38
39kernel.dump_on_oops [= MODE]
40   Dump ftrace on Oops. If MODE = 1 or omitted, dump trace buffer
41   on all CPUs. If MODE = 2, dump a buffer on a CPU which kicks Oops.
42
43kernel.traceoff_on_warning
44   Stop tracing if WARN_ON() occurs.
45
46kernel.fgraph_max_depth = MAX_DEPTH
47   Set MAX_DEPTH to maximum depth of fgraph tracer.
48
49kernel.fgraph_filters = FILTER[, FILTER2...]
50   Add fgraph tracing function filters.
51
52kernel.fgraph_notraces = FILTER[, FILTER2...]
53   Add fgraph non-tracing function filters.
54
55
56Ftrace Per-instance Options
57---------------------------
58
59These options can be used for each instance including global ftrace node.
60
61ftrace.[instance.INSTANCE.]options = OPT1[, OPT2[...]]
62   Enable given ftrace options.
63
64ftrace.[instance.INSTANCE.]tracing_on = 0|1
65   Enable/Disable tracing on this instance when starting boot-time tracing.
66   (you can enable it by the "traceon" event trigger action)
67
68ftrace.[instance.INSTANCE.]trace_clock = CLOCK
69   Set given CLOCK to ftrace's trace_clock.
70
71ftrace.[instance.INSTANCE.]buffer_size = SIZE
72   Configure ftrace buffer size to SIZE. You can use "KB" or "MB"
73   for that SIZE.
74
75ftrace.[instance.INSTANCE.]alloc_snapshot
76   Allocate snapshot buffer.
77
78ftrace.[instance.INSTANCE.]cpumask = CPUMASK
79   Set CPUMASK as trace cpu-mask.
80
81ftrace.[instance.INSTANCE.]events = EVENT[, EVENT2[...]]
82   Enable given events on boot. You can use a wild card in EVENT.
83
84ftrace.[instance.INSTANCE.]tracer = TRACER
85   Set TRACER to current tracer on boot. (e.g. function)
86
87ftrace.[instance.INSTANCE.]ftrace.filters
88   This will take an array of tracing function filter rules.
89
90ftrace.[instance.INSTANCE.]ftrace.notraces
91   This will take an array of NON-tracing function filter rules.
92
93
94Ftrace Per-Event Options
95------------------------
96
97These options are setting per-event options.
98
99ftrace.[instance.INSTANCE.]event.GROUP.EVENT.enable
100   Enable GROUP:EVENT tracing.
101
102ftrace.[instance.INSTANCE.]event.GROUP.EVENT.filter = FILTER
103   Set FILTER rule to the GROUP:EVENT.
104
105ftrace.[instance.INSTANCE.]event.GROUP.EVENT.actions = ACTION[, ACTION2[...]]
106   Set ACTIONs to the GROUP:EVENT.
107
108ftrace.[instance.INSTANCE.]event.kprobes.EVENT.probes = PROBE[, PROBE2[...]]
109   Defines new kprobe event based on PROBEs. It is able to define
110   multiple probes on one event, but those must have same type of
111   arguments. This option is available only for the event which
112   group name is "kprobes".
113
114ftrace.[instance.INSTANCE.]event.synthetic.EVENT.fields = FIELD[, FIELD2[...]]
115   Defines new synthetic event with FIELDs. Each field should be
116   "type varname".
117
118Note that kprobe and synthetic event definitions can be written under
119instance node, but those are also visible from other instances. So please
120take care for event name conflict.
121
122
123When to Start
124=============
125
126All boot-time tracing options starting with ``ftrace`` will be enabled at the
127end of core_initcall. This means you can trace the events from postcore_initcall.
128Most of the subsystems and architecture dependent drivers will be initialized
129after that (arch_initcall or subsys_initcall). Thus, you can trace those with
130boot-time tracing.
131If you want to trace events before core_initcall, you can use the options
132starting with ``kernel``. Some of them will be enabled eariler than the initcall
133processing (for example,. ``kernel.ftrace=function`` and ``kernel.trace_event``
134will start before the initcall.)
135
136
137Examples
138========
139
140For example, to add filter and actions for each event, define kprobe
141events, and synthetic events with histogram, write a boot config like
142below::
143
144  ftrace.event {
145        task.task_newtask {
146                filter = "pid < 128"
147                enable
148        }
149        kprobes.vfs_read {
150                probes = "vfs_read $arg1 $arg2"
151                filter = "common_pid < 200"
152                enable
153        }
154        synthetic.initcall_latency {
155                fields = "unsigned long func", "u64 lat"
156                actions = "hist:keys=func.sym,lat:vals=lat:sort=lat"
157        }
158        initcall.initcall_start {
159                actions = "hist:keys=func:ts0=common_timestamp.usecs"
160        }
161        initcall.initcall_finish {
162                actions = "hist:keys=func:lat=common_timestamp.usecs-$ts0:onmatch(initcall.initcall_start).initcall_latency(func,$lat)"
163        }
164  }
165
166Also, boot-time tracing supports "instance" node, which allows us to run
167several tracers for different purpose at once. For example, one tracer
168is for tracing functions starting with "user\_", and others tracing
169"kernel\_" functions, you can write boot config as below::
170
171  ftrace.instance {
172        foo {
173                tracer = "function"
174                ftrace.filters = "user_*"
175        }
176        bar {
177                tracer = "function"
178                ftrace.filters = "kernel_*"
179        }
180  }
181
182The instance node also accepts event nodes so that each instance
183can customize its event tracing.
184
185With the trigger action and kprobes, you can trace function-graph while
186a function is called. For example, this will trace all function calls in
187the pci_proc_init()::
188
189  ftrace {
190        tracing_on = 0
191        tracer = function_graph
192        event.kprobes {
193                start_event {
194                        probes = "pci_proc_init"
195                        actions = "traceon"
196                }
197                end_event {
198                        probes = "pci_proc_init%return"
199                        actions = "traceoff"
200                }
201        }
202  }
203
204
205This boot-time tracing also supports ftrace kernel parameters via boot
206config.
207For example, following kernel parameters::
208
209 trace_options=sym-addr trace_event=initcall:* tp_printk trace_buf_size=1M ftrace=function ftrace_filter="vfs*"
210
211This can be written in boot config like below::
212
213  kernel {
214        trace_options = sym-addr
215        trace_event = "initcall:*"
216        tp_printk
217        trace_buf_size = 1M
218        ftrace = function
219        ftrace_filter = "vfs*"
220  }
221
222Note that parameters start with "kernel" prefix instead of "ftrace".
223