1 .. SPDX-License-Identifier: GPL-2.0
2
3 =================
4 Boot-time tracing
5 =================
6
7 :Author: Masami Hiramatsu <mhiramat@kernel.org>
8
9 Overview
10 ========
11
12 Boot-time tracing allows users to trace boot-time process including
13 device initialization with full features of ftrace including per-event
14 filter and actions, histograms, kprobe-events and synthetic-events,
15 and trace instances.
16 Since kernel command line is not enough to control these complex features,
17 this uses bootconfig file to describe tracing feature programming.
18
19 Options in the Boot Config
20 ==========================
21
22 Here is the list of available options list for boot time tracing in
23 boot config file [1]_. All options are under "ftrace." or "kernel."
24 prefix. See kernel parameters for the options which starts
25 with "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
30 Ftrace Global Options
31 ---------------------
32
33 Ftrace global options have "kernel." prefix in boot config, which means
34 these options are passed as a part of kernel legacy command line.
35
36 kernel.tp_printk
37 Output trace-event data on printk buffer too.
38
39 kernel.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
43 kernel.traceoff_on_warning
44 Stop tracing if WARN_ON() occurs.
45
46 kernel.fgraph_max_depth = MAX_DEPTH
47 Set MAX_DEPTH to maximum depth of fgraph tracer.
48
49 kernel.fgraph_filters = FILTER[, FILTER2...]
50 Add fgraph tracing function filters.
51
52 kernel.fgraph_notraces = FILTER[, FILTER2...]
53 Add fgraph non-tracing function filters.
54
55
56 Ftrace Per-instance Options
57 ---------------------------
58
59 These options can be used for each instance including global ftrace node.
60
61 ftrace.[instance.INSTANCE.]options = OPT1[, OPT2[...]]
62 Enable given ftrace options.
63
64 ftrace.[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
68 ftrace.[instance.INSTANCE.]trace_clock = CLOCK
69 Set given CLOCK to ftrace's trace_clock.
70
71 ftrace.[instance.INSTANCE.]buffer_size = SIZE
72 Configure ftrace buffer size to SIZE. You can use "KB" or "MB"
73 for that SIZE.
74
75 ftrace.[instance.INSTANCE.]alloc_snapshot
76 Allocate snapshot buffer.
77
78 ftrace.[instance.INSTANCE.]cpumask = CPUMASK
79 Set CPUMASK as trace cpu-mask.
80
81 ftrace.[instance.INSTANCE.]events = EVENT[, EVENT2[...]]
82 Enable given events on boot. You can use a wild card in EVENT.
83
84 ftrace.[instance.INSTANCE.]tracer = TRACER
85 Set TRACER to current tracer on boot. (e.g. function)
86
87 ftrace.[instance.INSTANCE.]ftrace.filters
88 This will take an array of tracing function filter rules.
89
90 ftrace.[instance.INSTANCE.]ftrace.notraces
91 This will take an array of NON-tracing function filter rules.
92
93
94 Ftrace Per-Event Options
95 ------------------------
96
97 These options are setting per-event options.
98
99 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.enable
100 Enable GROUP:EVENT tracing.
101
102 ftrace.[instance.INSTANCE.]event.GROUP.enable
103 Enable all event tracing within GROUP.
104
105 ftrace.[instance.INSTANCE.]event.enable
106 Enable all event tracing.
107
108 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.filter = FILTER
109 Set FILTER rule to the GROUP:EVENT.
110
111 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.actions = ACTION[, ACTION2[...]]
112 Set ACTIONs to the GROUP:EVENT.
113
114 ftrace.[instance.INSTANCE.]event.kprobes.EVENT.probes = PROBE[, PROBE2[...]]
115 Defines new kprobe event based on PROBEs. It is able to define
116 multiple probes on one event, but those must have same type of
117 arguments. This option is available only for the event which
118 group name is "kprobes".
119
120 ftrace.[instance.INSTANCE.]event.synthetic.EVENT.fields = FIELD[, FIELD2[...]]
121 Defines new synthetic event with FIELDs. Each field should be
122 "type varname".
123
124 Note that kprobe and synthetic event definitions can be written under
125 instance node, but those are also visible from other instances. So please
126 take care for event name conflict.
127
128 Ftrace Histogram Options
129 ------------------------
130
131 Since it is too long to write a histogram action as a string for per-event
132 action option, there are tree-style options under per-event 'hist' subkey
133 for the histogram actions. For the detail of the each parameter,
134 please read the event histogram document (Documentation/trace/histogram.rst)
135
136 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]keys = KEY1[, KEY2[...]]
137 Set histogram key parameters. (Mandatory)
138 The 'N' is a digit string for the multiple histogram. You can omit it
139 if there is one histogram on the event.
140
141 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]values = VAL1[, VAL2[...]]
142 Set histogram value parameters.
143
144 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]sort = SORT1[, SORT2[...]]
145 Set histogram sort parameter options.
146
147 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]size = NR_ENTRIES
148 Set histogram size (number of entries).
149
150 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]name = NAME
151 Set histogram name.
152
153 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]var.VARIABLE = EXPR
154 Define a new VARIABLE by EXPR expression.
155
156 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<pause|continue|clear>
157 Set histogram control parameter. You can set one of them.
158
159 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmatch.[M.]event = GROUP.EVENT
160 Set histogram 'onmatch' handler matching event parameter.
161 The 'M' is a digit string for the multiple 'onmatch' handler. You can omit it
162 if there is one 'onmatch' handler on this histogram.
163
164 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmatch.[M.]trace = EVENT[, ARG1[...]]
165 Set histogram 'trace' action for 'onmatch'.
166 EVENT must be a synthetic event name, and ARG1... are parameters
167 for that event. Mandatory if 'onmatch.event' option is set.
168
169 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmax.[M.]var = VAR
170 Set histogram 'onmax' handler variable parameter.
171
172 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onchange.[M.]var = VAR
173 Set histogram 'onchange' handler variable parameter.
174
175 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<onmax|onchange>.[M.]save = ARG1[, ARG2[...]]
176 Set histogram 'save' action parameters for 'onmax' or 'onchange' handler.
177 This option or below 'snapshot' option is mandatory if 'onmax.var' or
178 'onchange.var' option is set.
179
180 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<onmax|onchange>.[M.]snapshot
181 Set histogram 'snapshot' action for 'onmax' or 'onchange' handler.
182 This option or above 'save' option is mandatory if 'onmax.var' or
183 'onchange.var' option is set.
184
185 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.filter = FILTER_EXPR
186 Set histogram filter expression. You don't need 'if' in the FILTER_EXPR.
187
188 Note that this 'hist' option can conflict with the per-event 'actions'
189 option if the 'actions' option has a histogram action.
190
191
192 When to Start
193 =============
194
195 All boot-time tracing options starting with ``ftrace`` will be enabled at the
196 end of core_initcall. This means you can trace the events from postcore_initcall.
197 Most of the subsystems and architecture dependent drivers will be initialized
198 after that (arch_initcall or subsys_initcall). Thus, you can trace those with
199 boot-time tracing.
200 If you want to trace events before core_initcall, you can use the options
201 starting with ``kernel``. Some of them will be enabled eariler than the initcall
202 processing (for example,. ``kernel.ftrace=function`` and ``kernel.trace_event``
203 will start before the initcall.)
204
205
206 Examples
207 ========
208
209 For example, to add filter and actions for each event, define kprobe
210 events, and synthetic events with histogram, write a boot config like
211 below::
212
213 ftrace.event {
214 task.task_newtask {
215 filter = "pid < 128"
216 enable
217 }
218 kprobes.vfs_read {
219 probes = "vfs_read $arg1 $arg2"
220 filter = "common_pid < 200"
221 enable
222 }
223 synthetic.initcall_latency {
224 fields = "unsigned long func", "u64 lat"
225 hist {
226 keys = func.sym, lat
227 values = lat
228 sort = lat
229 }
230 }
231 initcall.initcall_start.hist {
232 keys = func
233 var.ts0 = common_timestamp.usecs
234 }
235 initcall.initcall_finish.hist {
236 keys = func
237 var.lat = common_timestamp.usecs - $ts0
238 onmatch {
239 event = initcall.initcall_start
240 trace = initcall_latency, func, $lat
241 }
242 }
243 }
244
245 Also, boot-time tracing supports "instance" node, which allows us to run
246 several tracers for different purpose at once. For example, one tracer
247 is for tracing functions starting with "user\_", and others tracing
248 "kernel\_" functions, you can write boot config as below::
249
250 ftrace.instance {
251 foo {
252 tracer = "function"
253 ftrace.filters = "user_*"
254 }
255 bar {
256 tracer = "function"
257 ftrace.filters = "kernel_*"
258 }
259 }
260
261 The instance node also accepts event nodes so that each instance
262 can customize its event tracing.
263
264 With the trigger action and kprobes, you can trace function-graph while
265 a function is called. For example, this will trace all function calls in
266 the pci_proc_init()::
267
268 ftrace {
269 tracing_on = 0
270 tracer = function_graph
271 event.kprobes {
272 start_event {
273 probes = "pci_proc_init"
274 actions = "traceon"
275 }
276 end_event {
277 probes = "pci_proc_init%return"
278 actions = "traceoff"
279 }
280 }
281 }
282
283
284 This boot-time tracing also supports ftrace kernel parameters via boot
285 config.
286 For example, following kernel parameters::
287
288 trace_options=sym-addr trace_event=initcall:* tp_printk trace_buf_size=1M ftrace=function ftrace_filter="vfs*"
289
290 This can be written in boot config like below::
291
292 kernel {
293 trace_options = sym-addr
294 trace_event = "initcall:*"
295 tp_printk
296 trace_buf_size = 1M
297 ftrace = function
298 ftrace_filter = "vfs*"
299 }
300
301 Note that parameters start with "kernel" prefix instead of "ftrace".
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