Boot-time tracing

Author:Masami Hiramatsu <>


Boot-time tracing allows users to trace boot-time process including device initialization with full features of ftrace including per-event filter and actions, histograms, kprobe-events and synthetic-events, and trace instances. Since kernel command line is not enough to control these complex features, this uses bootconfig file to describe tracing feature programming.

Options in the Boot Config

Here is the list of available options list for boot time tracing in boot config file [1]. All options are under “ftrace.” or “kernel.” prefix. See kernel parameters for the options which starts with “kernel.” prefix [2].

[1]See Documentation/admin-guide/bootconfig.rst
[2]See Documentation/admin-guide/kernel-parameters.rst

Ftrace Global Options

Ftrace global options have “kernel.” prefix in boot config, which means these options are passed as a part of kernel legacy command line.

Output trace-event data on printk buffer too.
kernel.dump_on_oops [= MODE]
Dump ftrace on Oops. If MODE = 1 or omitted, dump trace buffer on all CPUs. If MODE = 2, dump a buffer on a CPU which kicks Oops.
Stop tracing if WARN_ON() occurs.
kernel.fgraph_max_depth = MAX_DEPTH
Set MAX_DEPTH to maximum depth of fgraph tracer.
kernel.fgraph_filters = FILTER[, FILTER2…]
Add fgraph tracing function filters.
kernel.fgraph_notraces = FILTER[, FILTER2…]
Add fgraph non-tracing function filters.

Ftrace Per-instance Options

These options can be used for each instance including global ftrace node.

ftrace.[instance.INSTANCE.]options = OPT1[, OPT2[…]]
Enable given ftrace options.
ftrace.[instance.INSTANCE.]trace_clock = CLOCK
Set given CLOCK to ftrace’s trace_clock.
ftrace.[instance.INSTANCE.]buffer_size = SIZE
Configure ftrace buffer size to SIZE. You can use “KB” or “MB” for that SIZE.
Allocate snapshot buffer.
ftrace.[instance.INSTANCE.]cpumask = CPUMASK
Set CPUMASK as trace cpu-mask.
ftrace.[instance.INSTANCE.]events = EVENT[, EVENT2[…]]
Enable given events on boot. You can use a wild card in EVENT.
ftrace.[instance.INSTANCE.]tracer = TRACER
Set TRACER to current tracer on boot. (e.g. function)
This will take an array of tracing function filter rules.
This will take an array of NON-tracing function filter rules.

Ftrace Per-Event Options

These options are setting per-event options.

Enable GROUP:EVENT tracing.
ftrace.[instance.INSTANCE.]event.GROUP.EVENT.filter = FILTER
Set FILTER rule to the GROUP:EVENT.
ftrace.[instance.INSTANCE.]event.GROUP.EVENT.actions = ACTION[, ACTION2[…]]
ftrace.[instance.INSTANCE.]event.kprobes.EVENT.probes = PROBE[, PROBE2[…]]
Defines new kprobe event based on PROBEs. It is able to define multiple probes on one event, but those must have same type of arguments. This option is available only for the event which group name is “kprobes”.
ftrace.[instance.INSTANCE.]event.synthetic.EVENT.fields = FIELD[, FIELD2[…]]
Defines new synthetic event with FIELDs. Each field should be “type varname”.

Note that kprobe and synthetic event definitions can be written under instance node, but those are also visible from other instances. So please take care for event name conflict.


For example, to add filter and actions for each event, define kprobe events, and synthetic events with histogram, write a boot config like below:

ftrace.event {
      task.task_newtask {
              filter = "pid < 128"
      kprobes.vfs_read {
              probes = "vfs_read $arg1 $arg2"
              filter = "common_pid < 200"
      synthetic.initcall_latency {
              fields = "unsigned long func", "u64 lat"
              actions = "hist:keys=func.sym,lat:vals=lat:sort=lat"
      initcall.initcall_start {
              actions = "hist:keys=func:ts0=common_timestamp.usecs"
      initcall.initcall_finish {
              actions = "hist:keys=func:lat=common_timestamp.usecs-$ts0:onmatch(initcall.initcall_start).initcall_latency(func,$lat)"

Also, boot-time tracing supports “instance” node, which allows us to run several tracers for different purpose at once. For example, one tracer is for tracing functions starting with “user_”, and others tracing “kernel_” functions, you can write boot config as below:

ftrace.instance {
      foo {
              tracer = "function"
              ftrace.filters = "user_*"
      bar {
              tracer = "function"
              ftrace.filters = "kernel_*"

The instance node also accepts event nodes so that each instance can customize its event tracing.

This boot-time tracing also supports ftrace kernel parameters via boot config. For example, following kernel parameters:

trace_options=sym-addr trace_event=initcall:* tp_printk trace_buf_size=1M ftrace=function ftrace_filter="vfs*"

This can be written in boot config like below:

kernel {
      trace_options = sym-addr
      trace_event = "initcall:*"
      trace_buf_size = 1M
      ftrace = function
      ftrace_filter = "vfs*"

Note that parameters start with “kernel” prefix instead of “ftrace”.