.. SPDX-License-Identifier: GPL-2.0

=================
Boot-time tracing
=================

:Author: Masami Hiramatsu <mhiramat@kernel.org>

Overview
========

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 :ref:`Documentation/admin-guide/bootconfig.rst <bootconfig>`
.. [2] See :ref:`Documentation/admin-guide/kernel-parameters.rst <kernelparameters>`

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.

kernel.tp_printk
   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.

kernel.traceoff_on_warning
   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.]tracing_on = 0|1
   Enable/Disable tracing on this instance when starting boot-time tracing.
   (you can enable it by the "traceon" event trigger action)

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.

ftrace.[instance.INSTANCE.]alloc_snapshot
   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)

ftrace.[instance.INSTANCE.]ftrace.filters
   This will take an array of tracing function filter rules.

ftrace.[instance.INSTANCE.]ftrace.notraces
   This will take an array of NON-tracing function filter rules.


Ftrace Per-Event Options
------------------------

These options are setting per-event options.

ftrace.[instance.INSTANCE.]event.GROUP.EVENT.enable
   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[...]]
   Set ACTIONs to the GROUP:EVENT.

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.


When to Start
=============

All boot-time tracing options starting with ``ftrace`` will be enabled at the
end of core_initcall. This means you can trace the events from postcore_initcall.
Most of the subsystems and architecture dependent drivers will be initialized
after that (arch_initcall or subsys_initcall). Thus, you can trace those with
boot-time tracing.
If you want to trace events before core_initcall, you can use the options
starting with ``kernel``. Some of them will be enabled eariler than the initcall
processing (for example,. ``kernel.ftrace=function`` and ``kernel.trace_event``
will start before the initcall.)


Examples
========

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"
                enable
        }
        kprobes.vfs_read {
                probes = "vfs_read $arg1 $arg2"
                filter = "common_pid < 200"
                enable
        }
        synthetic.initcall_latency {
                fields = "unsigned long func", "u64 lat"
                actions = "hist:keys=func.sym,lat:vals=lat:sort=lat"
        }
        initcall.initcall_start {
                actions = "hist:keys=func:ts0=common_timestamp.usecs"
        }
        initcall.initcall_finish {
                actions = "hist:keys=func:lat=common_timestamp.usecs-$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.

With the trigger action and kprobes, you can trace function-graph while
a function is called. For example, this will trace all function calls in
the pci_proc_init()::

  ftrace {
        tracing_on = 0
        tracer = function_graph
        event.kprobes {
                start_event {
                        probes = "pci_proc_init"
                        actions = "traceon"
                }
                end_event {
                        probes = "pci_proc_init%return"
                        actions = "traceoff"
                }
        }
  }


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:*"
        tp_printk
        trace_buf_size = 1M
        ftrace = function
        ftrace_filter = "vfs*"
  }

Note that parameters start with "kernel" prefix instead of "ftrace".