Documentation / admin-guide / dynamic-debug-howto.rst


Based on kernel version 5.8. Page generated on 2020-08-08 17:40 EST.

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Dynamic debug
+++++++++++++


Introduction
============

This document describes how to use the dynamic debug (dyndbg) feature.

Dynamic debug is designed to allow you to dynamically enable/disable
kernel code to obtain additional kernel information.  Currently, if
``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and
``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically
enabled per-callsite.

If you do not want to enable dynamic debug globally (i.e. in some embedded
system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
modules which you'd like to dynamically debug later.

If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
shortcut for ``print_hex_dump(KERN_DEBUG)``.

For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
its ``prefix_str`` argument, if it is constant string; or ``hexdump``
in case ``prefix_str`` is built dynamically.

Dynamic debug has even more useful features:

 * Simple query language allows turning on and off debugging
   statements by matching any combination of 0 or 1 of:

   - source filename
   - function name
   - line number (including ranges of line numbers)
   - module name
   - format string

 * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``
   which can be read to display the complete list of known debug
   statements, to help guide you

Controlling dynamic debug Behaviour
===================================

The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a
control file in the 'debugfs' filesystem. Thus, you must first mount
the debugfs filesystem, in order to make use of this feature.
Subsequently, we refer to the control file as:
``<debugfs>/dynamic_debug/control``. For example, if you want to enable
printing from source file ``svcsock.c``, line 1603 you simply do::

  nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
				<debugfs>/dynamic_debug/control

If you make a mistake with the syntax, the write will fail thus::

  nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
				<debugfs>/dynamic_debug/control
  -bash: echo: write error: Invalid argument

Note, for systems without 'debugfs' enabled, the control file can be
found in ``/proc/dynamic_debug/control``.

Viewing Dynamic Debug Behaviour
===============================

You can view the currently configured behaviour of all the debug
statements via::

  nullarbor:~ # cat <debugfs>/dynamic_debug/control
  # filename:lineno [module]function flags format
  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
  ...


You can also apply standard Unix text manipulation filters to this
data, e.g.::

  nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
  62

  nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
  42

The third column shows the currently enabled flags for each debug
statement callsite (see below for definitions of the flags).  The
default value, with no flags enabled, is ``=_``.  So you can view all
the debug statement callsites with any non-default flags::

  nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
  # filename:lineno [module]function flags format
  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"

Command Language Reference
==========================

At the lexical level, a command comprises a sequence of words separated
by spaces or tabs.  So these are all equivalent::

  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
				<debugfs>/dynamic_debug/control
  nullarbor:~ # echo -n '  file   svcsock.c     line  1603 +p  ' >
				<debugfs>/dynamic_debug/control
  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
				<debugfs>/dynamic_debug/control

Command submissions are bounded by a write() system call.
Multiple commands can be written together, separated by ``;`` or ``\n``::

  ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
     > <debugfs>/dynamic_debug/control

If your query set is big, you can batch them too::

  ~# cat query-batch-file > <debugfs>/dynamic_debug/control

Another way is to use wildcards. The match rule supports ``*`` (matches
zero or more characters) and ``?`` (matches exactly one character). For
example, you can match all usb drivers::

  ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control

At the syntactical level, a command comprises a sequence of match
specifications, followed by a flags change specification::

  command ::= match-spec* flags-spec

The match-spec's are used to choose a subset of the known pr_debug()
callsites to which to apply the flags-spec.  Think of them as a query
with implicit ANDs between each pair.  Note that an empty list of
match-specs will select all debug statement callsites.

A match specification comprises a keyword, which controls the
attribute of the callsite to be compared, and a value to compare
against.  Possible keywords are:::

  match-spec ::= 'func' string |
		 'file' string |
		 'module' string |
		 'format' string |
		 'line' line-range

  line-range ::= lineno |
		 '-'lineno |
		 lineno'-' |
		 lineno'-'lineno

  lineno ::= unsigned-int

.. note::

  ``line-range`` cannot contain space, e.g.
  "1-30" is valid range but "1 - 30" is not.


The meanings of each keyword are:

func
    The given string is compared against the function name
    of each callsite.  Example::

	func svc_tcp_accept

file
    The given string is compared against either the full pathname, the
    src-root relative pathname, or the basename of the source file of
    each callsite.  Examples::

	file svcsock.c
	file kernel/freezer.c
	file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c

module
    The given string is compared against the module name
    of each callsite.  The module name is the string as
    seen in ``lsmod``, i.e. without the directory or the ``.ko``
    suffix and with ``-`` changed to ``_``.  Examples::

	module sunrpc
	module nfsd

format
    The given string is searched for in the dynamic debug format
    string.  Note that the string does not need to match the
    entire format, only some part.  Whitespace and other
    special characters can be escaped using C octal character
    escape ``\ooo`` notation, e.g. the space character is ``\040``.
    Alternatively, the string can be enclosed in double quote
    characters (``"``) or single quote characters (``'``).
    Examples::

	format svcrdma:         // many of the NFS/RDMA server pr_debugs
	format readahead        // some pr_debugs in the readahead cache
	format nfsd:\040SETATTR // one way to match a format with whitespace
	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace

line
    The given line number or range of line numbers is compared
    against the line number of each ``pr_debug()`` callsite.  A single
    line number matches the callsite line number exactly.  A
    range of line numbers matches any callsite between the first
    and last line number inclusive.  An empty first number means
    the first line in the file, an empty last line number means the
    last line number in the file.  Examples::

	line 1603           // exactly line 1603
	line 1600-1605      // the six lines from line 1600 to line 1605
	line -1605          // the 1605 lines from line 1 to line 1605
	line 1600-          // all lines from line 1600 to the end of the file

The flags specification comprises a change operation followed
by one or more flag characters.  The change operation is one
of the characters::

  -    remove the given flags
  +    add the given flags
  =    set the flags to the given flags

The flags are::

  p    enables the pr_debug() callsite.
  f    Include the function name in the printed message
  l    Include line number in the printed message
  m    Include module name in the printed message
  t    Include thread ID in messages not generated from interrupt context
  _    No flags are set. (Or'd with others on input)

For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag
have meaning, other flags ignored.

For display, the flags are preceded by ``=``
(mnemonic: what the flags are currently equal to).

Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
To clear all flags at once, use ``=_`` or ``-flmpt``.


Debug messages during Boot Process
==================================

To activate debug messages for core code and built-in modules during
the boot process, even before userspace and debugfs exists, use
``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"``
(``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated).  QUERY follows
the syntax described above, but must not exceed 1023 characters.  Your
bootloader may impose lower limits.

These ``dyndbg`` params are processed just after the ddebug tables are
processed, as part of the arch_initcall.  Thus you can enable debug
messages in all code run after this arch_initcall via this boot
parameter.

On an x86 system for example ACPI enablement is a subsys_initcall and::

   dyndbg="file ec.c +p"

will show early Embedded Controller transactions during ACPI setup if
your machine (typically a laptop) has an Embedded Controller.
PCI (or other devices) initialization also is a hot candidate for using
this boot parameter for debugging purposes.

If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
boot time, without effect, but will be reprocessed when module is
loaded later. ``ddebug_query=`` and bare ``dyndbg=`` are only processed at
boot.


Debug Messages at Module Initialization Time
============================================

When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
``foo.params``, strips ``foo.``, and passes them to the kernel along with
params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
in the following order:

1. parameters given via ``/etc/modprobe.d/*.conf``::

	options foo dyndbg=+pt
	options foo dyndbg # defaults to +p

2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::

	foo.dyndbg=" func bar +p; func buz +mp"

3. args to modprobe::

	modprobe foo dyndbg==pmf # override previous settings

These ``dyndbg`` queries are applied in order, with last having final say.
This allows boot args to override or modify those from ``/etc/modprobe.d``
(sensible, since 1 is system wide, 2 is kernel or boot specific), and
modprobe args to override both.

In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
``foo`` is extracted from the param-name, and applied to each query in
``QUERY``, and only 1 match-spec of each type is allowed.

The ``dyndbg`` option is a "fake" module parameter, which means:

- modules do not need to define it explicitly
- every module gets it tacitly, whether they use pr_debug or not
- it doesn't appear in ``/sys/module/$module/parameters/``
  To see it, grep the control file, or inspect ``/proc/cmdline.``

For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
the debugfs interface if the debug messages are no longer needed::

   echo "module module_name -p" > <debugfs>/dynamic_debug/control

Examples
========

::

  // enable the message at line 1603 of file svcsock.c
  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
				<debugfs>/dynamic_debug/control

  // enable all the messages in file svcsock.c
  nullarbor:~ # echo -n 'file svcsock.c +p' >
				<debugfs>/dynamic_debug/control

  // enable all the messages in the NFS server module
  nullarbor:~ # echo -n 'module nfsd +p' >
				<debugfs>/dynamic_debug/control

  // enable all 12 messages in the function svc_process()
  nullarbor:~ # echo -n 'func svc_process +p' >
				<debugfs>/dynamic_debug/control

  // disable all 12 messages in the function svc_process()
  nullarbor:~ # echo -n 'func svc_process -p' >
				<debugfs>/dynamic_debug/control

  // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
  nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
				<debugfs>/dynamic_debug/control

  // enable messages in files of which the paths include string "usb"
  nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control

  // enable all messages
  nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control

  // add module, function to all enabled messages
  nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control

  // boot-args example, with newlines and comments for readability
  Kernel command line: ...
    // see whats going on in dyndbg=value processing
    dynamic_debug.verbose=1
    // enable pr_debugs in 2 builtins, #cmt is stripped
    dyndbg="module params +p #cmt ; module sys +p"
    // enable pr_debugs in 2 functions in a module loaded later
    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"