1Dynamic debug
2+++++++++++++
3
4
5Introduction
6============
7
8This document describes how to use the dynamic debug (dyndbg) feature.
9
10Dynamic debug is designed to allow you to dynamically enable/disable
11kernel code to obtain additional kernel information.  Currently, if
12``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and
13``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically
14enabled per-callsite.
15
16If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
17shortcut for ``print_hex_dump(KERN_DEBUG)``.
18
19For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
20its ``prefix_str`` argument, if it is constant string; or ``hexdump``
21in case ``prefix_str`` is built dynamically.
22
23Dynamic debug has even more useful features:
24
25 * Simple query language allows turning on and off debugging
26   statements by matching any combination of 0 or 1 of:
27
28   - source filename
29   - function name
30   - line number (including ranges of line numbers)
31   - module name
32   - format string
33
34 * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``
35   which can be read to display the complete list of known debug
36   statements, to help guide you
37
38Controlling dynamic debug Behaviour
39===================================
40
41The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a
42control file in the 'debugfs' filesystem. Thus, you must first mount
43the debugfs filesystem, in order to make use of this feature.
44Subsequently, we refer to the control file as:
45``<debugfs>/dynamic_debug/control``. For example, if you want to enable
46printing from source file ``svcsock.c``, line 1603 you simply do::
47
48  nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
49				<debugfs>/dynamic_debug/control
50
51If you make a mistake with the syntax, the write will fail thus::
52
53  nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
54				<debugfs>/dynamic_debug/control
55  -bash: echo: write error: Invalid argument
56
57Viewing Dynamic Debug Behaviour
58===============================
59
60You can view the currently configured behaviour of all the debug
61statements via::
62
63  nullarbor:~ # cat <debugfs>/dynamic_debug/control
64  # filename:lineno [module]function flags format
65  /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"
66  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
67  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
68  /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
69  ...
70
71
72You can also apply standard Unix text manipulation filters to this
73data, e.g.::
74
75  nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
76  62
77
78  nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
79  42
80
81The third column shows the currently enabled flags for each debug
82statement callsite (see below for definitions of the flags).  The
83default value, with no flags enabled, is ``=_``.  So you can view all
84the debug statement callsites with any non-default flags::
85
86  nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
87  # filename:lineno [module]function flags format
88  /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"
89
90Command Language Reference
91==========================
92
93At the lexical level, a command comprises a sequence of words separated
94by spaces or tabs.  So these are all equivalent::
95
96  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
97				<debugfs>/dynamic_debug/control
98  nullarbor:~ # echo -n '  file   svcsock.c     line  1603 +p  ' >
99				<debugfs>/dynamic_debug/control
100  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
101				<debugfs>/dynamic_debug/control
102
103Command submissions are bounded by a write() system call.
104Multiple commands can be written together, separated by ``;`` or ``\n``::
105
106  ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
107     > <debugfs>/dynamic_debug/control
108
109If your query set is big, you can batch them too::
110
111  ~# cat query-batch-file > <debugfs>/dynamic_debug/control
112
113Another way is to use wildcards. The match rule supports ``*`` (matches
114zero or more characters) and ``?`` (matches exactly one character). For
115example, you can match all usb drivers::
116
117  ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control
118
119At the syntactical level, a command comprises a sequence of match
120specifications, followed by a flags change specification::
121
122  command ::= match-spec* flags-spec
123
124The match-spec's are used to choose a subset of the known pr_debug()
125callsites to which to apply the flags-spec.  Think of them as a query
126with implicit ANDs between each pair.  Note that an empty list of
127match-specs will select all debug statement callsites.
128
129A match specification comprises a keyword, which controls the
130attribute of the callsite to be compared, and a value to compare
131against.  Possible keywords are:::
132
133  match-spec ::= 'func' string |
134		 'file' string |
135		 'module' string |
136		 'format' string |
137		 'line' line-range
138
139  line-range ::= lineno |
140		 '-'lineno |
141		 lineno'-' |
142		 lineno'-'lineno
143
144  lineno ::= unsigned-int
145
146.. note::
147
148  ``line-range`` cannot contain space, e.g.
149  "1-30" is valid range but "1 - 30" is not.
150
151
152The meanings of each keyword are:
153
154func
155    The given string is compared against the function name
156    of each callsite.  Example::
157
158	func svc_tcp_accept
159
160file
161    The given string is compared against either the full pathname, the
162    src-root relative pathname, or the basename of the source file of
163    each callsite.  Examples::
164
165	file svcsock.c
166	file kernel/freezer.c
167	file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
168
169module
170    The given string is compared against the module name
171    of each callsite.  The module name is the string as
172    seen in ``lsmod``, i.e. without the directory or the ``.ko``
173    suffix and with ``-`` changed to ``_``.  Examples::
174
175	module sunrpc
176	module nfsd
177
178format
179    The given string is searched for in the dynamic debug format
180    string.  Note that the string does not need to match the
181    entire format, only some part.  Whitespace and other
182    special characters can be escaped using C octal character
183    escape ``\ooo`` notation, e.g. the space character is ``\040``.
184    Alternatively, the string can be enclosed in double quote
185    characters (``"``) or single quote characters (``'``).
186    Examples::
187
188	format svcrdma:         // many of the NFS/RDMA server pr_debugs
189	format readahead        // some pr_debugs in the readahead cache
190	format nfsd:\040SETATTR // one way to match a format with whitespace
191	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
192	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
193
194line
195    The given line number or range of line numbers is compared
196    against the line number of each ``pr_debug()`` callsite.  A single
197    line number matches the callsite line number exactly.  A
198    range of line numbers matches any callsite between the first
199    and last line number inclusive.  An empty first number means
200    the first line in the file, an empty last line number means the
201    last line number in the file.  Examples::
202
203	line 1603           // exactly line 1603
204	line 1600-1605      // the six lines from line 1600 to line 1605
205	line -1605          // the 1605 lines from line 1 to line 1605
206	line 1600-          // all lines from line 1600 to the end of the file
207
208The flags specification comprises a change operation followed
209by one or more flag characters.  The change operation is one
210of the characters::
211
212  -    remove the given flags
213  +    add the given flags
214  =    set the flags to the given flags
215
216The flags are::
217
218  p    enables the pr_debug() callsite.
219  f    Include the function name in the printed message
220  l    Include line number in the printed message
221  m    Include module name in the printed message
222  t    Include thread ID in messages not generated from interrupt context
223  _    No flags are set. (Or'd with others on input)
224
225For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag
226have meaning, other flags ignored.
227
228For display, the flags are preceded by ``=``
229(mnemonic: what the flags are currently equal to).
230
231Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
232To clear all flags at once, use ``=_`` or ``-flmpt``.
233
234
235Debug messages during Boot Process
236==================================
237
238To activate debug messages for core code and built-in modules during
239the boot process, even before userspace and debugfs exists, use
240``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"``
241(``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated).  QUERY follows
242the syntax described above, but must not exceed 1023 characters.  Your
243bootloader may impose lower limits.
244
245These ``dyndbg`` params are processed just after the ddebug tables are
246processed, as part of the arch_initcall.  Thus you can enable debug
247messages in all code run after this arch_initcall via this boot
248parameter.
249
250On an x86 system for example ACPI enablement is a subsys_initcall and::
251
252   dyndbg="file ec.c +p"
253
254will show early Embedded Controller transactions during ACPI setup if
255your machine (typically a laptop) has an Embedded Controller.
256PCI (or other devices) initialization also is a hot candidate for using
257this boot parameter for debugging purposes.
258
259If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
260boot time, without effect, but will be reprocessed when module is
261loaded later. ``ddebug_query=`` and bare ``dyndbg=`` are only processed at
262boot.
263
264
265Debug Messages at Module Initialization Time
266============================================
267
268When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
269``foo.params``, strips ``foo.``, and passes them to the kernel along with
270params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
271in the following order:
272
2731. parameters given via ``/etc/modprobe.d/*.conf``::
274
275	options foo dyndbg=+pt
276	options foo dyndbg # defaults to +p
277
2782. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::
279
280	foo.dyndbg=" func bar +p; func buz +mp"
281
2823. args to modprobe::
283
284	modprobe foo dyndbg==pmf # override previous settings
285
286These ``dyndbg`` queries are applied in order, with last having final say.
287This allows boot args to override or modify those from ``/etc/modprobe.d``
288(sensible, since 1 is system wide, 2 is kernel or boot specific), and
289modprobe args to override both.
290
291In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
292``foo`` is extracted from the param-name, and applied to each query in
293``QUERY``, and only 1 match-spec of each type is allowed.
294
295The ``dyndbg`` option is a "fake" module parameter, which means:
296
297- modules do not need to define it explicitly
298- every module gets it tacitly, whether they use pr_debug or not
299- it doesn't appear in ``/sys/module/$module/parameters/``
300  To see it, grep the control file, or inspect ``/proc/cmdline.``
301
302For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
303enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
304the debugfs interface if the debug messages are no longer needed::
305
306   echo "module module_name -p" > <debugfs>/dynamic_debug/control
307
308Examples
309========
310
311::
312
313  // enable the message at line 1603 of file svcsock.c
314  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
315				<debugfs>/dynamic_debug/control
316
317  // enable all the messages in file svcsock.c
318  nullarbor:~ # echo -n 'file svcsock.c +p' >
319				<debugfs>/dynamic_debug/control
320
321  // enable all the messages in the NFS server module
322  nullarbor:~ # echo -n 'module nfsd +p' >
323				<debugfs>/dynamic_debug/control
324
325  // enable all 12 messages in the function svc_process()
326  nullarbor:~ # echo -n 'func svc_process +p' >
327				<debugfs>/dynamic_debug/control
328
329  // disable all 12 messages in the function svc_process()
330  nullarbor:~ # echo -n 'func svc_process -p' >
331				<debugfs>/dynamic_debug/control
332
333  // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
334  nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
335				<debugfs>/dynamic_debug/control
336
337  // enable messages in files of which the paths include string "usb"
338  nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control
339
340  // enable all messages
341  nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
342
343  // add module, function to all enabled messages
344  nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
345
346  // boot-args example, with newlines and comments for readability
347  Kernel command line: ...
348    // see whats going on in dyndbg=value processing
349    dynamic_debug.verbose=1
350    // enable pr_debugs in 2 builtins, #cmt is stripped
351    dyndbg="module params +p #cmt ; module sys +p"
352    // enable pr_debugs in 2 functions in a module loaded later
353    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
354