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