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Documentation / dynamic-debug-howto.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	
2	Introduction
3	============
4	
5	This document describes how to use the dynamic debug (dyndbg) feature.
6	
7	Dynamic debug is designed to allow you to dynamically enable/disable
8	kernel code to obtain additional kernel information.  Currently, if
9	CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() and
10	print_hex_dump_debug()/print_hex_dump_bytes() calls can be dynamically
11	enabled per-callsite.
12	
13	If CONFIG_DYNAMIC_DEBUG is not set, print_hex_dump_debug() is just
14	shortcut for print_hex_dump(KERN_DEBUG).
15	
16	For print_hex_dump_debug()/print_hex_dump_bytes(), format string is
17	its 'prefix_str' argument, if it is constant string; or "hexdump"
18	in case 'prefix_str' is build dynamically.
19	
20	Dynamic debug has even more useful features:
21	
22	 * Simple query language allows turning on and off debugging
23	   statements by matching any combination of 0 or 1 of:
24	
25	   - source filename
26	   - function name
27	   - line number (including ranges of line numbers)
28	   - module name
29	   - format string
30	
31	 * Provides a debugfs control file: <debugfs>/dynamic_debug/control
32	   which can be read to display the complete list of known debug
33	   statements, to help guide you
34	
35	Controlling dynamic debug Behaviour
36	===================================
37	
38	The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
39	control file in the 'debugfs' filesystem. Thus, you must first mount
40	the debugfs filesystem, in order to make use of this feature.
41	Subsequently, we refer to the control file as:
42	<debugfs>/dynamic_debug/control. For example, if you want to enable
43	printing from source file 'svcsock.c', line 1603 you simply do:
44	
45	nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
46					<debugfs>/dynamic_debug/control
47	
48	If you make a mistake with the syntax, the write will fail thus:
49	
50	nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
51					<debugfs>/dynamic_debug/control
52	-bash: echo: write error: Invalid argument
53	
54	Viewing Dynamic Debug Behaviour
55	===========================
56	
57	You can view the currently configured behaviour of all the debug
58	statements via:
59	
60	nullarbor:~ # cat <debugfs>/dynamic_debug/control
61	# filename:lineno [module]function flags format
62	/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"
63	/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
64	/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
65	/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
66	...
67	
68	
69	You can also apply standard Unix text manipulation filters to this
70	data, e.g.
71	
72	nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
73	62
74	
75	nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
76	42
77	
78	The third column shows the currently enabled flags for each debug
79	statement callsite (see below for definitions of the flags).  The
80	default value, with no flags enabled, is "=_".  So you can view all
81	the debug statement callsites with any non-default flags:
82	
83	nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
84	# filename:lineno [module]function flags format
85	/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"
86	
87	
88	Command Language Reference
89	==========================
90	
91	At the lexical level, a command comprises a sequence of words separated
92	by spaces or tabs.  So these are all equivalent:
93	
94	nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
95					<debugfs>/dynamic_debug/control
96	nullarbor:~ # echo -c '  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	
101	Command submissions are bounded by a write() system call.
102	Multiple commands can be written together, separated by ';' or '\n'.
103	
104	  ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
105	     > <debugfs>/dynamic_debug/control
106	
107	If your query set is big, you can batch them too:
108	
109	  ~# cat query-batch-file > <debugfs>/dynamic_debug/control
110	
111	At the syntactical level, a command comprises a sequence of match
112	specifications, followed by a flags change specification.
113	
114	command ::= match-spec* flags-spec
115	
116	The match-spec's are used to choose a subset of the known pr_debug()
117	callsites to which to apply the flags-spec.  Think of them as a query
118	with implicit ANDs between each pair.  Note that an empty list of
119	match-specs will select all debug statement callsites.
120	
121	A match specification comprises a keyword, which controls the
122	attribute of the callsite to be compared, and a value to compare
123	against.  Possible keywords are:
124	
125	match-spec ::= 'func' string |
126		       'file' string |
127		       'module' string |
128		       'format' string |
129		       'line' line-range
130	
131	line-range ::= lineno |
132		       '-'lineno |
133		       lineno'-' |
134		       lineno'-'lineno
135	// Note: line-range cannot contain space, e.g.
136	// "1-30" is valid range but "1 - 30" is not.
137	
138	lineno ::= unsigned-int
139	
140	The meanings of each keyword are:
141	
142	func
143	    The given string is compared against the function name
144	    of each callsite.  Example:
145	
146	    func svc_tcp_accept
147	
148	file
149	    The given string is compared against either the full pathname, the
150	    src-root relative pathname, or the basename of the source file of
151	    each callsite.  Examples:
152	
153	    file svcsock.c
154	    file kernel/freezer.c
155	    file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
156	
157	module
158	    The given string is compared against the module name
159	    of each callsite.  The module name is the string as
160	    seen in "lsmod", i.e. without the directory or the .ko
161	    suffix and with '-' changed to '_'.  Examples:
162	
163	    module sunrpc
164	    module nfsd
165	
166	format
167	    The given string is searched for in the dynamic debug format
168	    string.  Note that the string does not need to match the
169	    entire format, only some part.  Whitespace and other
170	    special characters can be escaped using C octal character
171	    escape \ooo notation, e.g. the space character is \040.
172	    Alternatively, the string can be enclosed in double quote
173	    characters (") or single quote characters (').
174	    Examples:
175	
176	    format svcrdma:	    // many of the NFS/RDMA server pr_debugs
177	    format readahead	    // some pr_debugs in the readahead cache
178	    format nfsd:\040SETATTR // one way to match a format with whitespace
179	    format "nfsd: SETATTR"  // a neater way to match a format with whitespace
180	    format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
181	
182	line
183	    The given line number or range of line numbers is compared
184	    against the line number of each pr_debug() callsite.  A single
185	    line number matches the callsite line number exactly.  A
186	    range of line numbers matches any callsite between the first
187	    and last line number inclusive.  An empty first number means
188	    the first line in the file, an empty line number means the
189	    last number in the file.  Examples:
190	
191	    line 1603	    // exactly line 1603
192	    line 1600-1605  // the six lines from line 1600 to line 1605
193	    line -1605	    // the 1605 lines from line 1 to line 1605
194	    line 1600-	    // all lines from line 1600 to the end of the file
195	
196	The flags specification comprises a change operation followed
197	by one or more flag characters.  The change operation is one
198	of the characters:
199	
200	  -    remove the given flags
201	  +    add the given flags
202	  =    set the flags to the given flags
203	
204	The flags are:
205	
206	  p    enables the pr_debug() callsite.
207	  f    Include the function name in the printed message
208	  l    Include line number in the printed message
209	  m    Include module name in the printed message
210	  t    Include thread ID in messages not generated from interrupt context
211	  _    No flags are set. (Or'd with others on input)
212	
213	For print_hex_dump_debug() and print_hex_dump_bytes(), only 'p' flag
214	have meaning, other flags ignored.
215	
216	For display, the flags are preceded by '='
217	(mnemonic: what the flags are currently equal to).
218	
219	Note the regexp ^[-+=][flmpt_]+$ matches a flags specification.
220	To clear all flags at once, use "=_" or "-flmpt".
221	
222	
223	Debug messages during Boot Process
224	==================================
225	
226	To activate debug messages for core code and built-in modules during
227	the boot process, even before userspace and debugfs exists, use
228	dyndbg="QUERY", module.dyndbg="QUERY", or ddebug_query="QUERY"
229	(ddebug_query is obsoleted by dyndbg, and deprecated).  QUERY follows
230	the syntax described above, but must not exceed 1023 characters.  Your
231	bootloader may impose lower limits.
232	
233	These dyndbg params are processed just after the ddebug tables are
234	processed, as part of the arch_initcall.  Thus you can enable debug
235	messages in all code run after this arch_initcall via this boot
236	parameter.
237	
238	On an x86 system for example ACPI enablement is a subsys_initcall and
239	   dyndbg="file ec.c +p"
240	will show early Embedded Controller transactions during ACPI setup if
241	your machine (typically a laptop) has an Embedded Controller.
242	PCI (or other devices) initialization also is a hot candidate for using
243	this boot parameter for debugging purposes.
244	
245	If foo module is not built-in, foo.dyndbg will still be processed at
246	boot time, without effect, but will be reprocessed when module is
247	loaded later.  dyndbg_query= and bare dyndbg= are only processed at
248	boot.
249	
250	
251	Debug Messages at Module Initialization Time
252	============================================
253	
254	When "modprobe foo" is called, modprobe scans /proc/cmdline for
255	foo.params, strips "foo.", and passes them to the kernel along with
256	params given in modprobe args or /etc/modprob.d/*.conf files,
257	in the following order:
258	
259	1. # parameters given via /etc/modprobe.d/*.conf
260	   options foo dyndbg=+pt
261	   options foo dyndbg # defaults to +p
262	
263	2. # foo.dyndbg as given in boot args, "foo." is stripped and passed
264	   foo.dyndbg=" func bar +p; func buz +mp"
265	
266	3. # args to modprobe
267	   modprobe foo dyndbg==pmf # override previous settings
268	
269	These dyndbg queries are applied in order, with last having final say.
270	This allows boot args to override or modify those from /etc/modprobe.d
271	(sensible, since 1 is system wide, 2 is kernel or boot specific), and
272	modprobe args to override both.
273	
274	In the foo.dyndbg="QUERY" form, the query must exclude "module foo".
275	"foo" is extracted from the param-name, and applied to each query in
276	"QUERY", and only 1 match-spec of each type is allowed.
277	
278	The dyndbg option is a "fake" module parameter, which means:
279	
280	- modules do not need to define it explicitly
281	- every module gets it tacitly, whether they use pr_debug or not
282	- it doesn't appear in /sys/module/$module/parameters/
283	  To see it, grep the control file, or inspect /proc/cmdline.
284	
285	For CONFIG_DYNAMIC_DEBUG kernels, any settings given at boot-time (or
286	enabled by -DDEBUG flag during compilation) can be disabled later via
287	the sysfs interface if the debug messages are no longer needed:
288	
289	   echo "module module_name -p" > <debugfs>/dynamic_debug/control
290	
291	Examples
292	========
293	
294	// enable the message at line 1603 of file svcsock.c
295	nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
296					<debugfs>/dynamic_debug/control
297	
298	// enable all the messages in file svcsock.c
299	nullarbor:~ # echo -n 'file svcsock.c +p' >
300					<debugfs>/dynamic_debug/control
301	
302	// enable all the messages in the NFS server module
303	nullarbor:~ # echo -n 'module nfsd +p' >
304					<debugfs>/dynamic_debug/control
305	
306	// enable all 12 messages in the function svc_process()
307	nullarbor:~ # echo -n 'func svc_process +p' >
308					<debugfs>/dynamic_debug/control
309	
310	// disable all 12 messages in the function svc_process()
311	nullarbor:~ # echo -n 'func svc_process -p' >
312					<debugfs>/dynamic_debug/control
313	
314	// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
315	nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
316					<debugfs>/dynamic_debug/control
317	
318	// enable all messages
319	nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
320	
321	// add module, function to all enabled messages
322	nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
323	
324	// boot-args example, with newlines and comments for readability
325	Kernel command line: ...
326	  // see whats going on in dyndbg=value processing
327	  dynamic_debug.verbose=1
328	  // enable pr_debugs in 2 builtins, #cmt is stripped
329	  dyndbg="module params +p #cmt ; module sys +p"
330	  // enable pr_debugs in 2 functions in a module loaded later
331	  pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
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