5 This document describes how to use the dynamic debug (ddebug) feature.
7 Dynamic debug is designed to allow you to dynamically enable/disable kernel
8 code to obtain additional kernel information. Currently, if
9 CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_debug() calls can be
10 dynamically enabled per-callsite.
12 Dynamic debug has even more useful features:
14 * Simple query language allows turning on and off debugging statements by
15 matching any combination of:
19 - line number (including ranges of line numbers)
23 * Provides a debugfs control file: <debugfs>/dynamic_debug/control which can be
24 read to display the complete list of known debug statements, to help guide you
26 Controlling dynamic debug Behaviour
27 ===============================
29 The behaviour of pr_debug()/dev_debug()s are controlled via writing to a
30 control file in the 'debugfs' filesystem. Thus, you must first mount the debugfs
31 filesystem, in order to make use of this feature. Subsequently, we refer to the
32 control file as: <debugfs>/dynamic_debug/control. For example, if you want to
33 enable printing from source file 'svcsock.c', line 1603 you simply do:
35 nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
36 <debugfs>/dynamic_debug/control
38 If you make a mistake with the syntax, the write will fail thus:
40 nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
41 <debugfs>/dynamic_debug/control
42 -bash: echo: write error: Invalid argument
44 Viewing Dynamic Debug Behaviour
45 ===========================
47 You can view the currently configured behaviour of all the debug statements
50 nullarbor:~ # cat <debugfs>/dynamic_debug/control
51 # filename:lineno [module]function flags format
52 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup - "SVCRDMA\040Module\040Removed,\040deregister\040RPC\040RDMA\040transport\012"
53 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline\040\040\040\040\040\040\040:\040%d\012"
54 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init - "\011sq_depth\040\040\040\040\040\040\040\040\040:\040%d\012"
55 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init - "\011max_requests\040\040\040\040\040:\040%d\012"
59 You can also apply standard Unix text manipulation filters to this
62 nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l
65 nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
68 Note in particular that the third column shows the enabled behaviour
69 flags for each debug statement callsite (see below for definitions of the
70 flags). The default value, no extra behaviour enabled, is "-". So
71 you can view all the debug statement callsites with any non-default flags:
73 nullarbor:~ # awk '$3 != "-"' <debugfs>/dynamic_debug/control
74 # filename:lineno [module]function flags format
75 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process:\040st_sendto\040returned\040%d\012"
78 Command Language Reference
79 ==========================
81 At the lexical level, a command comprises a sequence of words separated
82 by whitespace characters. Note that newlines are treated as word
83 separators and do *not* end a command or allow multiple commands to
84 be done together. So these are all equivalent:
86 nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
87 <debugfs>/dynamic_debug/control
88 nullarbor:~ # echo -c ' file svcsock.c line 1603 +p ' >
89 <debugfs>/dynamic_debug/control
90 nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >
91 <debugfs>/dynamic_debug/control
92 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
93 <debugfs>/dynamic_debug/control
95 Commands are bounded by a write() system call. If you want to do
96 multiple commands you need to do a separate "echo" for each, like:
98 nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
99 > echo 'file svcsock.c line 1563 +p' > /proc/dprintk
104 > echo 'file svcsock.c line 1603 +p' ;\
105 > echo 'file svcsock.c line 1563 +p' ;\
108 At the syntactical level, a command comprises a sequence of match
109 specifications, followed by a flags change specification.
111 command ::= match-spec* flags-spec
113 The match-spec's are used to choose a subset of the known dprintk()
114 callsites to which to apply the flags-spec. Think of them as a query
115 with implicit ANDs between each pair. Note that an empty list of
116 match-specs is possible, but is not very useful because it will not
117 match any debug statement callsites.
119 A match specification comprises a keyword, which controls the attribute
120 of the callsite to be compared, and a value to compare against. Possible
123 match-spec ::= 'func' string |
129 line-range ::= lineno |
133 // Note: line-range cannot contain space, e.g.
134 // "1-30" is valid range but "1 - 30" is not.
136 lineno ::= unsigned-int
138 The meanings of each keyword are:
141 The given string is compared against the function name
142 of each callsite. Example:
147 The given string is compared against either the full
148 pathname or the basename of the source file of each
152 file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
155 The given string is compared against the module name
156 of each callsite. The module name is the string as
157 seen in "lsmod", i.e. without the directory or the .ko
158 suffix and with '-' changed to '_'. Examples:
164 The given string is searched for in the dynamic debug format
165 string. Note that the string does not need to match the
166 entire format, only some part. Whitespace and other
167 special characters can be escaped using C octal character
168 escape \ooo notation, e.g. the space character is \040.
171 format svcrdma: // many of the NFS/RDMA server dprintks
172 format readahead // some dprintks in the readahead cache
173 format nfsd:\040SETATTR // how to match a format with whitespace
176 The given line number or range of line numbers is compared
177 against the line number of each dprintk() callsite. A single
178 line number matches the callsite line number exactly. A
179 range of line numbers matches any callsite between the first
180 and last line number inclusive. An empty first number means
181 the first line in the file, an empty line number means the
182 last number in the file. Examples:
184 line 1603 // exactly line 1603
185 line 1600-1605 // the six lines from line 1600 to line 1605
186 line -1605 // the 1605 lines from line 1 to line 1605
187 line 1600- // all lines from line 1600 to the end of the file
189 The flags specification comprises a change operation followed
190 by one or more flag characters. The change operation is one
194 remove the given flags
200 set the flags to the given flags
205 Causes a printk() message to be emitted to dmesg
207 Note the regexp ^[-+=][scp]+$ matches a flags specification.
208 Note also that there is no convenient syntax to remove all
209 the flags at once, you need to use "-psc".
214 // enable the message at line 1603 of file svcsock.c
215 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
216 <debugfs>/dynamic_debug/control
218 // enable all the messages in file svcsock.c
219 nullarbor:~ # echo -n 'file svcsock.c +p' >
220 <debugfs>/dynamic_debug/control
222 // enable all the messages in the NFS server module
223 nullarbor:~ # echo -n 'module nfsd +p' >
224 <debugfs>/dynamic_debug/control
226 // enable all 12 messages in the function svc_process()
227 nullarbor:~ # echo -n 'func svc_process +p' >
228 <debugfs>/dynamic_debug/control
230 // disable all 12 messages in the function svc_process()
231 nullarbor:~ # echo -n 'func svc_process -p' >
232 <debugfs>/dynamic_debug/control