Documentation/dynamic-debug-howto.txt

   1
   2 Introduction
   3 ============
   4
   5 This document describes how to use the dynamic debug (ddebug) feature.
   6
   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.
  11
  12 Dynamic debug has even more useful features:
  13
  14  * Simple query language allows turning on and off debugging statements by
  15    matching any combination of:
  16
  17    - source filename
  18    - function name
  19    - line number (including ranges of line numbers)
  20    - module name
  21    - format string
  22
  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
  25
  26 Controlling dynamic debug Behaviour
  27 ===============================
  28
  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:
  34
  35 nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
  36                                 <debugfs>/dynamic_debug/control
  37
  38 If you make a mistake with the syntax, the write will fail thus:
  39
  40 nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
  41                                 <debugfs>/dynamic_debug/control
  42 -bash: echo: write error: Invalid argument
  43
  44 Viewing Dynamic Debug Behaviour
  45 ===========================
  46
  47 You can view the currently configured behaviour of all the debug statements
  48 via:
  49
  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 Module Removed, deregister RPC RDMA transport\012"
  53 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline       : %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         : %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     : %d\012"
  56 ...
  57
  58
  59 You can also apply standard Unix text manipulation filters to this
  60 data, e.g.
  61
  62 nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
  63 62
  64
  65 nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
  66 42
  67
  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:
  72
  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: st_sendto returned %d\012"
  76
  77
  78 Command Language Reference
  79 ==========================
  80
  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:
  85
  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
  94
  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:
  97
  98 nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
  99 > echo 'file svcsock.c line 1563 +p' > /proc/dprintk
 100
 101 or even like:
 102
 103 nullarbor:~ # (
 104 > echo 'file svcsock.c line 1603 +p' ;\
 105 > echo 'file svcsock.c line 1563 +p' ;\
 106 > ) > /proc/dprintk
 107
 108 At the syntactical level, a command comprises a sequence of match
 109 specifications, followed by a flags change specification.
 110
 111 command ::= match-spec* flags-spec
 112
 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.
 118
 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
 121 keywords are:
 122
 123 match-spec ::= 'func' string |
 124                'file' string |
 125                'module' string |
 126                'format' string |
 127                'line' line-range
 128
 129 line-range ::= lineno |
 130                '-'lineno |
 131                lineno'-' |
 132                lineno'-'lineno
 133 // Note: line-range cannot contain space, e.g.
 134 // "1-30" is valid range but "1 - 30" is not.
 135
 136 lineno ::= unsigned-int
 137
 138 The meanings of each keyword are:
 139
 140 func
 141     The given string is compared against the function name
 142     of each callsite.  Example:
 143
 144     func svc_tcp_accept
 145
 146 file
 147     The given string is compared against either the full
 148     pathname or the basename of the source file of each
 149     callsite.  Examples:
 150
 151     file svcsock.c
 152     file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
 153
 154 module
 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:
 159
 160     module sunrpc
 161     module nfsd
 162
 163 format
 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.
 169     Alternatively, the string can be enclosed in double quote
 170     characters (") or single quote characters (').
 171     Examples:
 172
 173     format svcrdma:         // many of the NFS/RDMA server dprintks
 174     format readahead        // some dprintks in the readahead cache
 175     format nfsd:\040SETATTR // one way to match a format with whitespace
 176     format "nfsd: SETATTR"  // a neater way to match a format with whitespace
 177     format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
 178
 179 line
 180     The given line number or range of line numbers is compared
 181     against the line number of each dprintk() callsite.  A single
 182     line number matches the callsite line number exactly.  A
 183     range of line numbers matches any callsite between the first
 184     and last line number inclusive.  An empty first number means
 185     the first line in the file, an empty line number means the
 186     last number in the file.  Examples:
 187
 188     line 1603       // exactly line 1603
 189     line 1600-1605  // the six lines from line 1600 to line 1605
 190     line -1605      // the 1605 lines from line 1 to line 1605
 191     line 1600-      // all lines from line 1600 to the end of the file
 192
 193 The flags specification comprises a change operation followed
 194 by one or more flag characters.  The change operation is one
 195 of the characters:
 196
 197 -
 198     remove the given flags
 199
 200 +
 201     add the given flags
 202
 203 =
 204     set the flags to the given flags
 205
 206 The flags are:
 207
 208 p
 209     Causes a printk() message to be emitted to dmesg
 210
 211 Note the regexp ^[-+=][scp]+$ matches a flags specification.
 212 Note also that there is no convenient syntax to remove all
 213 the flags at once, you need to use "-psc".
 214
 215 Examples
 216 ========
 217
 218 // enable the message at line 1603 of file svcsock.c
 219 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
 220                                 <debugfs>/dynamic_debug/control
 221
 222 // enable all the messages in file svcsock.c
 223 nullarbor:~ # echo -n 'file svcsock.c +p' >
 224                                 <debugfs>/dynamic_debug/control
 225
 226 // enable all the messages in the NFS server module
 227 nullarbor:~ # echo -n 'module nfsd +p' >
 228                                 <debugfs>/dynamic_debug/control
 229
 230 // enable all 12 messages in the function svc_process()
 231 nullarbor:~ # echo -n 'func svc_process +p' >
 232                                 <debugfs>/dynamic_debug/control
 233
 234 // disable all 12 messages in the function svc_process()
 235 nullarbor:~ # echo -n 'func svc_process -p' >
 236                                 <debugfs>/dynamic_debug/control
 237
 238 // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
 239 nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
 240                                 <debugfs>/dynamic_debug/control