Documentation/dynamic-debug-howto.txt

   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 doesnt 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"