1 Dynamic debug 2 +++++++++++++ 3 4 5 Introduction 6 ============ 7 8 Dynamic debug allows you to dynamically enable/disable kernel 9 debug-print code to obtain additional kernel information. 10 11 If ``/proc/dynamic_debug/control`` exists, your kernel has dynamic 12 debug. You'll need root access (sudo su) to use this. 13 14 Dynamic debug provides: 15 16 * a Catalog of all *prdbgs* in your kernel. 17 ``cat /proc/dynamic_debug/control`` to see them. 18 19 * a Simple query/command language to alter *prdbgs* by selecting on 20 any combination of 0 or 1 of: 21 22 - source filename 23 - function name 24 - line number (including ranges of line numbers) 25 - module name 26 - format string 27 - class name (as known/declared by each module) 28 29 NOTE: To actually get the debug-print output on the console, you may 30 need to adjust the kernel ``loglevel=``, or use ``ignore_loglevel``. 31 Read about these kernel parameters in 32 Documentation/admin-guide/kernel-parameters.rst. 33 34 Viewing Dynamic Debug Behaviour 35 =============================== 36 37 You can view the currently configured behaviour in the *prdbg* catalog:: 38 39 :#> head -n7 /proc/dynamic_debug/control 40 # filename:lineno [module]function flags format 41 init/main.c:1179 [main]initcall_blacklist =_ "blacklisting initcall %s\012 42 init/main.c:1218 [main]initcall_blacklisted =_ "initcall %s blacklisted\012" 43 init/main.c:1424 [main]run_init_process =_ " with arguments:\012" 44 init/main.c:1426 [main]run_init_process =_ " %s\012" 45 init/main.c:1427 [main]run_init_process =_ " with environment:\012" 46 init/main.c:1429 [main]run_init_process =_ " %s\012" 47 48 The 3rd space-delimited column shows the current flags, preceded by 49 a ``=`` for easy use with grep/cut. ``=p`` shows enabled callsites. 50 51 Controlling dynamic debug Behaviour 52 =================================== 53 54 The behaviour of *prdbg* sites are controlled by writing 55 query/commands to the control file. Example:: 56 57 # grease the interface 58 :#> alias ddcmd='echo $* > /proc/dynamic_debug/control' 59 60 :#> ddcmd '-p; module main func run* +p' 61 :#> grep =p /proc/dynamic_debug/control 62 init/main.c:1424 [main]run_init_process =p " with arguments:\012" 63 init/main.c:1426 [main]run_init_process =p " %s\012" 64 init/main.c:1427 [main]run_init_process =p " with environment:\012" 65 init/main.c:1429 [main]run_init_process =p " %s\012" 66 67 Error messages go to console/syslog:: 68 69 :#> ddcmd mode foo +p 70 dyndbg: unknown keyword "mode" 71 dyndbg: query parse failed 72 bash: echo: write error: Invalid argument 73 74 If debugfs is also enabled and mounted, ``dynamic_debug/control`` is 75 also under the mount-dir, typically ``/sys/kernel/debug/``. 76 77 Command Language Reference 78 ========================== 79 80 At the basic lexical level, a command is a sequence of words separated 81 by spaces or tabs. So these are all equivalent:: 82 83 :#> ddcmd file svcsock.c line 1603 +p 84 :#> ddcmd "file svcsock.c line 1603 +p" 85 :#> ddcmd ' file svcsock.c line 1603 +p ' 86 87 Command submissions are bounded by a write() system call. 88 Multiple commands can be written together, separated by ``;`` or ``\n``:: 89 90 :#> ddcmd "func pnpacpi_get_resources +p; func pnp_assign_mem +p" 91 :#> ddcmd <<"EOC" 92 func pnpacpi_get_resources +p 93 func pnp_assign_mem +p 94 EOC 95 :#> cat query-batch-file > /proc/dynamic_debug/control 96 97 You can also use wildcards in each query term. The match rule supports 98 ``*`` (matches zero or more characters) and ``?`` (matches exactly one 99 character). For example, you can match all usb drivers:: 100 101 :#> ddcmd file "drivers/usb/*" +p # "" to suppress shell expansion 102 103 Syntactically, a command is pairs of keyword values, followed by a 104 flags change or setting:: 105 106 command ::= match-spec* flags-spec 107 108 The match-spec's select *prdbgs* from the catalog, upon which to apply 109 the flags-spec, all constraints are ANDed together. An absent keyword 110 is the same as keyword "*". 111 112 113 A match specification is a keyword, which selects the attribute of 114 the callsite to be compared, and a value to compare against. Possible 115 keywords are::: 116 117 match-spec ::= 'func' string | 118 'file' string | 119 'module' string | 120 'format' string | 121 'class' string | 122 'line' line-range 123 124 line-range ::= lineno | 125 '-'lineno | 126 lineno'-' | 127 lineno'-'lineno 128 129 lineno ::= unsigned-int 130 131 .. note:: 132 133 ``line-range`` cannot contain space, e.g. 134 "1-30" is valid range but "1 - 30" is not. 135 136 137 The meanings of each keyword are: 138 139 func 140 The given string is compared against the function name 141 of each callsite. Example:: 142 143 func svc_tcp_accept 144 func *recv* # in rfcomm, bluetooth, ping, tcp 145 146 file 147 The given string is compared against either the src-root relative 148 pathname, or the basename of the source file of each callsite. 149 Examples:: 150 151 file svcsock.c 152 file kernel/freezer.c # ie column 1 of control file 153 file drivers/usb/* # all callsites under it 154 file inode.c:start_* # parse :tail as a func (above) 155 file inode.c:1-100 # parse :tail as a line-range (above) 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 module drm* # both drm, drm_kms_helper 166 167 format 168 The given string is searched for in the dynamic debug format 169 string. Note that the string does not need to match the 170 entire format, only some part. Whitespace and other 171 special characters can be escaped using C octal character 172 escape ``\ooo`` notation, e.g. the space character is ``\040``. 173 Alternatively, the string can be enclosed in double quote 174 characters (``"``) or single quote characters (``'``). 175 Examples:: 176 177 format svcrdma: // many of the NFS/RDMA server pr_debugs 178 format readahead // some pr_debugs in the readahead cache 179 format nfsd:\040SETATTR // one way to match a format with whitespace 180 format "nfsd: SETATTR" // a neater way to match a format with whitespace 181 format 'nfsd: SETATTR' // yet another way to match a format with whitespace 182 183 class 184 The given class_name is validated against each module, which may 185 have declared a list of known class_names. If the class_name is 186 found for a module, callsite & class matching and adjustment 187 proceeds. Examples:: 188 189 class DRM_UT_KMS # a DRM.debug category 190 class JUNK # silent non-match 191 // class TLD_* # NOTICE: no wildcard in class names 192 193 line 194 The given line number or range of line numbers is compared 195 against the line number of each ``pr_debug()`` callsite. A single 196 line number matches the callsite line number exactly. A 197 range of line numbers matches any callsite between the first 198 and last line number inclusive. An empty first number means 199 the first line in the file, an empty last line number means the 200 last line number in the file. Examples:: 201 202 line 1603 // exactly line 1603 203 line 1600-1605 // the six lines from line 1600 to line 1605 204 line -1605 // the 1605 lines from line 1 to line 1605 205 line 1600- // all lines from line 1600 to the end of the file 206 207 The flags specification comprises a change operation followed 208 by one or more flag characters. The change operation is one 209 of the characters:: 210 211 - remove the given flags 212 + add the given flags 213 = set the flags to the given flags 214 215 The flags are:: 216 217 p enables the pr_debug() callsite. 218 _ enables no flags. 219 220 Decorator flags add to the message-prefix, in order: 221 t Include thread ID, or <intr> 222 m Include module name 223 f Include the function name 224 s Include the source file name 225 l Include line number 226 227 For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only 228 the ``p`` flag has meaning, other flags are ignored. 229 230 Note the regexp ``^[-+=][fslmpt_]+$`` matches a flags specification. 231 To clear all flags at once, use ``=_`` or ``-fslmpt``. 232 233 234 Debug messages during Boot Process 235 ================================== 236 237 To activate debug messages for core code and built-in modules during 238 the boot process, even before userspace and debugfs exists, use 239 ``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``. QUERY follows 240 the syntax described above, but must not exceed 1023 characters. Your 241 bootloader may impose lower limits. 242 243 These ``dyndbg`` params are processed just after the ddebug tables are 244 processed, as part of the early_initcall. Thus you can enable debug 245 messages in all code run after this early_initcall via this boot 246 parameter. 247 248 On an x86 system for example ACPI enablement is a subsys_initcall and:: 249 250 dyndbg="file ec.c +p" 251 252 will show early Embedded Controller transactions during ACPI setup if 253 your machine (typically a laptop) has an Embedded Controller. 254 PCI (or other devices) initialization also is a hot candidate for using 255 this boot parameter for debugging purposes. 256 257 If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at 258 boot time, without effect, but will be reprocessed when module is 259 loaded later. Bare ``dyndbg=`` is only processed at boot. 260 261 262 Debug Messages at Module Initialization Time 263 ============================================ 264 265 When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for 266 ``foo.params``, strips ``foo.``, and passes them to the kernel along with 267 params given in modprobe args or ``/etc/modprobe.d/*.conf`` files, 268 in the following order: 269 270 1. parameters given via ``/etc/modprobe.d/*.conf``:: 271 272 options foo dyndbg=+pt 273 options foo dyndbg # defaults to +p 274 275 2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed:: 276 277 foo.dyndbg=" func bar +p; func buz +mp" 278 279 3. args to modprobe:: 280 281 modprobe foo dyndbg==pmf # override previous settings 282 283 These ``dyndbg`` queries are applied in order, with last having final say. 284 This allows boot args to override or modify those from ``/etc/modprobe.d`` 285 (sensible, since 1 is system wide, 2 is kernel or boot specific), and 286 modprobe args to override both. 287 288 In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``. 289 ``foo`` is extracted from the param-name, and applied to each query in 290 ``QUERY``, and only 1 match-spec of each type is allowed. 291 292 The ``dyndbg`` option is a "fake" module parameter, which means: 293 294 - modules do not need to define it explicitly 295 - every module gets it tacitly, whether they use pr_debug or not 296 - it doesn't appear in ``/sys/module/$module/parameters/`` 297 To see it, grep the control file, or inspect ``/proc/cmdline.`` 298 299 For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or 300 enabled by ``-DDEBUG`` flag during compilation) can be disabled later via 301 the debugfs interface if the debug messages are no longer needed:: 302 303 echo "module module_name -p" > /proc/dynamic_debug/control 304 305 Examples 306 ======== 307 308 :: 309 310 // enable the message at line 1603 of file svcsock.c 311 :#> ddcmd 'file svcsock.c line 1603 +p' 312 313 // enable all the messages in file svcsock.c 314 :#> ddcmd 'file svcsock.c +p' 315 316 // enable all the messages in the NFS server module 317 :#> ddcmd 'module nfsd +p' 318 319 // enable all 12 messages in the function svc_process() 320 :#> ddcmd 'func svc_process +p' 321 322 // disable all 12 messages in the function svc_process() 323 :#> ddcmd 'func svc_process -p' 324 325 // enable messages for NFS calls READ, READLINK, READDIR and READDIR+. 326 :#> ddcmd 'format "nfsd: READ" +p' 327 328 // enable messages in files of which the paths include string "usb" 329 :#> ddcmd 'file *usb* +p' 330 331 // enable all messages 332 :#> ddcmd '+p' 333 334 // add module, function to all enabled messages 335 :#> ddcmd '+mf' 336 337 // boot-args example, with newlines and comments for readability 338 Kernel command line: ... 339 // see what's going on in dyndbg=value processing 340 dynamic_debug.verbose=3 341 // enable pr_debugs in the btrfs module (can be builtin or loadable) 342 btrfs.dyndbg="+p" 343 // enable pr_debugs in all files under init/ 344 // and the function parse_one, #cmt is stripped 345 dyndbg="file init/* +p #cmt ; func parse_one +p" 346 // enable pr_debugs in 2 functions in a module loaded later 347 pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p" 348 349 Kernel Configuration 350 ==================== 351 352 Dynamic Debug is enabled via kernel config items:: 353 354 CONFIG_DYNAMIC_DEBUG=y # build catalog, enables CORE 355 CONFIG_DYNAMIC_DEBUG_CORE=y # enable mechanics only, skip catalog 356 357 If you do not want to enable dynamic debug globally (i.e. in some embedded 358 system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic 359 debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any 360 modules which you'd like to dynamically debug later. 361 362 363 Kernel *prdbg* API 364 ================== 365 366 The following functions are cataloged and controllable when dynamic 367 debug is enabled:: 368 369 pr_debug() 370 dev_dbg() 371 print_hex_dump_debug() 372 print_hex_dump_bytes() 373 374 Otherwise, they are off by default; ``ccflags += -DDEBUG`` or 375 ``#define DEBUG`` in a source file will enable them appropriately. 376 377 If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is 378 just a shortcut for ``print_hex_dump(KERN_DEBUG)``. 379 380 For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is 381 its ``prefix_str`` argument, if it is constant string; or ``hexdump`` 382 in case ``prefix_str`` is built dynamically.
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