1Dynamic debug 2+++++++++++++ 3 4 5Introduction 6============ 7 8This document describes how to use the dynamic debug (dyndbg) feature. 9 10Dynamic debug is designed to allow you to dynamically enable/disable 11kernel code to obtain additional kernel information. Currently, if 12``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and 13``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically 14enabled per-callsite. 15 16If you do not want to enable dynamic debug globally (i.e. in some embedded 17system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic 18debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any 19modules which you'd like to dynamically debug later. 20 21If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just 22shortcut for ``print_hex_dump(KERN_DEBUG)``. 23 24For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is 25its ``prefix_str`` argument, if it is constant string; or ``hexdump`` 26in case ``prefix_str`` is built dynamically. 27 28Dynamic debug has even more useful features: 29 30 * Simple query language allows turning on and off debugging 31 statements by matching any combination of 0 or 1 of: 32 33 - source filename 34 - function name 35 - line number (including ranges of line numbers) 36 - module name 37 - format string 38 39 * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control`` 40 which can be read to display the complete list of known debug 41 statements, to help guide you 42 43Controlling dynamic debug Behaviour 44=================================== 45 46The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a 47control file in the 'debugfs' filesystem. Thus, you must first mount 48the debugfs filesystem, in order to make use of this feature. 49Subsequently, we refer to the control file as: 50``<debugfs>/dynamic_debug/control``. For example, if you want to enable 51printing from source file ``svcsock.c``, line 1603 you simply do:: 52 53 nullarbor:~ # echo 'file svcsock.c line 1603 +p' > 54 <debugfs>/dynamic_debug/control 55 56If you make a mistake with the syntax, the write will fail thus:: 57 58 nullarbor:~ # echo 'file svcsock.c wtf 1 +p' > 59 <debugfs>/dynamic_debug/control 60 -bash: echo: write error: Invalid argument 61 62Note, for systems without 'debugfs' enabled, the control file can be 63found in ``/proc/dynamic_debug/control``. 64 65Viewing Dynamic Debug Behaviour 66=============================== 67 68You can view the currently configured behaviour of all the debug 69statements via:: 70 71 nullarbor:~ # cat <debugfs>/dynamic_debug/control 72 # filename:lineno [module]function flags format 73 net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012" 74 net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline : %d\012" 75 net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth : %d\012" 76 net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests : %d\012" 77 ... 78 79 80You can also apply standard Unix text manipulation filters to this 81data, e.g.:: 82 83 nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l 84 62 85 86 nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l 87 42 88 89The third column shows the currently enabled flags for each debug 90statement callsite (see below for definitions of the flags). The 91default value, with no flags enabled, is ``=_``. So you can view all 92the debug statement callsites with any non-default flags:: 93 94 nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control 95 # filename:lineno [module]function flags format 96 net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012" 97 98Command Language Reference 99========================== 100 101At the lexical level, a command comprises a sequence of words separated 102by spaces or tabs. So these are all equivalent:: 103 104 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' > 105 <debugfs>/dynamic_debug/control 106 nullarbor:~ # echo -n ' file svcsock.c line 1603 +p ' > 107 <debugfs>/dynamic_debug/control 108 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' > 109 <debugfs>/dynamic_debug/control 110 111Command submissions are bounded by a write() system call. 112Multiple commands can be written together, separated by ``;`` or ``\n``:: 113 114 ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \ 115 > <debugfs>/dynamic_debug/control 116 117If your query set is big, you can batch them too:: 118 119 ~# cat query-batch-file > <debugfs>/dynamic_debug/control 120 121Another way is to use wildcards. The match rule supports ``*`` (matches 122zero or more characters) and ``?`` (matches exactly one character). For 123example, you can match all usb drivers:: 124 125 ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control 126 127At the syntactical level, a command comprises a sequence of match 128specifications, followed by a flags change specification:: 129 130 command ::= match-spec* flags-spec 131 132The match-spec's are used to choose a subset of the known pr_debug() 133callsites to which to apply the flags-spec. Think of them as a query 134with implicit ANDs between each pair. Note that an empty list of 135match-specs will select all debug statement callsites. 136 137A match specification comprises a keyword, which controls the 138attribute of the callsite to be compared, and a value to compare 139against. Possible keywords are::: 140 141 match-spec ::= 'func' string | 142 'file' string | 143 'module' string | 144 'format' string | 145 'line' line-range 146 147 line-range ::= lineno | 148 '-'lineno | 149 lineno'-' | 150 lineno'-'lineno 151 152 lineno ::= unsigned-int 153 154.. note:: 155 156 ``line-range`` cannot contain space, e.g. 157 "1-30" is valid range but "1 - 30" is not. 158 159 ``module=foo`` combined keyword=value form is interchangably accepted 160 161The meanings of each keyword are: 162 163func 164 The given string is compared against the function name 165 of each callsite. Example:: 166 167 func svc_tcp_accept 168 func *recv* # in rfcomm, bluetooth, ping, tcp 169 170file 171 The given string is compared against either the src-root relative 172 pathname, or the basename of the source file of each callsite. 173 Examples:: 174 175 file svcsock.c 176 file kernel/freezer.c # ie column 1 of control file 177 file drivers/usb/* # all callsites under it 178 file inode.c:start_* # parse :tail as a func (above) 179 file inode.c:1-100 # parse :tail as a line-range (above) 180 181module 182 The given string is compared against the module name 183 of each callsite. The module name is the string as 184 seen in ``lsmod``, i.e. without the directory or the ``.ko`` 185 suffix and with ``-`` changed to ``_``. Examples:: 186 187 module sunrpc 188 module nfsd 189 module drm* # both drm, drm_kms_helper 190 191format 192 The given string is searched for in the dynamic debug format 193 string. Note that the string does not need to match the 194 entire format, only some part. Whitespace and other 195 special characters can be escaped using C octal character 196 escape ``\ooo`` notation, e.g. the space character is ``\040``. 197 Alternatively, the string can be enclosed in double quote 198 characters (``"``) or single quote characters (``'``). 199 Examples:: 200 201 format svcrdma: // many of the NFS/RDMA server pr_debugs 202 format readahead // some pr_debugs in the readahead cache 203 format nfsd:\040SETATTR // one way to match a format with whitespace 204 format "nfsd: SETATTR" // a neater way to match a format with whitespace 205 format 'nfsd: SETATTR' // yet another way to match a format with whitespace 206 207line 208 The given line number or range of line numbers is compared 209 against the line number of each ``pr_debug()`` callsite. A single 210 line number matches the callsite line number exactly. A 211 range of line numbers matches any callsite between the first 212 and last line number inclusive. An empty first number means 213 the first line in the file, an empty last line number means the 214 last line number in the file. Examples:: 215 216 line 1603 // exactly line 1603 217 line 1600-1605 // the six lines from line 1600 to line 1605 218 line -1605 // the 1605 lines from line 1 to line 1605 219 line 1600- // all lines from line 1600 to the end of the file 220 221The flags specification comprises a change operation followed 222by one or more flag characters. The change operation is one 223of the characters:: 224 225 - remove the given flags 226 + add the given flags 227 = set the flags to the given flags 228 229The flags are:: 230 231 p enables the pr_debug() callsite. 232 f Include the function name in the printed message 233 l Include line number in the printed message 234 m Include module name in the printed message 235 t Include thread ID in messages not generated from interrupt context 236 _ No flags are set. (Or'd with others on input) 237 238For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag 239have meaning, other flags ignored. 240 241For display, the flags are preceded by ``=`` 242(mnemonic: what the flags are currently equal to). 243 244Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification. 245To clear all flags at once, use ``=_`` or ``-flmpt``. 246 247 248Debug messages during Boot Process 249================================== 250 251To activate debug messages for core code and built-in modules during 252the boot process, even before userspace and debugfs exists, use 253``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"`` 254(``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated). QUERY follows 255the syntax described above, but must not exceed 1023 characters. Your 256bootloader may impose lower limits. 257 258These ``dyndbg`` params are processed just after the ddebug tables are 259processed, as part of the early_initcall. Thus you can enable debug 260messages in all code run after this early_initcall via this boot 261parameter. 262 263On an x86 system for example ACPI enablement is a subsys_initcall and:: 264 265 dyndbg="file ec.c +p" 266 267will show early Embedded Controller transactions during ACPI setup if 268your machine (typically a laptop) has an Embedded Controller. 269PCI (or other devices) initialization also is a hot candidate for using 270this boot parameter for debugging purposes. 271 272If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at 273boot time, without effect, but will be reprocessed when module is 274loaded later. ``ddebug_query=`` and bare ``dyndbg=`` are only processed at 275boot. 276 277 278Debug Messages at Module Initialization Time 279============================================ 280 281When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for 282``foo.params``, strips ``foo.``, and passes them to the kernel along with 283params given in modprobe args or ``/etc/modprob.d/*.conf`` files, 284in the following order: 285 2861. parameters given via ``/etc/modprobe.d/*.conf``:: 287 288 options foo dyndbg=+pt 289 options foo dyndbg # defaults to +p 290 2912. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed:: 292 293 foo.dyndbg=" func bar +p; func buz +mp" 294 2953. args to modprobe:: 296 297 modprobe foo dyndbg==pmf # override previous settings 298 299These ``dyndbg`` queries are applied in order, with last having final say. 300This allows boot args to override or modify those from ``/etc/modprobe.d`` 301(sensible, since 1 is system wide, 2 is kernel or boot specific), and 302modprobe args to override both. 303 304In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``. 305``foo`` is extracted from the param-name, and applied to each query in 306``QUERY``, and only 1 match-spec of each type is allowed. 307 308The ``dyndbg`` option is a "fake" module parameter, which means: 309 310- modules do not need to define it explicitly 311- every module gets it tacitly, whether they use pr_debug or not 312- it doesn't appear in ``/sys/module/$module/parameters/`` 313 To see it, grep the control file, or inspect ``/proc/cmdline.`` 314 315For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or 316enabled by ``-DDEBUG`` flag during compilation) can be disabled later via 317the debugfs interface if the debug messages are no longer needed:: 318 319 echo "module module_name -p" > <debugfs>/dynamic_debug/control 320 321Examples 322======== 323 324:: 325 326 // enable the message at line 1603 of file svcsock.c 327 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' > 328 <debugfs>/dynamic_debug/control 329 330 // enable all the messages in file svcsock.c 331 nullarbor:~ # echo -n 'file svcsock.c +p' > 332 <debugfs>/dynamic_debug/control 333 334 // enable all the messages in the NFS server module 335 nullarbor:~ # echo -n 'module nfsd +p' > 336 <debugfs>/dynamic_debug/control 337 338 // enable all 12 messages in the function svc_process() 339 nullarbor:~ # echo -n 'func svc_process +p' > 340 <debugfs>/dynamic_debug/control 341 342 // disable all 12 messages in the function svc_process() 343 nullarbor:~ # echo -n 'func svc_process -p' > 344 <debugfs>/dynamic_debug/control 345 346 // enable messages for NFS calls READ, READLINK, READDIR and READDIR+. 347 nullarbor:~ # echo -n 'format "nfsd: READ" +p' > 348 <debugfs>/dynamic_debug/control 349 350 // enable messages in files of which the paths include string "usb" 351 nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control 352 353 // enable all messages 354 nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control 355 356 // add module, function to all enabled messages 357 nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control 358 359 // boot-args example, with newlines and comments for readability 360 Kernel command line: ... 361 // see whats going on in dyndbg=value processing 362 dynamic_debug.verbose=1 363 // enable pr_debugs in 2 builtins, #cmt is stripped 364 dyndbg="module params +p #cmt ; module sys +p" 365 // enable pr_debugs in 2 functions in a module loaded later 366 pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p" 367