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 /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" 74 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline : %d\012" 75 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth : %d\012" 76 /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/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 /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" 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 160The meanings of each keyword are: 161 162func 163 The given string is compared against the function name 164 of each callsite. Example:: 165 166 func svc_tcp_accept 167 168file 169 The given string is compared against either the full pathname, the 170 src-root relative pathname, or the basename of the source file of 171 each callsite. Examples:: 172 173 file svcsock.c 174 file kernel/freezer.c 175 file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c 176 177module 178 The given string is compared against the module name 179 of each callsite. The module name is the string as 180 seen in ``lsmod``, i.e. without the directory or the ``.ko`` 181 suffix and with ``-`` changed to ``_``. Examples:: 182 183 module sunrpc 184 module nfsd 185 186format 187 The given string is searched for in the dynamic debug format 188 string. Note that the string does not need to match the 189 entire format, only some part. Whitespace and other 190 special characters can be escaped using C octal character 191 escape ``\ooo`` notation, e.g. the space character is ``\040``. 192 Alternatively, the string can be enclosed in double quote 193 characters (``"``) or single quote characters (``'``). 194 Examples:: 195 196 format svcrdma: // many of the NFS/RDMA server pr_debugs 197 format readahead // some pr_debugs in the readahead cache 198 format nfsd:\040SETATTR // one way to match a format with whitespace 199 format "nfsd: SETATTR" // a neater way to match a format with whitespace 200 format 'nfsd: SETATTR' // yet another way to match a format with whitespace 201 202line 203 The given line number or range of line numbers is compared 204 against the line number of each ``pr_debug()`` callsite. A single 205 line number matches the callsite line number exactly. A 206 range of line numbers matches any callsite between the first 207 and last line number inclusive. An empty first number means 208 the first line in the file, an empty last line number means the 209 last line number in the file. Examples:: 210 211 line 1603 // exactly line 1603 212 line 1600-1605 // the six lines from line 1600 to line 1605 213 line -1605 // the 1605 lines from line 1 to line 1605 214 line 1600- // all lines from line 1600 to the end of the file 215 216The flags specification comprises a change operation followed 217by one or more flag characters. The change operation is one 218of the characters:: 219 220 - remove the given flags 221 + add the given flags 222 = set the flags to the given flags 223 224The flags are:: 225 226 p enables the pr_debug() callsite. 227 f Include the function name in the printed message 228 l Include line number in the printed message 229 m Include module name in the printed message 230 t Include thread ID in messages not generated from interrupt context 231 _ No flags are set. (Or'd with others on input) 232 233For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag 234have meaning, other flags ignored. 235 236For display, the flags are preceded by ``=`` 237(mnemonic: what the flags are currently equal to). 238 239Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification. 240To clear all flags at once, use ``=_`` or ``-flmpt``. 241 242 243Debug messages during Boot Process 244================================== 245 246To activate debug messages for core code and built-in modules during 247the boot process, even before userspace and debugfs exists, use 248``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"`` 249(``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated). QUERY follows 250the syntax described above, but must not exceed 1023 characters. Your 251bootloader may impose lower limits. 252 253These ``dyndbg`` params are processed just after the ddebug tables are 254processed, as part of the arch_initcall. Thus you can enable debug 255messages in all code run after this arch_initcall via this boot 256parameter. 257 258On an x86 system for example ACPI enablement is a subsys_initcall and:: 259 260 dyndbg="file ec.c +p" 261 262will show early Embedded Controller transactions during ACPI setup if 263your machine (typically a laptop) has an Embedded Controller. 264PCI (or other devices) initialization also is a hot candidate for using 265this boot parameter for debugging purposes. 266 267If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at 268boot time, without effect, but will be reprocessed when module is 269loaded later. ``ddebug_query=`` and bare ``dyndbg=`` are only processed at 270boot. 271 272 273Debug Messages at Module Initialization Time 274============================================ 275 276When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for 277``foo.params``, strips ``foo.``, and passes them to the kernel along with 278params given in modprobe args or ``/etc/modprob.d/*.conf`` files, 279in the following order: 280 2811. parameters given via ``/etc/modprobe.d/*.conf``:: 282 283 options foo dyndbg=+pt 284 options foo dyndbg # defaults to +p 285 2862. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed:: 287 288 foo.dyndbg=" func bar +p; func buz +mp" 289 2903. args to modprobe:: 291 292 modprobe foo dyndbg==pmf # override previous settings 293 294These ``dyndbg`` queries are applied in order, with last having final say. 295This allows boot args to override or modify those from ``/etc/modprobe.d`` 296(sensible, since 1 is system wide, 2 is kernel or boot specific), and 297modprobe args to override both. 298 299In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``. 300``foo`` is extracted from the param-name, and applied to each query in 301``QUERY``, and only 1 match-spec of each type is allowed. 302 303The ``dyndbg`` option is a "fake" module parameter, which means: 304 305- modules do not need to define it explicitly 306- every module gets it tacitly, whether they use pr_debug or not 307- it doesn't appear in ``/sys/module/$module/parameters/`` 308 To see it, grep the control file, or inspect ``/proc/cmdline.`` 309 310For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or 311enabled by ``-DDEBUG`` flag during compilation) can be disabled later via 312the debugfs interface if the debug messages are no longer needed:: 313 314 echo "module module_name -p" > <debugfs>/dynamic_debug/control 315 316Examples 317======== 318 319:: 320 321 // enable the message at line 1603 of file svcsock.c 322 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' > 323 <debugfs>/dynamic_debug/control 324 325 // enable all the messages in file svcsock.c 326 nullarbor:~ # echo -n 'file svcsock.c +p' > 327 <debugfs>/dynamic_debug/control 328 329 // enable all the messages in the NFS server module 330 nullarbor:~ # echo -n 'module nfsd +p' > 331 <debugfs>/dynamic_debug/control 332 333 // enable all 12 messages in the function svc_process() 334 nullarbor:~ # echo -n 'func svc_process +p' > 335 <debugfs>/dynamic_debug/control 336 337 // disable all 12 messages in the function svc_process() 338 nullarbor:~ # echo -n 'func svc_process -p' > 339 <debugfs>/dynamic_debug/control 340 341 // enable messages for NFS calls READ, READLINK, READDIR and READDIR+. 342 nullarbor:~ # echo -n 'format "nfsd: READ" +p' > 343 <debugfs>/dynamic_debug/control 344 345 // enable messages in files of which the paths include string "usb" 346 nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control 347 348 // enable all messages 349 nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control 350 351 // add module, function to all enabled messages 352 nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control 353 354 // boot-args example, with newlines and comments for readability 355 Kernel command line: ... 356 // see whats going on in dyndbg=value processing 357 dynamic_debug.verbose=1 358 // enable pr_debugs in 2 builtins, #cmt is stripped 359 dyndbg="module params +p #cmt ; module sys +p" 360 // enable pr_debugs in 2 functions in a module loaded later 361 pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p" 362