linux/Documentation/dynamic-debug-howto.txt
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   1
   2Introduction
   3============
   4
   5This document describes how to use the dynamic debug (dyndbg) feature.
   6
   7Dynamic debug is designed to allow you to dynamically enable/disable
   8kernel code to obtain additional kernel information.  Currently, if
   9CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() and
  10print_hex_dump_debug()/print_hex_dump_bytes() calls can be dynamically
  11enabled per-callsite.
  12
  13If CONFIG_DYNAMIC_DEBUG is not set, print_hex_dump_debug() is just
  14shortcut for print_hex_dump(KERN_DEBUG).
  15
  16For print_hex_dump_debug()/print_hex_dump_bytes(), format string is
  17its 'prefix_str' argument, if it is constant string; or "hexdump"
  18in case 'prefix_str' is build dynamically.
  19
  20Dynamic 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
  35Controlling dynamic debug Behaviour
  36===================================
  37
  38The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
  39control file in the 'debugfs' filesystem. Thus, you must first mount
  40the debugfs filesystem, in order to make use of this feature.
  41Subsequently, we refer to the control file as:
  42<debugfs>/dynamic_debug/control. For example, if you want to enable
  43printing from source file 'svcsock.c', line 1603 you simply do:
  44
  45nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
  46                                <debugfs>/dynamic_debug/control
  47
  48If you make a mistake with the syntax, the write will fail thus:
  49
  50nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
  51                                <debugfs>/dynamic_debug/control
  52-bash: echo: write error: Invalid argument
  53
  54Viewing Dynamic Debug Behaviour
  55===========================
  56
  57You can view the currently configured behaviour of all the debug
  58statements via:
  59
  60nullarbor:~ # 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
  69You can also apply standard Unix text manipulation filters to this
  70data, e.g.
  71
  72nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
  7362
  74
  75nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
  7642
  77
  78The third column shows the currently enabled flags for each debug
  79statement callsite (see below for definitions of the flags).  The
  80default value, with no flags enabled, is "=_".  So you can view all
  81the debug statement callsites with any non-default flags:
  82
  83nullarbor:~ # 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
  88Command Language Reference
  89==========================
  90
  91At the lexical level, a command comprises a sequence of words separated
  92by spaces or tabs.  So these are all equivalent:
  93
  94nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
  95                                <debugfs>/dynamic_debug/control
  96nullarbor:~ # echo -c '  file   svcsock.c     line  1603 +p  ' >
  97                                <debugfs>/dynamic_debug/control
  98nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
  99                                <debugfs>/dynamic_debug/control
 100
 101Command submissions are bounded by a write() system call.
 102Multiple 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
 107If your query set is big, you can batch them too:
 108
 109  ~# cat query-batch-file > <debugfs>/dynamic_debug/control
 110
 111At the syntactical level, a command comprises a sequence of match
 112specifications, followed by a flags change specification.
 113
 114command ::= match-spec* flags-spec
 115
 116The match-spec's are used to choose a subset of the known pr_debug()
 117callsites to which to apply the flags-spec.  Think of them as a query
 118with implicit ANDs between each pair.  Note that an empty list of
 119match-specs will select all debug statement callsites.
 120
 121A match specification comprises a keyword, which controls the
 122attribute of the callsite to be compared, and a value to compare
 123against.  Possible keywords are:
 124
 125match-spec ::= 'func' string |
 126               'file' string |
 127               'module' string |
 128               'format' string |
 129               'line' line-range
 130
 131line-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
 138lineno ::= unsigned-int
 139
 140The meanings of each keyword are:
 141
 142func
 143    The given string is compared against the function name
 144    of each callsite.  Example:
 145
 146    func svc_tcp_accept
 147
 148file
 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
 157module
 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
 166format
 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
 182line
 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
 196The flags specification comprises a change operation followed
 197by one or more flag characters.  The change operation is one
 198of the characters:
 199
 200  -    remove the given flags
 201  +    add the given flags
 202  =    set the flags to the given flags
 203
 204The 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
 213For print_hex_dump_debug() and print_hex_dump_bytes(), only 'p' flag
 214have meaning, other flags ignored.
 215
 216For display, the flags are preceded by '='
 217(mnemonic: what the flags are currently equal to).
 218
 219Note the regexp ^[-+=][flmpt_]+$ matches a flags specification.
 220To clear all flags at once, use "=_" or "-flmpt".
 221
 222
 223Debug messages during Boot Process
 224==================================
 225
 226To activate debug messages for core code and built-in modules during
 227the boot process, even before userspace and debugfs exists, use
 228dyndbg="QUERY", module.dyndbg="QUERY", or ddebug_query="QUERY"
 229(ddebug_query is obsoleted by dyndbg, and deprecated).  QUERY follows
 230the syntax described above, but must not exceed 1023 characters.  Your
 231bootloader may impose lower limits.
 232
 233These dyndbg params are processed just after the ddebug tables are
 234processed, as part of the arch_initcall.  Thus you can enable debug
 235messages in all code run after this arch_initcall via this boot
 236parameter.
 237
 238On an x86 system for example ACPI enablement is a subsys_initcall and
 239   dyndbg="file ec.c +p"
 240will show early Embedded Controller transactions during ACPI setup if
 241your machine (typically a laptop) has an Embedded Controller.
 242PCI (or other devices) initialization also is a hot candidate for using
 243this boot parameter for debugging purposes.
 244
 245If foo module is not built-in, foo.dyndbg will still be processed at
 246boot time, without effect, but will be reprocessed when module is
 247loaded later.  dyndbg_query= and bare dyndbg= are only processed at
 248boot.
 249
 250
 251Debug Messages at Module Initialization Time
 252============================================
 253
 254When "modprobe foo" is called, modprobe scans /proc/cmdline for
 255foo.params, strips "foo.", and passes them to the kernel along with
 256params given in modprobe args or /etc/modprob.d/*.conf files,
 257in the following order:
 258
 2591. # parameters given via /etc/modprobe.d/*.conf
 260   options foo dyndbg=+pt
 261   options foo dyndbg # defaults to +p
 262
 2632. # foo.dyndbg as given in boot args, "foo." is stripped and passed
 264   foo.dyndbg=" func bar +p; func buz +mp"
 265
 2663. # args to modprobe
 267   modprobe foo dyndbg==pmf # override previous settings
 268
 269These dyndbg queries are applied in order, with last having final say.
 270This 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
 272modprobe args to override both.
 273
 274In 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
 278The 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 doesn't appear in /sys/module/$module/parameters/
 283  To see it, grep the control file, or inspect /proc/cmdline.
 284
 285For CONFIG_DYNAMIC_DEBUG kernels, any settings given at boot-time (or
 286enabled by -DDEBUG flag during compilation) can be disabled later via
 287the sysfs interface if the debug messages are no longer needed:
 288
 289   echo "module module_name -p" > <debugfs>/dynamic_debug/control
 290
 291Examples
 292========
 293
 294// enable the message at line 1603 of file svcsock.c
 295nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
 296                                <debugfs>/dynamic_debug/control
 297
 298// enable all the messages in file svcsock.c
 299nullarbor:~ # echo -n 'file svcsock.c +p' >
 300                                <debugfs>/dynamic_debug/control
 301
 302// enable all the messages in the NFS server module
 303nullarbor:~ # echo -n 'module nfsd +p' >
 304                                <debugfs>/dynamic_debug/control
 305
 306// enable all 12 messages in the function svc_process()
 307nullarbor:~ # echo -n 'func svc_process +p' >
 308                                <debugfs>/dynamic_debug/control
 309
 310// disable all 12 messages in the function svc_process()
 311nullarbor:~ # echo -n 'func svc_process -p' >
 312                                <debugfs>/dynamic_debug/control
 313
 314// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
 315nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
 316                                <debugfs>/dynamic_debug/control
 317
 318// enable all messages
 319nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
 320
 321// add module, function to all enabled messages
 322nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
 323
 324// boot-args example, with newlines and comments for readability
 325Kernel 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"
 332
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