Introduction¶

Dynamic debug allows you to dynamically enable/disable kerneldebug-print code to obtain additional kernel information.

If/proc/dynamic_debug/control exists, your kernel has dynamicdebug. You’ll need root access (sudo su) to use this.

Dynamic debug provides:

• a Catalog of allprdbgs in your kernel.cat/proc/dynamic_debug/control to see them.

• a Simple query/command language to alterprdbgs by selecting onany combination of 0 or 1 of:

  • source filename

  • function name

  • line number (including ranges of line numbers)

  • module name

  • format string

  • class name (as known/declared by each module)

NOTE: To actually get the debug-print output on the console, you mayneed to adjust the kernelloglevel=, or useignore_loglevel.Read about these kernel parameters inThe kernel’s command-line parameters.

Viewing Dynamic Debug Behaviour¶

You can view the currently configured behaviour in theprdbg catalog:

:#> head -n7 /proc/dynamic_debug/control# filename:lineno [module]function flags formatinit/main.c:1179 [main]initcall_blacklist =_ "blacklisting initcall %s\012init/main.c:1218 [main]initcall_blacklisted =_ "initcall %s blacklisted\012"init/main.c:1424 [main]run_init_process =_ "  with arguments:\012"init/main.c:1426 [main]run_init_process =_ "    %s\012"init/main.c:1427 [main]run_init_process =_ "  with environment:\012"init/main.c:1429 [main]run_init_process =_ "    %s\012"

The 3rd space-delimited column shows the current flags, preceded bya= for easy use with grep/cut.=p shows enabled callsites.

Controlling dynamic debug Behaviour¶

The behaviour ofprdbg sites are controlled by writingquery/commands to the control file. Example:

# grease the interface:#> alias ddcmd='echo $* > /proc/dynamic_debug/control':#> ddcmd '-p; module main func run* +p':#> grep =p /proc/dynamic_debug/controlinit/main.c:1424 [main]run_init_process =p "  with arguments:\012"init/main.c:1426 [main]run_init_process =p "    %s\012"init/main.c:1427 [main]run_init_process =p "  with environment:\012"init/main.c:1429 [main]run_init_process =p "    %s\012"

Error messages go to console/syslog:

:#> ddcmd mode foo +pdyndbg: unknown keyword "mode"dyndbg: query parse failedbash: echo: write error: Invalid argument

If debugfs is also enabled and mounted,dynamic_debug/control isalso under the mount-dir, typically/sys/kernel/debug/.

Command Language Reference¶

At the basic lexical level, a command is a sequence of words separatedby spaces or tabs. So these are all equivalent:

:#> ddcmd file svcsock.c line 1603 +p:#> ddcmd "file svcsock.c line 1603 +p":#> ddcmd '  file   svcsock.c     line  1603 +p  '

Command submissions are bounded by a write() system call.Multiple commands can be written together, separated by; or\n:

:#> ddcmd "func pnpacpi_get_resources +p; func pnp_assign_mem +p":#> ddcmd <<"EOC"func pnpacpi_get_resources +pfunc pnp_assign_mem +pEOC:#> cat query-batch-file > /proc/dynamic_debug/control

You can also use wildcards in each query term. The match rule supports* (matches zero or more characters) and? (matches exactly onecharacter). For example, you can match all usb drivers:

:#> ddcmd file "drivers/usb/*" +p     # "" to suppress shell expansion

Syntactically, a command is pairs of keyword values, followed by aflags change or setting:

command ::= match-spec* flags-spec

The match-spec’s selectprdbgs from the catalog, upon which to applythe flags-spec, all constraints are ANDed together. An absent keywordis the same as keyword “*”.

A match specification is a keyword, which selects the attribute ofthe callsite to be compared, and a value to compare against. Possiblekeywords are::

match-spec ::= 'func' string |               'file' string |               'module' string |               'format' string |               'class' string |               'line' line-rangeline-range ::= lineno |               '-'lineno |               lineno'-' |               lineno'-'linenolineno ::= unsigned-int

The meanings of each keyword are:

func

The given string is compared against the function nameof each callsite. Example:

func svc_tcp_acceptfunc *recv*             # in rfcomm, bluetooth, ping, tcp

file

The given string is compared against either the src-root relativepathname, or the basename of the source file of each callsite.Examples:

file svcsock.cfile kernel/freezer.c   # ie column 1 of control filefile drivers/usb/*      # all callsites under itfile inode.c:start_*    # parse :tail as a func (above)file inode.c:1-100      # parse :tail as a line-range (above)

module

The given string is compared against the module nameof each callsite. The module name is the string asseen inlsmod, i.e. without the directory or the.kosuffix and with- changed to_. Examples:

module sunrpcmodule nfsdmodule drm*     # both drm, drm_kms_helper

format

The given string is searched for in the dynamic debug formatstring. Note that the string does not need to match theentire format, only some part. Whitespace and otherspecial characters can be escaped using C octal characterescape\ooo notation, e.g. the space character is\040.Alternatively, the string can be enclosed in double quotecharacters (") or single quote characters (').Examples:

format svcrdma:         // many of the NFS/RDMA server pr_debugsformat readahead        // some pr_debugs in the readahead cacheformat nfsd:\040SETATTR // one way to match a format with whitespaceformat "nfsd: SETATTR"  // a neater way to match a format with whitespaceformat 'nfsd: SETATTR'  // yet another way to match a format with whitespace

class

The given class_name is validated against each module, which mayhave declared a list of known class_names. If the class_name isfound for a module, callsite & class matching and adjustmentproceeds. Examples:

class DRM_UT_KMS        # a DRM.debug categoryclass JUNK              # silent non-match// class TLD_*          # NOTICE: no wildcard in class names

line

The given line number or range of line numbers is comparedagainst the line number of eachpr_debug() callsite. A singleline number matches the callsite line number exactly. Arange of line numbers matches any callsite between the firstand last line number inclusive. An empty first number meansthe first line in the file, an empty last line number means thelast line number in the file. Examples:

line 1603           // exactly line 1603line 1600-1605      // the six lines from line 1600 to line 1605line -1605          // the 1605 lines from line 1 to line 1605line 1600-          // all lines from line 1600 to the end of the file

The flags specification comprises a change operation followedby one or more flag characters. The change operation is oneof the characters:

-    remove the given flags+    add the given flags=    set the flags to the given flags

The flags are:

p    enables the pr_debug() callsite._    enables no flags.Decorator flags add to the message-prefix, in order:t    Include thread ID, or <intr>m    Include module namef    Include the function names    Include the source file namel    Include line numberd    Include call trace

Forprint_hex_dump_debug() andprint_hex_dump_bytes(), onlythep flag has meaning, other flags are ignored.

Note the regexp^[-+=][fslmptd_]+$ matches a flags specification.To clear all flags at once, use=_ or-fslmptd.

Debug messages during Boot Process¶

To activate debug messages for core code and built-in modules duringthe boot process, even before userspace and debugfs exists, usedyndbg="QUERY" ormodule.dyndbg="QUERY". QUERY followsthe syntax described above, but must not exceed 1023 characters. Yourbootloader may impose lower limits.

Thesedyndbg params are processed just after the ddebug tables areprocessed, as part of the early_initcall. Thus you can enable debugmessages in all code run after this early_initcall via this bootparameter.

On an x86 system for example ACPI enablement is a subsys_initcall and:

dyndbg="file ec.c +p"

will show early Embedded Controller transactions during ACPI setup ifyour machine (typically a laptop) has an Embedded Controller.PCI (or other devices) initialization also is a hot candidate for usingthis boot parameter for debugging purposes.

Iffoo module is not built-in,foo.dyndbg will still be processed atboot time, without effect, but will be reprocessed when module isloaded later. Baredyndbg= is only processed at boot.

Debug Messages at Module Initialization Time¶

Whenmodprobefoo is called, modprobe scans/proc/cmdline forfoo.params, stripsfoo., and passes them to the kernel along withparams given in modprobe args or/etc/modprobe.d/*.conf files,in the following order:

1. parameters given via/etc/modprobe.d/*.conf:

   options foo dyndbg=+ptoptions foo dyndbg # defaults to +p

2. foo.dyndbg as given in boot args,foo. is stripped and passed:

   foo.dyndbg=" func bar +p; func buz +mp"

3. args to modprobe:

   modprobe foo dyndbg==pmf # override previous settings

Thesedyndbg queries are applied in order, with last having final say.This allows boot args to override or modify those from/etc/modprobe.d(sensible, since 1 is system wide, 2 is kernel or boot specific), andmodprobe args to override both.

In thefoo.dyndbg="QUERY" form, the query must excludemodulefoo.foo is extracted from the param-name, and applied to each query inQUERY, and only 1 match-spec of each type is allowed.

Thedyndbg option is a “fake” module parameter, which means:

• modules do not need to define it explicitly

• every module gets it tacitly, whether they use pr_debug or not

• it doesn’t appear in/sys/module/$module/parameters/To see it, grep the control file, or inspect/proc/cmdline.

ForCONFIG_DYNAMIC_DEBUG kernels, any settings given at boot-time (orenabled by-DDEBUG flag during compilation) can be disabled later viathe debugfs interface if the debug messages are no longer needed:

echo "module module_name -p" > /proc/dynamic_debug/control

Examples¶

// enable the message at line 1603 of file svcsock.c:#> ddcmd 'file svcsock.c line 1603 +p'// enable all the messages in file svcsock.c:#> ddcmd 'file svcsock.c +p'// enable all the messages in the NFS server module:#> ddcmd 'module nfsd +p'// enable all 12 messages in the function svc_process():#> ddcmd 'func svc_process +p'// disable all 12 messages in the function svc_process():#> ddcmd 'func svc_process -p'// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.:#> ddcmd 'format "nfsd: READ" +p'// enable messages in files of which the paths include string "usb":#> ddcmd 'file *usb* +p'// enable all messages:#> ddcmd '+p'// add module, function to all enabled messages:#> ddcmd '+mf'// boot-args example, with newlines and comments for readabilityKernel command line: ...  // see what's going on in dyndbg=value processing  dynamic_debug.verbose=3  // enable pr_debugs in the btrfs module (can be builtin or loadable)  btrfs.dyndbg="+p"  // enable pr_debugs in all files under init/  // and the function parse_one, #cmt is stripped  dyndbg="file init/* +p #cmt ; func parse_one +p"  // enable pr_debugs in 2 functions in a module loaded later  pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"

Kernel Configuration¶

Dynamic Debug is enabled via kernel config items:

CONFIG_DYNAMIC_DEBUG=y        # build catalog, enables CORECONFIG_DYNAMIC_DEBUG_CORE=y   # enable mechanics only, skip catalog

If you do not want to enable dynamic debug globally (i.e. in some embeddedsystem), you may setCONFIG_DYNAMIC_DEBUG_CORE as basic support of dynamicdebug and addccflags:=-DDYNAMIC_DEBUG_MODULE into the Makefile of anymodules which you’d like to dynamically debug later.

Kernelprdbg API¶

The following functions are cataloged and controllable when dynamicdebug is enabled:

pr_debug()dev_dbg()print_hex_dump_debug()print_hex_dump_bytes()

Otherwise, they are off by default;ccflags+=-DDEBUG or#defineDEBUG in a source file will enable them appropriately.

IfCONFIG_DYNAMIC_DEBUG is not set,print_hex_dump_debug() isjust a shortcut forprint_hex_dump(KERN_DEBUG).

Forprint_hex_dump_debug()/print_hex_dump_bytes(), format string isitsprefix_str argument, if it is constant string; orhexdumpin caseprefix_str is built dynamically.