Introduction¶

This document describes how to use the dynamic debug (dyndbg) feature.

Dynamic debug is designed to allow you to dynamically enable/disablekernel code to obtain additional kernel information. Currently, ifCONFIG_DYNAMIC_DEBUG is set, then allpr_debug()/dev_dbg() andprint_hex_dump_debug()/print_hex_dump_bytes() calls can be dynamicallyenabled per-callsite.

IfCONFIG_DYNAMIC_DEBUG is not set,print_hex_dump_debug() is justshortcut 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.

Dynamic debug has even more useful features:

• Simple query language allows turning on and off debuggingstatements by matching any combination of 0 or 1 of:
  • source filename
  • function name
  • line number (including ranges of line numbers)
  • module name
  • format string
• Provides a debugfs control file:<debugfs>/dynamic_debug/controlwhich can be read to display the complete list of known debugstatements, to help guide you

Controlling dynamic debug Behaviour¶

The behaviour ofpr_debug()/dev_dbg() are controlled via writing to acontrol file in the ‘debugfs’ filesystem. Thus, you must first mountthe debugfs filesystem, in order to make use of this feature.Subsequently, we refer to the control file as:<debugfs>/dynamic_debug/control. For example, if you want to enableprinting from source filesvcsock.c, line 1603 you simply do:

nullarbor:~ # echo 'file svcsock.c line 1603 +p' >                              <debugfs>/dynamic_debug/control

If you make a mistake with the syntax, the write will fail thus:

nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >                              <debugfs>/dynamic_debug/control-bash: echo: write error: Invalid argument

Viewing Dynamic Debug Behaviour¶

You can view the currently configured behaviour of all the debugstatements via:

nullarbor:~ # cat <debugfs>/dynamic_debug/control# filename:lineno [module]function flags format/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"/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"...

You can also apply standard Unix text manipulation filters to thisdata, e.g.:

nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l62nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l42

The third column shows the currently enabled flags for each debugstatement callsite (see below for definitions of the flags). Thedefault value, with no flags enabled, is=_. So you can view allthe debug statement callsites with any non-default flags:

nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control# filename:lineno [module]function flags format/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"

Command Language Reference¶

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

nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >                              <debugfs>/dynamic_debug/controlnullarbor:~ # echo -n '  file   svcsock.c     line  1603 +p  ' >                              <debugfs>/dynamic_debug/controlnullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >                              <debugfs>/dynamic_debug/control

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

~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \   > <debugfs>/dynamic_debug/control

If your query set is big, you can batch them too:

~# cat query-batch-file > <debugfs>/dynamic_debug/control

A another way is to use wildcard. The match rule support* (matcheszero or more characters) and? (matches exactly one character).Forexample, you can match all usb drivers:

~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control

At the syntactical level, a command comprises a sequence of matchspecifications, followed by a flags change specification:

command ::= match-spec* flags-spec

The match-spec’s are used to choose a subset of the known pr_debug()callsites to which to apply the flags-spec. Think of them as a querywith implicit ANDs between each pair. Note that an empty list ofmatch-specs will select all debug statement callsites.

A match specification comprises a keyword, which controls theattribute of the callsite to be compared, and a value to compareagainst. Possible keywords are::

match-spec ::= 'func' string |               'file' string |               'module' string |               'format' 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_accept

file

The given string is compared against either the full pathname, thesrc-root relative pathname, or the basename of the source file ofeach callsite. Examples:

file svcsock.cfile kernel/freezer.cfile /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c

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 nfsd

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

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.f    Include the function name in the printed messagel    Include line number in the printed messagem    Include module name in the printed messaget    Include thread ID in messages not generated from interrupt context_    No flags are set. (Or'd with others on input)

Forprint_hex_dump_debug() andprint_hex_dump_bytes(), onlyp flaghave meaning, other flags ignored.

For display, the flags are preceded by=(mnemonic: what the flags are currently equal to).

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

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",module.dyndbg="QUERY", orddebug_query="QUERY"(ddebug_query is obsoleted bydyndbg, and deprecated). 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 arch_initcall. Thus you can enable debugmessages in all code run after this arch_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.dyndbg_query= and baredyndbg= are only processed atboot.

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/modprob.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 sysfs interface if the debug messages are no longer needed:

echo "module module_name -p" > <debugfs>/dynamic_debug/control

Examples¶

// enable the message at line 1603 of file svcsock.cnullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >                              <debugfs>/dynamic_debug/control// enable all the messages in file svcsock.cnullarbor:~ # echo -n 'file svcsock.c +p' >                              <debugfs>/dynamic_debug/control// enable all the messages in the NFS server modulenullarbor:~ # echo -n 'module nfsd +p' >                              <debugfs>/dynamic_debug/control// enable all 12 messages in the function svc_process()nullarbor:~ # echo -n 'func svc_process +p' >                              <debugfs>/dynamic_debug/control// disable all 12 messages in the function svc_process()nullarbor:~ # echo -n 'func svc_process -p' >                              <debugfs>/dynamic_debug/control// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.nullarbor:~ # echo -n 'format "nfsd: READ" +p' >                              <debugfs>/dynamic_debug/control// enable messages in files of which the paths include string "usb"nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control// enable all messagesnullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control// add module, function to all enabled messagesnullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control// boot-args example, with newlines and comments for readabilityKernel command line: ...  // see whats going on in dyndbg=value processing  dynamic_debug.verbose=1  // enable pr_debugs in 2 builtins, #cmt is stripped  dyndbg="module params +p #cmt ; module sys +p"  // enable pr_debugs in 2 functions in a module loaded later  pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"