This document describes how to use the dynamic debug (dyndbg) feature.
Dynamic debug is designed to allow you to dynamically enable/disable
kernel code to obtain additional kernel information. Currently, if
CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() and
print_hex_dump_debug()/print_hex_dump_bytes() calls can be dynamically
enabled per-callsite.
If CONFIG_DYNAMIC_DEBUG is not set, print_hex_dump_debug() is just
shortcut for print_hex_dump(KERN_DEBUG).
For print_hex_dump_debug()/print_hex_dump_bytes(), format string is
its prefix_str argument, if it is constant string; or hexdump
in case prefix_str is build dynamically.
Dynamic debug has even more useful features:
- Simple query language allows turning on and off debugging statements 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 debug statements, to help guide you
The behaviour of pr_debug()/dev_dbg() are controlled via writing to a
control file in the 'debugfs' filesystem. Thus, you must first mount
the 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 enable
printing from source file svcsock.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
You can view the currently configured behaviour of all the debug statements 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 this data, e.g.:
nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l 62 nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l 42
The third column shows the currently enabled flags for each debug
statement callsite (see below for definitions of the flags). The
default value, with no flags enabled, is =_. So you can view all
the 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"
At the lexical level, a command comprises a sequence of words separated by spaces or tabs. So these are all equivalent:
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
<debugfs>/dynamic_debug/control
nullarbor:~ # echo -n ' file svcsock.c line 1603 +p ' >
<debugfs>/dynamic_debug/control
nullarbor:~ # 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 * (matches
zero or more characters) and ? (matches exactly one character).For
example, 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 match specifications, 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 query with implicit ANDs between each pair. Note that an empty list of match-specs will select all debug statement callsites.
A match specification comprises a keyword, which controls the attribute of the callsite to be compared, and a value to compare against. Possible keywords are::
match-spec ::= 'func' string |
'file' string |
'module' string |
'format' string |
'line' line-range
line-range ::= lineno |
'-'lineno |
lineno'-' |
lineno'-'lineno
lineno ::= unsigned-int
Note
line-range cannot contain space, e.g.
"1-30" is valid range but "1 - 30" is not.
The meanings of each keyword are:
- func
The given string is compared against the function name of each callsite. Example:
func svc_tcp_accept
- file
The given string is compared against either the full pathname, the src-root relative pathname, or the basename of the source file of each callsite. Examples:
file svcsock.c file kernel/freezer.c file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
- module
The given string is compared against the module name of each callsite. The module name is the string as seen in
lsmod, i.e. without the directory or the.kosuffix and with-changed to_. Examples:module sunrpc module nfsd
- format
The given string is searched for in the dynamic debug format string. Note that the string does not need to match the entire format, only some part. Whitespace and other special characters can be escaped using C octal character escape
\ooonotation, e.g. the space character is\040. Alternatively, the string can be enclosed in double quote characters (") or single quote characters ('). Examples:format svcrdma: // many of the NFS/RDMA server pr_debugs format readahead // some pr_debugs in the readahead cache format nfsd:\040SETATTR // one way to match a format with whitespace format "nfsd: SETATTR" // a neater way to match a format with whitespace format 'nfsd: SETATTR' // yet another way to match a format with whitespace
- line
The given line number or range of line numbers is compared against the line number of each
pr_debug()callsite. A single line number matches the callsite line number exactly. A range of line numbers matches any callsite between the first and last line number inclusive. An empty first number means the first line in the file, an empty line number means the last number in the file. Examples:line 1603 // exactly line 1603 line 1600-1605 // the six lines from line 1600 to line 1605 line -1605 // the 1605 lines from line 1 to line 1605 line 1600- // all lines from line 1600 to the end of the file
The flags specification comprises a change operation followed by one or more flag characters. The change operation is one of 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 message l Include line number in the printed message m Include module name in the printed message t Include thread ID in messages not generated from interrupt context _ No flags are set. (Or'd with others on input)
For print_hex_dump_debug() and print_hex_dump_bytes(), only p flag
have 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.
To activate debug messages for core code and built-in modules during
the boot process, even before userspace and debugfs exists, use
dyndbg="QUERY", module.dyndbg="QUERY", or ddebug_query="QUERY"
(ddebug_query is obsoleted by dyndbg, and deprecated). QUERY follows
the syntax described above, but must not exceed 1023 characters. Your
bootloader may impose lower limits.
These dyndbg params are processed just after the ddebug tables are
processed, as part of the arch_initcall. Thus you can enable debug
messages in all code run after this arch_initcall via this boot
parameter.
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 if your machine (typically a laptop) has an Embedded Controller. PCI (or other devices) initialization also is a hot candidate for using this boot parameter for debugging purposes.
If foo module is not built-in, foo.dyndbg will still be processed at
boot time, without effect, but will be reprocessed when module is
loaded later. dyndbg_query= and bare dyndbg= are only processed at
boot.
When modprobe foo is called, modprobe scans /proc/cmdline for
foo.params, strips foo., and passes them to the kernel along with
params given in modprobe args or /etc/modprob.d/*.conf files,
in the following order:
parameters given via
/etc/modprobe.d/*.conf:options foo dyndbg=+pt options foo dyndbg # defaults to +p
foo.dyndbgas given in boot args,foo.is stripped and passed:foo.dyndbg=" func bar +p; func buz +mp"
args to modprobe:
modprobe foo dyndbg==pmf # override previous settings
These dyndbg 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), and
modprobe args to override both.
In the foo.dyndbg="QUERY" form, the query must exclude module foo.
foo is extracted from the param-name, and applied to each query in
QUERY, and only 1 match-spec of each type is allowed.
The dyndbg 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.
For CONFIG_DYNAMIC_DEBUG kernels, any settings given at boot-time (or
enabled by -DDEBUG flag during compilation) can be disabled later via
the sysfs interface if the debug messages are no longer needed:
echo "module module_name -p" > <debugfs>/dynamic_debug/control
// enable the message at line 1603 of file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
<debugfs>/dynamic_debug/control
// enable all the messages in file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c +p' >
<debugfs>/dynamic_debug/control
// enable all the messages in the NFS server module
nullarbor:~ # 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 messages
nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
// add module, function to all enabled messages
nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
// boot-args example, with newlines and comments for readability
Kernel 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"