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This document describes how to use the dynamic debug (ddebug) 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() calls can be
dynamically enabled per-callsite.
Dynamic debug has even more useful features:
* Simple query language allows turning on and off debugging statements by
matching any combination of:
- source filename
- function name
- line number (including ranges of line numbers)
- module name
- format string
* Provides a debugfs control file: <debugfs>/dynamic_debug/control which can be
read to display the complete list of known debug statements, to help guide you
Controlling dynamic debug Behaviour
The behaviour of pr_debug()/dev_dbg()s 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' >
If you make a mistake with the syntax, the write will fail thus:
nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
-bash: echo: write error: Invalid argument
Viewing Dynamic Debug Behaviour
You can view the currently configured behaviour of all the debug statements
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
nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
Note in particular that the third column shows the enabled behaviour
flags for each debug statement callsite (see below for definitions of the
flags). The default value, no extra behaviour 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"
Command Language Reference
At the lexical level, a command comprises a sequence of words separated
by whitespace characters. Note that newlines are treated as word
separators and do *not* end a command or allow multiple commands to
be done together. So these are all equivalent:
nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
nullarbor:~ # echo -c ' file svcsock.c line 1603 +p ' >
nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
Commands are bounded by a write() system call. If you want to do
multiple commands you need to do a separate "echo" for each, like:
nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
> echo 'file svcsock.c line 1563 +p' > /proc/dprintk
or even like:
nullarbor:~ # (
> echo 'file svcsock.c line 1603 +p' ;\
> echo 'file svcsock.c line 1563 +p' ;\
> ) > /proc/dprintk
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 dprintk()
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 is possible, but is not very useful because it will not
match any 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'-' |
// Note: line-range cannot contain space, e.g.
// "1-30" is valid range but "1 - 30" is not.
lineno ::= unsigned-int
The meanings of each keyword are:
The given string is compared against the function name
of each callsite. Example:
func svc_tcp_accept
The given string is compared against either the full
pathname or the basename of the source file of each
callsite. Examples:
file svcsock.c
file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
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 .ko
suffix and with '-' changed to '_'. Examples:
module sunrpc
module nfsd
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 \ooo notation, e.g. the space character is \040.
Alternatively, the string can be enclosed in double quote
characters (") or single quote characters (').
format svcrdma: // many of the NFS/RDMA server dprintks
format readahead // some dprintks 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
The given line number or range of line numbers is compared
against the line number of each dprintk() 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:
Include the function name in the printed message
Include line number in the printed message
Include module name in the printed message
Causes a printk() message to be emitted to dmesg
Include thread ID in messages not generated from interrupt context
Note the regexp ^[-+=][flmpt]+$ matches a flags specification.
Note also that there is no convenient syntax to remove all
the flags at once, you need to use "-flmpt".
Debug messages during boot process
To be able to activate debug messages during the boot process,
even before userspace and debugfs exists, use the boot parameter:
QUERY follows the syntax described above, but must not exceed 1023
characters. The enablement of debug messages is done as an arch_initcall.
Thus you can enable debug messages in all code processed after this
arch_initcall via this boot parameter.
On an x86 system for example ACPI enablement is a subsys_initcall and
ddebug_query="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.
// enable the message at line 1603 of file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
// enable all the messages in file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c +p' >
// enable all the messages in the NFS server module
nullarbor:~ # echo -n 'module nfsd +p' >
// enable all 12 messages in the function svc_process()
nullarbor:~ # echo -n 'func svc_process +p' >
// disable all 12 messages in the function svc_process()
nullarbor:~ # echo -n 'func svc_process -p' >
// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
nullarbor:~ # echo -n 'format "nfsd: READ" +p' >