path: root/Documentation/dynamic-debug-howto.txt
diff options
Diffstat (limited to 'Documentation/dynamic-debug-howto.txt')
1 files changed, 331 insertions, 0 deletions
diff --git a/Documentation/dynamic-debug-howto.txt b/Documentation/dynamic-debug-howto.txt
new file mode 100644
index 00000000..72322c6d
--- /dev/null
+++ b/Documentation/dynamic-debug-howto.txt
@@ -0,0 +1,331 @@
+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/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' >
+ <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 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
+nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
+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"
+Command Language Reference
+At the lexical level, a command comprises a sequence of words separated
+by spaces or tabs. So these are all equivalent:
+nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
+ <debugfs>/dynamic_debug/control
+nullarbor:~ # echo -c ' 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
+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
+// 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, 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
+ 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 (').
+ 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
+ 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".
+Debug messages during Boot Process
+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
+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
+Debug Messages at Module Initialization Time
+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:
+1. # parameters given via /etc/modprobe.d/*.conf
+ options foo dyndbg=+pt
+ options 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
+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 doesnt 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 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"