Dynamic debug +++++++++++++ Introduction ============ Dynamic debug allows you to dynamically enable/disable kernel debug-print code to obtain additional kernel information. If ``/proc/dynamic_debug/control`` exists, your kernel has dynamic debug. You'll need root access (sudo su) to use this. Dynamic debug provides: * a Catalog of all *prdbgs* in your kernel. ``cat /proc/dynamic_debug/control`` to see them. * a Simple query/command language to alter *prdbgs* by selecting on any 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) Viewing Dynamic Debug Behaviour =============================== You can view the currently configured behaviour in the *prdbg* catalog:: :#> head -n7 /proc/dynamic_debug/control # filename:lineno [module]function flags format init/main.c:1179 [main]initcall_blacklist =_ "blacklisting initcall %s\012 init/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 by a ``=`` for easy use with grep/cut. ``=p`` shows enabled callsites. Controlling dynamic debug Behaviour =================================== The behaviour of *prdbg* sites are controlled by writing query/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/control init/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 +p dyndbg: unknown keyword "mode" dyndbg: query parse failed bash: echo: write error: Invalid argument If debugfs is also enabled and mounted, ``dynamic_debug/control`` is also under the mount-dir, typically ``/sys/kernel/debug/``. Command Language Reference ========================== At the basic lexical level, a command is a sequence of words separated by 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 +p func pnp_assign_mem +p EOC :#> 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 one character). 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 a flags change or setting:: command ::= match-spec* flags-spec The match-spec's select *prdbgs* from the catalog, upon which to apply the flags-spec, all constraints are ANDed together. An absent keyword is the same as keyword "*". A match specification is a keyword, which selects 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 | 'class' 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 func *recv* # in rfcomm, bluetooth, ping, tcp file The given string is compared against either the src-root relative pathname, or the basename of the source file of each callsite. Examples:: file svcsock.c file kernel/freezer.c # ie column 1 of control file file drivers/usb/* # all callsites under it file 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 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 module drm* # both drm, drm_kms_helper 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 ``\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 class The given class_name is validated against each module, which may have declared a list of known class_names. If the class_name is found for a module, callsite & class matching and adjustment proceeds. Examples:: class DRM_UT_KMS # a DRM.debug category class JUNK # silent non-match // class TLD_* # NOTICE: no wildcard in class names 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 last line number means the last line 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. _ enables no flags. Decorator flags add to the message-prefix, in order: t Include thread ID, or m Include module name f Include the function name s Include the source file name l Include line number For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only the ``p`` flag has meaning, other flags are ignored. Note the regexp ``^[-+=][fslmpt_]+$`` matches a flags specification. To clear all flags at once, use ``=_`` or ``-fslmpt``. 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"`` or ``module.dyndbg="QUERY"``. 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 early_initcall. Thus you can enable debug messages in all code run after this early_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. Bare ``dyndbg=`` is only processed at boot. 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/modprobe.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 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 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 readability Kernel 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 CORE CONFIG_DYNAMIC_DEBUG_CORE=y # enable mechanics only, skip catalog If you do not want to enable dynamic debug globally (i.e. in some embedded system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any modules which you'd like to dynamically debug later. Kernel *prdbg* API ================== The following functions are cataloged and controllable when dynamic debug is enabled:: pr_debug() dev_dbg() print_hex_dump_debug() print_hex_dump_bytes() Otherwise, they are off by default; ``ccflags += -DDEBUG`` or ``#define DEBUG`` in a source file will enable them appropriately. If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just a 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 built dynamically.