367 lines
13 KiB
ReStructuredText
367 lines
13 KiB
ReStructuredText
Dynamic debug
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+++++++++++++
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Introduction
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============
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This document describes how to use the dynamic debug (dyndbg) feature.
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Dynamic debug is designed to allow you to dynamically enable/disable
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kernel code to obtain additional kernel information. Currently, if
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``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and
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``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically
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enabled per-callsite.
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If you do not want to enable dynamic debug globally (i.e. in some embedded
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system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
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debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
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modules which you'd like to dynamically debug later.
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If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
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shortcut for ``print_hex_dump(KERN_DEBUG)``.
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For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
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its ``prefix_str`` argument, if it is constant string; or ``hexdump``
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in case ``prefix_str`` is built dynamically.
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Dynamic debug has even more useful features:
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* Simple query language allows turning on and off debugging
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statements by matching any combination of 0 or 1 of:
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- source filename
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- function name
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- line number (including ranges of line numbers)
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- module name
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- format string
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* Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``
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which can be read to display the complete list of known debug
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statements, to help guide you
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Controlling dynamic debug Behaviour
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===================================
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The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a
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control file in the 'debugfs' filesystem. Thus, you must first mount
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the debugfs filesystem, in order to make use of this feature.
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Subsequently, we refer to the control file as:
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``<debugfs>/dynamic_debug/control``. For example, if you want to enable
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printing from source file ``svcsock.c``, line 1603 you simply do::
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nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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If you make a mistake with the syntax, the write will fail thus::
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nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
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<debugfs>/dynamic_debug/control
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-bash: echo: write error: Invalid argument
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Note, for systems without 'debugfs' enabled, the control file can be
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found in ``/proc/dynamic_debug/control``.
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Viewing Dynamic Debug Behaviour
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===============================
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You can view the currently configured behaviour of all the debug
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statements via::
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nullarbor:~ # cat <debugfs>/dynamic_debug/control
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# filename:lineno [module]function flags format
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net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
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net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline : %d\012"
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net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth : %d\012"
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net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests : %d\012"
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...
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You can also apply standard Unix text manipulation filters to this
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data, e.g.::
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nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l
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62
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nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
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42
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The third column shows the currently enabled flags for each debug
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statement callsite (see below for definitions of the flags). The
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default value, with no flags enabled, is ``=_``. So you can view all
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the debug statement callsites with any non-default flags::
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nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
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# filename:lineno [module]function flags format
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net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"
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Command Language Reference
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==========================
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At the lexical level, a command comprises a sequence of words separated
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by spaces or tabs. So these are all equivalent::
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nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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nullarbor:~ # echo -n ' file svcsock.c line 1603 +p ' >
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<debugfs>/dynamic_debug/control
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nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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Command submissions are bounded by a write() system call.
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Multiple commands can be written together, separated by ``;`` or ``\n``::
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~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
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> <debugfs>/dynamic_debug/control
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If your query set is big, you can batch them too::
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~# cat query-batch-file > <debugfs>/dynamic_debug/control
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Another way is to use wildcards. The match rule supports ``*`` (matches
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zero or more characters) and ``?`` (matches exactly one character). For
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example, you can match all usb drivers::
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~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control
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At the syntactical level, a command comprises a sequence of match
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specifications, followed by a flags change specification::
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command ::= match-spec* flags-spec
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The match-spec's are used to choose a subset of the known pr_debug()
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callsites to which to apply the flags-spec. Think of them as a query
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with implicit ANDs between each pair. Note that an empty list of
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match-specs will select all debug statement callsites.
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A match specification comprises a keyword, which controls the
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attribute of the callsite to be compared, and a value to compare
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against. Possible keywords are:::
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match-spec ::= 'func' string |
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'file' string |
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'module' string |
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'format' string |
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'line' line-range
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line-range ::= lineno |
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'-'lineno |
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lineno'-' |
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lineno'-'lineno
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lineno ::= unsigned-int
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.. note::
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``line-range`` cannot contain space, e.g.
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"1-30" is valid range but "1 - 30" is not.
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The meanings of each keyword are:
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func
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The given string is compared against the function name
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of each callsite. Example::
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func svc_tcp_accept
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func *recv* # in rfcomm, bluetooth, ping, tcp
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file
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The given string is compared against either the src-root relative
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pathname, or the basename of the source file of each callsite.
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Examples::
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file svcsock.c
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file kernel/freezer.c # ie column 1 of control file
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file drivers/usb/* # all callsites under it
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file inode.c:start_* # parse :tail as a func (above)
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file inode.c:1-100 # parse :tail as a line-range (above)
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module
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The given string is compared against the module name
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of each callsite. The module name is the string as
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seen in ``lsmod``, i.e. without the directory or the ``.ko``
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suffix and with ``-`` changed to ``_``. Examples::
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module sunrpc
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module nfsd
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module drm* # both drm, drm_kms_helper
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format
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The given string is searched for in the dynamic debug format
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string. Note that the string does not need to match the
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entire format, only some part. Whitespace and other
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special characters can be escaped using C octal character
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escape ``\ooo`` notation, e.g. the space character is ``\040``.
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Alternatively, the string can be enclosed in double quote
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characters (``"``) or single quote characters (``'``).
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Examples::
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format svcrdma: // many of the NFS/RDMA server pr_debugs
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format readahead // some pr_debugs in the readahead cache
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format nfsd:\040SETATTR // one way to match a format with whitespace
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format "nfsd: SETATTR" // a neater way to match a format with whitespace
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format 'nfsd: SETATTR' // yet another way to match a format with whitespace
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line
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The given line number or range of line numbers is compared
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against the line number of each ``pr_debug()`` callsite. A single
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line number matches the callsite line number exactly. A
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range of line numbers matches any callsite between the first
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and last line number inclusive. An empty first number means
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the first line in the file, an empty last line number means the
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last line number in the file. Examples::
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line 1603 // exactly line 1603
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line 1600-1605 // the six lines from line 1600 to line 1605
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line -1605 // the 1605 lines from line 1 to line 1605
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line 1600- // all lines from line 1600 to the end of the file
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The flags specification comprises a change operation followed
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by one or more flag characters. The change operation is one
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of the characters::
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- remove the given flags
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+ add the given flags
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= set the flags to the given flags
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The flags are::
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p enables the pr_debug() callsite.
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f Include the function name in the printed message
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l Include line number in the printed message
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m Include module name in the printed message
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t Include thread ID in messages not generated from interrupt context
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_ No flags are set. (Or'd with others on input)
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For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag
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have meaning, other flags ignored.
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For display, the flags are preceded by ``=``
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(mnemonic: what the flags are currently equal to).
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Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
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To clear all flags at once, use ``=_`` or ``-flmpt``.
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Debug messages during Boot Process
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==================================
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To activate debug messages for core code and built-in modules during
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the boot process, even before userspace and debugfs exists, use
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``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``. QUERY follows
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the syntax described above, but must not exceed 1023 characters. Your
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bootloader may impose lower limits.
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These ``dyndbg`` params are processed just after the ddebug tables are
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processed, as part of the early_initcall. Thus you can enable debug
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messages in all code run after this early_initcall via this boot
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parameter.
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On an x86 system for example ACPI enablement is a subsys_initcall and::
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dyndbg="file ec.c +p"
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will show early Embedded Controller transactions during ACPI setup if
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your machine (typically a laptop) has an Embedded Controller.
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PCI (or other devices) initialization also is a hot candidate for using
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this boot parameter for debugging purposes.
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If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
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boot time, without effect, but will be reprocessed when module is
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loaded later. Bare ``dyndbg=`` is only processed at boot.
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Debug Messages at Module Initialization Time
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============================================
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When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
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``foo.params``, strips ``foo.``, and passes them to the kernel along with
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params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
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in the following order:
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1. parameters given via ``/etc/modprobe.d/*.conf``::
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options foo dyndbg=+pt
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options foo dyndbg # defaults to +p
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2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::
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foo.dyndbg=" func bar +p; func buz +mp"
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3. args to modprobe::
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modprobe foo dyndbg==pmf # override previous settings
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These ``dyndbg`` queries are applied in order, with last having final say.
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This allows boot args to override or modify those from ``/etc/modprobe.d``
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(sensible, since 1 is system wide, 2 is kernel or boot specific), and
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modprobe args to override both.
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In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
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``foo`` is extracted from the param-name, and applied to each query in
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``QUERY``, and only 1 match-spec of each type is allowed.
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The ``dyndbg`` option is a "fake" module parameter, which means:
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- modules do not need to define it explicitly
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- every module gets it tacitly, whether they use pr_debug or not
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- it doesn't appear in ``/sys/module/$module/parameters/``
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To see it, grep the control file, or inspect ``/proc/cmdline.``
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For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
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enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
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the debugfs interface if the debug messages are no longer needed::
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echo "module module_name -p" > <debugfs>/dynamic_debug/control
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Examples
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========
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::
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// enable the message at line 1603 of file svcsock.c
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nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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// enable all the messages in file svcsock.c
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nullarbor:~ # echo -n 'file svcsock.c +p' >
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<debugfs>/dynamic_debug/control
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// enable all the messages in the NFS server module
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nullarbor:~ # echo -n 'module nfsd +p' >
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<debugfs>/dynamic_debug/control
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// enable all 12 messages in the function svc_process()
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nullarbor:~ # echo -n 'func svc_process +p' >
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<debugfs>/dynamic_debug/control
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// disable all 12 messages in the function svc_process()
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nullarbor:~ # echo -n 'func svc_process -p' >
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<debugfs>/dynamic_debug/control
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// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
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nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
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<debugfs>/dynamic_debug/control
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// enable messages in files of which the paths include string "usb"
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nullarbor:~ # echo -n 'file *usb* +p' > <debugfs>/dynamic_debug/control
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// enable all messages
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nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
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// add module, function to all enabled messages
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nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
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// boot-args example, with newlines and comments for readability
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Kernel command line: ...
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// see whats going on in dyndbg=value processing
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dynamic_debug.verbose=3
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// enable pr_debugs in the btrfs module (can be builtin or loadable)
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btrfs.dyndbg="+p"
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// enable pr_debugs in all files under init/
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// and the function parse_one, #cmt is stripped
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dyndbg="file init/* +p #cmt ; func parse_one +p"
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// enable pr_debugs in 2 functions in a module loaded later
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pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
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