dynamic debug: update docs

Introduction

============

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_debug() 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_debug()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\040Module\040Removed,\040deregister\040RPC\040RDMA\040transport\012"

/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline\040\040\040\040\040\040\040:\040%d\012"

/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init - "\011sq_depth\040\040\040\040\040\040\040\040\040:\040%d\012"

/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init - "\011max_requests\040\040\040\040\040:\040%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

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:\040st_sendto\040returned\040%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' >

				<debugfs>/dynamic_debug/control

nullarbor:~ # echo -c '  file   svcsock.c     line  1603 +p  ' >

				<debugfs>/dynamic_debug/control

nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >

				<debugfs>/dynamic_debug/control

nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >

				<debugfs>/dynamic_debug/control

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'-' |

	       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:

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 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

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

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.

    Examples:

    format svcrdma:	    // many of the NFS/RDMA server dprintks

    format readahead	    // some dprintks in the readahead cache

    format nfsd:\040SETATTR // how to match a format with whitespace

line

    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:

p

    Causes a printk() message to be emitted to dmesg

Note the regexp ^[-+=][scp]+$ matches a flags specification.

Note also that there is no convenient syntax to remove all

the flags at once, you need to use "-psc".

Examples

========

// 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

	  Say N if you are unsure.

config DYNAMIC_DEBUG

	bool "Enable dynamic printk() call support"

	bool "Enable dynamic printk() support"

	default n

	depends on PRINTK

	depends on DEBUG_FS

	select PRINTK_DEBUG

	help

	  Compiles debug level messages into the kernel, which would not

	  otherwise be available at runtime. These messages can then be

	  enabled/disabled on a per module basis. This mechanism implicitly

	  enables all pr_debug() and dev_dbg() calls. The impact of this

	  compile option is a larger kernel text size of about 2%.

	  enabled/disabled based on various levels of scope - per source file,

	  function, module, format string, and line number. This mechanism

	  implicitly enables all pr_debug() and dev_dbg() calls. The impact of

	  this compile option is a larger kernel text size of about 2%.

	  Usage:

	  Dynamic debugging is controlled by the debugfs file,

	  dynamic_printk/modules. This file contains a list of the modules that

	  can be enabled. The format of the file is the module name, followed

	  by a set of flags that can be enabled. The first flag is always the

	  'enabled' flag. For example:

	  Dynamic debugging is controlled via the 'dynamic_debug/ddebug' file,

	  which is contained in the 'debugfs' filesystem. Thus, the debugfs

	  filesystem must first be mounted before making use of this feature.

	  We refer the control file as: <debugfs>/dynamic_debug/ddebug. This

	  file contains a list of the debug statements that can be enabled. The

	  format for each line of the file is:

		<module_name> <enabled=0/1>

				.

				.

				.

		filename:lineno [module]function flags format

	  <module_name> : Name of the module in which the debug call resides

	  <enabled=0/1> : whether the messages are enabled or not

	  filename : source file of the debug statement

	  lineno : line number of the debug statement

	  module : module that contains the debug statement

	  function : function that contains the debug statement

          flags : 'p' means the line is turned 'on' for printing

          format : the format used for the debug statement

	  From a live system:

		snd_hda_intel enabled=0

		fixup enabled=0

		driver enabled=0

		nullarbor:~ # cat <debugfs>/dynamic_debug/ddebug

		# filename:lineno [module]function flags format

		fs/aio.c:222 [aio]__put_ioctx - "__put_ioctx:\040freeing\040%p\012"

		fs/aio.c:248 [aio]ioctx_alloc - "ENOMEM:\040nr_events\040too\040high\012"

		fs/aio.c:1770 [aio]sys_io_cancel - "calling\040cancel\012"

	  Enable a module:

	  Example usage:

	  	$echo "set enabled=1 <module_name>" > dynamic_printk/modules

		// enable the message at line 1603 of file svcsock.c

		nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >

						<debugfs>/dynamic_debug/ddebug

	  Disable a module:

		// enable all the messages in file svcsock.c

		nullarbor:~ # echo -n 'file svcsock.c +p' >

						<debugfs>/dynamic_debug/ddebug

	  	$echo "set enabled=0 <module_name>" > dynamic_printk/modules

		// enable all the messages in the NFS server module

		nullarbor:~ # echo -n 'module nfsd +p' >

						<debugfs>/dynamic_debug/ddebug

	  Enable all modules:

		// enable all 12 messages in the function svc_process()

		nullarbor:~ # echo -n 'func svc_process +p' >

						<debugfs>/dynamic_debug/ddebug

		$echo "set enabled=1 all" > dynamic_printk/modules

		// disable all 12 messages in the function svc_process()

		nullarbor:~ # echo -n 'func svc_process -p' >

						<debugfs>/dynamic_debug/ddebug

	  Disable all modules:

		$echo "set enabled=0 all" > dynamic_printk/modules

	  Finally, passing "dynamic_printk" at the command line enables

	  debugging for all modules. This mode can be turned off via the above

	  disable command.

	  See Documentation/dynamic-debug-howto.txt for additional information.

source "samples/Kconfig"