CONFIG_DYNAMIC_DEBUG: Enable dynamic printk() support

General informations

The Linux kernel configuration item CONFIG_DYNAMIC_DEBUG:

• prompt: Enable dynamic printk() support
• type: bool
• depends on: ( CONFIG_PRINTK ) && (( CONFIG_DEBUG_FS || CONFIG_PROC_FS ))
• defined in lib/Kconfig.debug
• found in Linux kernels: 2.6.30–2.6.39, 3.0–3.19, 4.0–4.20, 5.0–5.19, 6.0–6.8, 6.9-rc+HEAD

Help text

Compiles debug level messages into the kernel, which would not otherwise be available at runtime. These messages can then be enabled/disabled based on various levels of scope - per source file, function, module, format string, and line number. This mechanism implicitly compiles in all pr_debug() and dev_dbg() calls, which enlarges the kernel text size by about 2%.

If a source file is compiled with DEBUG flag set, any pr_debug() calls in it are enabled by default, but can be disabled at runtime as below. Note that DEBUG flag is turned on by many *DEBUG* options.

Usage:

Dynamic debugging is controlled via the 'dynamic_debug/control' file, which is contained in the 'debugfs' filesystem or procfs. Thus, the debugfs or procfs filesystem must first be mounted before making use of this feature. We refer the control file as: <debugfs>/dynamic_debug/control. This file contains a list of the debug statements that can be enabled. The format for each line of the file is:

filename:lineno [module]function flags format

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:

nullarbor:~ # cat <debugfs>/dynamic_debug/control # 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"

Example usage:

// 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

See Documentation/admin-guide/dynamic-debug-howto.rst for additional information.

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The data is retrived from:

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• Linux Kernel Driver DataBase (LKDDb)

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