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 CONFIG_*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.