Commit db05021d authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
Browse files

ftrace: Update the kconfig for DYNAMIC_FTRACE 



The prompt to enable DYNAMIC_FTRACE (the ability to nop and
enable function tracing at run time) had a confusing statement:

 "enable/disable ftrace tracepoints dynamically"

This was written before tracepoints were added to the kernel,
but now that tracepoints have been added, this is very confusing
and has confused people enough to give wrong information during
presentations.

Not only that, I looked at the help text, and it still references
that dreaded daemon that use to wake up once a second to update
the nop locations and brick NICs, that hasn't been around for over
five years.

Time to bring the text up to the current decade.

Cc: stable@vger.kernel.org
Reported-by: default avatarEzequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent ff1fb5f6

kernel/trace/Kconfig

+14 −10
Original line number Diff line number Diff line
@@ -429,24 +429,28 @@ config PROBE_EVENTS 
	def_bool n



config DYNAMIC_FTRACE

	bool "enable/disable ftrace tracepoints dynamically"

	bool "enable/disable function tracing dynamically"

	depends on FUNCTION_TRACER

	depends on HAVE_DYNAMIC_FTRACE

	default y

	help

          This option will modify all the calls to ftrace dynamically

	  (will patch them out of the binary image and replace them

	  with a No-Op instruction) as they are called. A table is

	  created to dynamically enable them again.

	  This option will modify all the calls to function tracing

	  dynamically (will patch them out of the binary image and

	  replace them with a No-Op instruction) on boot up. During

	  compile time, a table is made of all the locations that ftrace

	  can function trace, and this table is linked into the kernel

	  image. When this is enabled, functions can be individually

	  enabled, and the functions not enabled will not affect

	  performance of the system.



	  See the files in /sys/kernel/debug/tracing:

	    available_filter_functions

	    set_ftrace_filter

	    set_ftrace_notrace



	  This way a CONFIG_FUNCTION_TRACER kernel is slightly larger, but

	  otherwise has native performance as long as no tracing is active.



	  The changes to the code are done by a kernel thread that

	  wakes up once a second and checks to see if any ftrace calls

	  were made. If so, it runs stop_machine (stops all CPUS)

	  and modifies the code to jump over the call to ftrace.



config DYNAMIC_FTRACE_WITH_REGS

	def_bool y

	depends on DYNAMIC_FTRACE

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