ftrace: add quick function trace stop

Impact: quick start and stop of function tracer

This patch adds a way to disable the function tracer quickly without
the need to run kstop_machine. It adds a new variable called
function_trace_stop which will stop the calls to functions from mcount
when set.  This is just an on/off switch and does not handle recursion
like preempt_disable().

It's main purpose is to help other tracers/debuggers start and stop tracing
fuctions without the need to call kstop_machine.

The config option HAVE_FUNCTION_TRACE_MCOUNT_TEST is added for archs
that implement the testing of the function_trace_stop in the mcount
arch dependent code. Otherwise, the test is done in the C code.

x86 is the only arch at the moment that supports this.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 33dbefd471e8..fc4febc3334a 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -9,6 +9,13 @@ config NOP_TRACER
 config HAVE_FUNCTION_TRACER
         bool
 
+config HAVE_FUNCTION_TRACE_MCOUNT_TEST
+        bool
+        help
+         This gets selected when the arch tests the function_trace_stop
+         variable at the mcount call site. Otherwise, this variable
+         is tested by the called function.
+
 config HAVE_DYNAMIC_FTRACE
         bool