tracing: Add -mcount-nop option support

-mcount-nop gcc option generates the calls to the profiling functions
as nops which allows to avoid patching mcount jump with NOP instructions
initially.

-mcount-nop gcc option will be activated if platform selects
HAVE_NOP_MCOUNT and gcc actually supports it.
In addition to that CC_USING_NOP_MCOUNT is defined and could be used by
architectures to adapt ftrace patching behavior.

Link: http://lkml.kernel.org/r/patch-3.thread-aa7b8d.git-e02ed2dc082b.your-ad-here.call-01533557518-ext-9465@work.hours

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Makefile

@@ -749,6 +749,12 @@ ifdef CONFIG_FTRACE_MCOUNT_RECORD
     CC_FLAGS_FTRACE	+= -mrecord-mcount
     export CC_USING_RECORD_MCOUNT := 1
   endif
+  ifdef CONFIG_HAVE_NOP_MCOUNT
+    ifeq ($(call cc-option-yn, -mnop-mcount),y)
+      CC_FLAGS_FTRACE	+= -mnop-mcount
+      CC_FLAGS_USING	+= -DCC_USING_NOP_MCOUNT
+    endif
+  endif
 endif
 ifdef CONFIG_HAVE_FENTRY
   ifeq ($(call cc-option-yn, -mfentry),y)

kernel/trace/Kconfig

@@ -47,6 +47,11 @@ config HAVE_FENTRY
 	help
 	  Arch supports the gcc options -pg with -mfentry
 
+config HAVE_NOP_MCOUNT
+	bool
+	help
+	  Arch supports the gcc options -pg with -mrecord-mcount and -nop-mcount
+
 config HAVE_C_RECORDMCOUNT
 	bool
 	help

kernel/trace/ftrace.c

@@ -2950,12 +2950,14 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
 			p = &pg->records[i];
 			p->flags = rec_flags;
 
+#ifndef CC_USING_NOP_MCOUNT
 			/*
 			 * Do the initial record conversion from mcount jump
 			 * to the NOP instructions.
 			 */
 			if (!ftrace_code_disable(mod, p))
 				break;
+#endif
 
 			update_cnt++;
 		}