tracing: optimize trace_printk()

Impact: micro-optimization

trace_printk() does this unconditionally:

	trace_printk_fmt = fmt;

Where trace_printk_fmt is an entry into a global array. This is
very SMP-unfriendly.

So only write it once per bootup.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1236356510-8381-5-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/kernel.h

@@ -454,7 +454,10 @@ do {									\
 do {									\
 	static const char *trace_printk_fmt				\
 	__attribute__((section("__trace_printk_fmt")));			\
-	trace_printk_fmt = fmt;					\
+									\
+	if (!trace_printk_fmt)						\
+		trace_printk_fmt = fmt;					\
+									\
 	__trace_printk_check_format(fmt, ##args);			\
 	__trace_printk(_THIS_IP_, trace_printk_fmt, ##args);		\
 } while (0)
@@ -467,7 +470,10 @@ __trace_printk(unsigned long ip, const char *fmt, ...)
 do {									\
 	static const char *trace_printk_fmt				\
 	__attribute__((section("__trace_printk_fmt")));			\
-	trace_printk_fmt = fmt;					\
+									\
+	if (!trace_printk_fmt)						\
+		trace_printk_fmt = fmt;					\
+									\
 	__ftrace_vprintk(_THIS_IP_, trace_printk_fmt, vargs);		\
 } while (0)