tracing, perf: Adjust code layout in get_recursion_context()

In an XDP redirect applications using tracepoint xdp:xdp_redirect to
diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
was consuming 2% CPU. This was reduced to 1.85% with this simple
change.

Looking at the annotated asm code, it was clear that the unlikely case
in_nmi() test was chosen (by the compiler) as the most likely
event/branch.  This small adjustment makes the compiler (GCC version
7.1.1 20170622 (Red Hat 7.1.1-3)) put in_nmi() as an unlikely branch.
Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/150342256382.16595.986861478681783732.stgit@firesoulSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/events/internal.h

@@ -208,7 +208,7 @@ static inline int get_recursion_context(int *recursion)
 {
 	int rctx;
 
-	if (in_nmi())
+	if (unlikely(in_nmi()))
 		rctx = 3;
 	else if (in_irq())
 		rctx = 2;