perf: Optimize get_recursion_context()

  "Look ma, no branches!"

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lkml.kernel.org/r/20201030151955.187580298@infradead.org

Author: Peter Zijlstra

kernel/events/internal.h

@@ -205,16 +205,12 @@ DEFINE_OUTPUT_COPY(__output_copy_user, arch_perf_out_copy_user)
 
 static inline int get_recursion_context(int *recursion)
 {
-	int rctx;
-
-	if (unlikely(in_nmi()))
-		rctx = 3;
-	else if (in_irq())
-		rctx = 2;
-	else if (in_serving_softirq())
-		rctx = 1;
-	else
-		rctx = 0;
+	unsigned int pc = preempt_count();
+	unsigned char rctx = 0;
+
+	rctx += !!(pc & (NMI_MASK));
+	rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK));
+	rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
 
 	if (recursion[rctx])
 		return -1;