diff --git a/src/pkg/testing/testing.go b/src/pkg/testing/testing.go
index d971f838e2..357d6f5f27 100644
--- a/src/pkg/testing/testing.go
+++ b/src/pkg/testing/testing.go
@@ -33,6 +33,7 @@
 //         }
 //     }
 //
+// The benchmark function must run the target code b.N times.
 // The benchmark package will vary b.N until the benchmark function lasts
 // long enough to be timed reliably.  The output
 //     BenchmarkHello    10000000    282 ns/op