diff --git a/tools/funcslower.py b/tools/funcslower.py
index b8ef230ae9e4..681e687f44c7 100755
--- a/tools/funcslower.py
+++ b/tools/funcslower.py
@@ -11,6 +11,9 @@
 # return from each function. For commonly-invoked functions like memory allocs
 # or file writes, this can be extremely expensive. Mind the overhead.
 #
+# NOTE: This tool cannot trace nested functions in the same invocation
+# due to instrumentation specifics, only innermost calls will be visible.
+#
 # By default, a minimum millisecond threshold of 1 is used.
 #
 # Copyright 2017, Sasha Goldshtein