diff --git a/man7/cgroups.7 b/man7/cgroups.7
index 8b0aab3cc..0627206c8 100644
--- a/man7/cgroups.7
+++ b/man7/cgroups.7
@@ -1307,6 +1307,10 @@ The threads of a process must all reside in the same threaded subtree.
 .PP
 The
 .I cgroup.threads
+file is writable only for the cgroups inside a threaded subtree.
+.PP
+The
+.I cgroup.threads
 file is present in each cgroup (including
 .I domain
 cgroups) and can be read in order to discover the set of threads