diff --git a/tools/testing/selftests/net/pmtu.sh b/tools/testing/selftests/net/pmtu.sh
index 543ad7513a8e..2e8972573d91 100755
--- a/tools/testing/selftests/net/pmtu.sh
+++ b/tools/testing/selftests/net/pmtu.sh
@@ -865,7 +865,6 @@ setup_ovs_bridge() {
 setup() {
 	[ "$(id -u)" -ne 0 ] && echo "  need to run as root" && return $ksft_skip
 
-	cleanup
 	for arg do
 		eval setup_${arg} || { echo "  ${arg} not supported"; return 1; }
 	done
@@ -876,7 +875,7 @@ trace() {
 
 	for arg do
 		[ "${ns_cmd}" = "" ] && ns_cmd="${arg}" && continue
-		${ns_cmd} tcpdump -s 0 -i "${arg}" -w "${name}_${arg}.pcap" 2> /dev/null &
+		${ns_cmd} tcpdump --immediate-mode -s 0 -i "${arg}" -w "${name}_${arg}.pcap" 2> /dev/null &
 		tcpdump_pids="${tcpdump_pids} $!"
 		ns_cmd=
 	done
@@ -1836,6 +1835,10 @@ run_test() {
 
 	unset IFS
 
+	# Since cleanup() relies on variables modified by this subshell, it
+	# has to run in this context.
+	trap cleanup EXIT
+
 	if [ "$VERBOSE" = "1" ]; then
 		printf "\n##########################################################################\n\n"
 	fi