diff --git a/t/perf/p0000-perf-lib-sanity.sh b/t/perf/p0000-perf-lib-sanity.sh
index f8dd536709..cf8e1efce7 100755
--- a/t/perf/p0000-perf-lib-sanity.sh
+++ b/t/perf/p0000-perf-lib-sanity.sh
@@ -38,9 +38,18 @@ test_expect_success 'test_export works with weird vars' '
 	test "$bar" = "weird # variable"
 '
 
+test_perf 'important variables available in subshells' '
+	test -n "$HOME" &&
+	test -n "$TEST_DIRECTORY" &&
+	test -n "$TRASH_DIRECTORY" &&
+	test -n "$GIT_BUILD_DIR"
+'
+
 test_perf 'test-lib-functions correctly loaded in subshells' '
 	: >a &&
-	test_path_is_file a
+	test_path_is_file a &&
+	: >b &&
+	test_cmp a b
 '
 
 test_done
diff --git a/t/perf/perf-lib.sh b/t/perf/perf-lib.sh
index bcc0131646..5580c22812 100644
--- a/t/perf/perf-lib.sh
+++ b/t/perf/perf-lib.sh
@@ -45,6 +45,10 @@ TEST_NO_CREATE_REPO=t
 
 . ../test-lib.sh
 
+# Variables from test-lib that are normally internal to the tests; we
+# need to export them for test_perf subshells
+export TEST_DIRECTORY TRASH_DIRECTORY GIT_BUILD_DIR GIT_TEST_CMP
+
 perf_results_dir=$TEST_OUTPUT_DIRECTORY/test-results
 mkdir -p "$perf_results_dir"
 rm -f "$perf_results_dir"/$(basename "$0" .sh).subtests