diff --git a/tools/cgroup-share.sh b/tools/cgroup-share.sh
new file mode 100755
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--- /dev/null
+++ b/tools/cgroup-share.sh
@@ -0,0 +1,91 @@
+#!/usr/bin/env bash
+#
+# Does cpu.weight reach the scheduler?
+#
+# Not an A/B on latency -- cgroup weight changes fairness between groups, so
+# what is measured is CPU time each group actually got. Two cgroups, each with
+# nproc spinners so the CPUs are genuinely oversubscribed: with one spinner per
+# group on a 4-thread box both simply get a core and the weight never comes
+# into play.
+#
+# Run the scheduler in another terminal, once with --cgrp-weight-levels 0 and
+# once with 6, and compare the ratio.
+#
+#   sudo ./tools/cgroup-share.sh [seconds] [spinners-per-group]
+#
+# Cgroups are created directly rather than through systemd-run. That started as
+# a workaround -- transient units were not surviving reliably here -- but it is
+# the better test anyway: making a fresh cgroup while the scheduler is attached
+# exercises ops.cgroup_init, which is the path being checked.
+
+set -euo pipefail
+
+DUR=${1:-30}
+# Spinners per group. The default matters more than it looks: with exactly
+# nproc per group, a hi task whose slice expires is not yet back in its DSQ
+# when that CPU calls ops.dispatch, and the other three are on CPUs, so level
+# 19 is momentarily empty and dispatch legitimately falls through to lo. The
+# ratio then measures the test's shape as much as the weight. Oversubscribe a
+# group and its queue stops emptying.
+N=${2:-$(nproc)}
+HI=1000
+LO=10
+ROOT=/sys/fs/cgroup/soryu-test
+
+if [ "$(id -u)" -ne 0 ]; then
+	echo "needs root: sudo $0 $*" >&2
+	exit 1
+fi
+
+PIDS=()
+
+cleanup() {
+	for p in ${PIDS+"${PIDS[@]}"}; do kill "$p" 2>/dev/null || true; done
+	sleep 0.3
+	rmdir "$ROOT/hi" "$ROOT/lo" "$ROOT" 2>/dev/null || true
+}
+trap cleanup EXIT
+cleanup
+
+mkdir -p "$ROOT"
+# The parent must delegate cpu before its children can have cpu.weight. The
+# root already delegates it, or nothing on this system would have a weight.
+grep -qw cpu "$ROOT/cgroup.subtree_control" || echo "+cpu" > "$ROOT/cgroup.subtree_control"
+
+mkdir -p "$ROOT/hi" "$ROOT/lo"
+echo "$HI" > "$ROOT/hi/cpu.weight"
+echo "$LO" > "$ROOT/lo/cpu.weight"
+
+echo "hi cpu.weight = $(cat "$ROOT/hi/cpu.weight")   lo cpu.weight = $(cat "$ROOT/lo/cpu.weight")"
+
+spawn() {
+	local dir=$1 i
+	for i in $(seq "$N"); do
+		( while :; do :; done ) &
+		PIDS+=("$!")
+		# One pid per write; cgroup.procs takes a single value.
+		echo "$!" > "$dir/cgroup.procs"
+	done
+}
+
+spawn "$ROOT/hi"
+spawn "$ROOT/lo"
+
+usage() { awk '/^usage_usec/{print $2}' "$1/cpu.stat"; }
+
+# Let both groups reach steady state before the first sample.
+sleep 3
+
+h0=$(usage "$ROOT/hi"); l0=$(usage "$ROOT/lo")
+echo "running $((N * 2)) spinners ($N per group) on $(nproc) CPUs for ${DUR}s..."
+sleep "$DUR"
+h1=$(usage "$ROOT/hi"); l1=$(usage "$ROOT/lo")
+
+dh=$((h1 - h0)); dl=$((l1 - l0))
+
+printf '\n  hi  cpu.weight=%-5s  %8s ms\n' "$HI" "$((dh / 1000))"
+printf '  lo  cpu.weight=%-5s  %8s ms\n' "$LO" "$((dl / 1000))"
+awk -v a="$dh" -v b="$dl" 'BEGIN {
+	if (b > 0) printf "\n  ratio hi:lo = %.2f : 1\n", a / b
+	else       printf "\n  ratio hi:lo = inf (lo got nothing)\n"
+}'