core/cgroup: fix TasksMaxScale percentage serialization (#41011)

bus_cgroup_set_tasks_max_scale() used a hand-rolled percentage format
that produced values ~10x too small (e.g., "TasksMax=4.0%" instead of
"TasksMax=40.00%").

On daemon-reload, the incorrect value was re-read, silently reducing
the effective TasksMax by ~10x and causing fork rejections on systems
with high thread counts.

Fix by using the existing PERMYRIAD macros, consistent with memory
property handlers (MemoryMax, MemoryHigh, MemoryLow, etc.).

Fixes: #41009

src/core/dbus-cgroup.c

@@ -1010,9 +1010,9 @@ static int bus_cgroup_set_tasks_max_scale(
                 *p = (CGroupTasksMax) { v, UINT32_MAX }; /* .scale is not 0, so this is interpreted as v/UINT32_MAX. */
                 unit_invalidate_cgroup(u, CGROUP_MASK_PIDS);
 
-                uint32_t scaled = DIV_ROUND_UP((uint64_t) v * 100U, (uint64_t) UINT32_MAX);
-                unit_write_settingf(u, flags, name, "%s=%" PRIu32 ".%" PRIu32 "%%", "TasksMax",
-                                    scaled / 10, scaled % 10);
+                int scaled = UINT32_SCALE_TO_PERMYRIAD(v);
+                unit_write_settingf(u, flags, name, "TasksMax=" PERMYRIAD_AS_PERCENT_FORMAT_STR,
+                                   PERMYRIAD_AS_PERCENT_FORMAT_VAL(scaled));
         }
 
         return 1;

test/units/TEST-19-CGROUP.tasks-max-scale.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -ex
+set -o pipefail
+
+# shellcheck source=test/units/test-control.sh
+. "$(dirname "$0")"/test-control.sh
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+testcase_tasks_max_scale_serialize() {
+    # Regression test for https://github.com/systemd/systemd/issues/41009
+    # TasksMaxScale was serialized as 4.0% instead of 40.00%, causing 10x reduction on daemon-reload
+
+    local unit="test-tasks-max.slice"
+    local unit_file="/run/systemd/system/${unit}"
+    local dropin_dir="/run/systemd/system.control/${unit}.d"
+
+    # shellcheck disable=SC2329
+    cleanup() (
+        set +e
+        rm -rf "${dropin_dir}"
+        rm -f "${unit_file}"
+        systemctl daemon-reload
+    )
+    trap cleanup RETURN
+
+    printf '[Slice]\n' >"${unit_file}"
+
+    systemctl daemon-reload
+
+    # Set TasksMax=40% via D-Bus — exercises bus_cgroup_set_tasks_max_scale()
+    systemctl set-property --runtime "${unit}" TasksMax=40%
+
+    # Verify drop-in file contains correct percentage (40.00%, not 4.0%)
+    grep -q '^TasksMax=40\.00%$' "${dropin_dir}/50-TasksMaxScale.conf"
+
+    # Capture value before daemon-reload
+    local tasks_max_before
+    tasks_max_before=$(systemctl show -P TasksMax "${unit}")
+
+    # Reload and verify value is preserved (the actual bug: value dropped 10x here)
+    systemctl daemon-reload
+
+    local tasks_max_after
+    tasks_max_after=$(systemctl show -P TasksMax "${unit}")
+    assert_eq "${tasks_max_before}" "${tasks_max_after}"
+}
+
+run_testcases