How to set cpumask at boot with systemd?

I have the following service:

[Unit]
Description=foo

[Service]
Type=oneshot
RemainAfterExit=true
ExecStop=/opt/script/bar.sh

[Install]
WantedBy=multi-user.target

And bar.sh:

#!/bin/bash
echo 1 > /sys/bus/workqueue/devices/writeback/cpumask
cat /sys/bus/workqueue/devices/writeback/cpumask > /tmp/writeback

Then after I rebooted, cat /tmp/writeback looks good: 000000,00000000,00000000,00000000,00000000,00000000,00000000,00000001

But /sys/bus/workqueue/devices/writeback/cpumask got overwritten back to default:

ffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff

Thought?