A mainline question about cgroup writeback

cgroup writeback support the filesystem both ext4 and ext2. but, it appears to be not work when I test the function in the ext4. The example is as follows:
echo "8:0 1048576" > blkio.throttle.write_bps_device
echo $$ > cgroup.procs
dd if=/dev/zero of=/test.c bs=1M count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 49.8796 s, 215 MB/s

Furthering test find that the ratelimit will be limited if it write data to disk directly. Additionly, The read ratelimit is limited whther it go through the page cache or not.