Hi Vivek,

To explore the possibility of an integrated async write cgroup IO
controller in balance_dirty_pages(), I did the attached patches.
They should serve it well to illustrate the basic ideas.

It's based on Andrea's two supporting patches and a slightly
simplified and improved version of this v6 patchset.

        root@fat ~# cat test-blkio-cgroup.sh
        #!/bin/sh

        mount /dev/sda7 /fs  

        rmdir /cgroup/async_write
        mkdir /cgroup/async_write
        echo $$ > /cgroup/async_write/tasks
        # echo "8:16  1048576" > /cgroup/async_write/blkio.throttle.read_bps_device

        dd if=/dev/zero of=/fs/zero1 bs=1M count=100 &
        dd if=/dev/zero of=/fs/zero2 bs=1M count=100 &

2-dd case:

        root@fat ~# 100+0 records in
        100+0 records out
        104857600 bytes (105 MB) copied100+0 records in
        100+0 records out
        , 11.9477 s, 8.8 MB/s
        104857600 bytes (105 MB) copied, 11.9496 s, 8.8 MB/s

1-dd case:

        root@fat ~# 100+0 records in
        100+0 records out
        104857600 bytes (105 MB) copied, 6.21919 s, 16.9 MB/s

The patch hard codes a limit of 16MiB/s or 16.8MB/s.  So the 1-dd case
is pretty accurate, and the 2-dd case is a bit leaked due to the time
to take the throttle bandwidth from its initial value 16MiB/s to
8MiB/s. This could be compensated by some position control in future,
so that it won't leak in normal cases.

The main bits, blkcg_update_throttle_bandwidth() is in fact a minimal
version of bdi_update_throttle_bandwidth(); blkcg_update_bandwidth()
is also a cut-down version of bdi_update_bandwidth().

Thanks,
Fengguang