writeback: implement backing_dev_info->tot_write_bandwidth

cgroup writeback support needs to keep track of the sum of
avg_write_bandwidth of all wb's (bdi_writeback's) with dirty inodes to
distribute write workload.  This patch adds bdi->tot_write_bandwidth
and updates inode_wb_list_move_locked(), inode_wb_list_del_locked()
and wb_update_write_bandwidth() to adjust it as wb's gain and lose
dirty inodes and its avg_write_bandwidth gets updated.

As the update events are not synchronized with each other,
bdi->tot_write_bandwidth is an atomic_long_t.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e31dea9..c95eb24 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -881,6 +881,9 @@ static void wb_update_write_bandwidth(struct bdi_writeback *wb,
 		avg += (old - avg) >> 3;
 
 out:
+	if (wb_has_dirty_io(wb))
+		atomic_long_add(avg - wb->avg_write_bandwidth,
+				&wb->bdi->tot_write_bandwidth);
 	wb->write_bandwidth = bw;
 	wb->avg_write_bandwidth = avg;
 }