On Fri 22-05-15 17:13:40, Tejun Heo wrote:

Currently, balance_dirty_pages() always work on bdi->wb.  This patch
updates it to work on the wb (bdi_writeback) matching memcg and blkcg
of the current task as that's what the inode is being dirtied against.

balance_dirty_pages_ratelimited() now pins the current wb and passes
it to balance_dirty_pages().

As no filesystem has FS_CGROUP_WRITEBACK yet, this doesn't lead to
visible behavior differences.

...

 void balance_dirty_pages_ratelimited(struct address_space *mapping)
 {
-	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-	struct bdi_writeback *wb = &bdi->wb;
+	struct inode *inode = mapping->host;
+	struct backing_dev_info *bdi = inode_to_bdi(inode);
+	struct bdi_writeback *wb = NULL;
 	int ratelimit;
 	int *p;
 
 	if (!bdi_cap_account_dirty(bdi))
 		return;
 
+	if (inode_cgwb_enabled(inode))
+		wb = wb_get_create_current(bdi, GFP_KERNEL);
+	if (!wb)
+		wb = &bdi->wb;
+

So this effectively adds a radix tree lookup (of wb belonging to memcg) for
every set_page_dirty() call. That seems relatively costly to me. And all
that just to check wb->dirty_exceeded. Cannot we just use inode_to_wb()
instead? I understand results may be different if multiple memcgs share an
inode and that's the reason why you use wb_get_create_current(), right?
But for dirty_exceeded check it may be good enough?

								Honza

-- 
Jan Kara [off-list ref]
SUSE Labs, CR

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