Hello, Jan.

On Tue, Jun 30, 2015 at 04:31:00PM +0200, Jan Kara wrote:
...

quoted

+	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

Hmmm... idk, radix tree lookups should be cheap especially when
shallow and set_page_dirty().  It's a glorified array indexing.  If
not, we should really be improving radix tree implementation.  That
said,

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?

Yeah, that probably should work.  I'll think more about it.

Thanks.

-- 
tejun

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