linux-kernel - Re: [PATCH 26/51] writeback: let balance_dirty_pages() work on the matching cgroup bdi

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Date:	Wed, 1 Jul 2015 21:26:51 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org, hch@...radead.org,
	hannes@...xchg.org, linux-fsdevel@...r.kernel.org,
	vgoyal@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-mm@...ck.org, mhocko@...e.cz, clm@...com,
	fengguang.wu@...el.com, david@...morbit.com, gthelen@...gle.com,
	khlebnikov@...dex-team.ru
Subject: Re: [PATCH 26/51] writeback: let balance_dirty_pages() work on the
 matching cgroup bdi_writeback

Hello, Jan.

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