linux-kernel - Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware

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Date:	Tue, 7 Jun 2011 15:42:44 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	containers@...ts.osdl.org, linux-fsdevel@...r.kernel.org,
	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ciju Rajan K <ciju@...ux.vnet.ibm.com>,
	David Rientjes <rientjes@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v8 11/12] writeback: make background writeback cgroup
 aware

On Tue, Jun 07, 2011 at 03:38:35PM -0400, Vivek Goyal wrote:
> On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote:
> > When the system is under background dirty memory threshold but a cgroup
> > is over its background dirty memory threshold, then only writeback
> > inodes associated with the over-limit cgroup(s).
> > 
> 
> [..]
> > -static inline bool over_bground_thresh(void)
> > +static inline bool over_bground_thresh(struct bdi_writeback *wb,
> > +				       struct writeback_control *wbc)
> >  {
> >  	unsigned long background_thresh, dirty_thresh;
> >  
> >  	global_dirty_limits(&background_thresh, &dirty_thresh);
> >  
> > -	return (global_page_state(NR_FILE_DIRTY) +
> > -		global_page_state(NR_UNSTABLE_NFS) > background_thresh);
> > +	if (global_page_state(NR_FILE_DIRTY) +
> > +	    global_page_state(NR_UNSTABLE_NFS) > background_thresh) {
> > +		wbc->for_cgroup = 0;
> > +		return true;
> > +	}
> > +
> > +	wbc->for_cgroup = 1;
> > +	wbc->shared_inodes = 1;
> > +	return mem_cgroups_over_bground_dirty_thresh();
> >  }
> 
> Hi Greg,
> 
> So all the logic of writeout from mem cgroup works only if system is
> below background limit. The moment we cross background limit, looks
> like we will fall back to existing way of writting inodes?

If yes, then from design point of view it is little odd that as long
as we are below background limit, we share the bdi between different
cgroups. The moment we are above background limit, we fall back to
algorithm of sharing the disk among individual inodes and forget
about memory cgroups. Kind of awkward.

Thanks
Vivek
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