linux-kernel - Re: [patch 2/8] mm: memcg-aware global reclaim

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Date:	Thu, 9 Jun 2011 05:26:17 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Ying Han <yinghan@...gle.com>, Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/8] mm: memcg-aware global reclaim

On Wed, Jun 08, 2011 at 11:30:46AM +0200, Johannes Weiner wrote:
> On Tue, Jun 07, 2011 at 08:25:19AM -0400, Christoph Hellwig wrote:
> > A few small nitpicks:
> > 
> > > +struct mem_cgroup *mem_cgroup_hierarchy_walk(struct mem_cgroup *root,
> > > +					     struct mem_cgroup *prev)
> > > +{
> > > +	struct mem_cgroup *mem;
> > > +
> > > +	if (mem_cgroup_disabled())
> > > +		return NULL;
> > > +
> > > +	if (!root)
> > > +		root = root_mem_cgroup;
> > > +	/*
> > > +	 * Even without hierarchy explicitely enabled in the root
> > > +	 * memcg, it is the ultimate parent of all memcgs.
> > > +	 */
> > > +	if (!(root == root_mem_cgroup || root->use_hierarchy))
> > > +		return root;
> > 
> > The logic here reads a bit weird, why not simply:
> > 
> > 	 /*
> > 	  * Even without hierarchy explicitely enabled in the root
> > 	  * memcg, it is the ultimate parent of all memcgs.
> > 	  */
> > 	if (!root || root == root_mem_cgroup)
> > 		return root_mem_cgroup;
> > 	if (root->use_hierarchy)
> > 		return root;
> 
> What you are proposing is not equivalent, so... case in point!  It's
> meant to do the hierarchy walk for when foo->use_hierarchy, obviously,
> but ALSO for root_mem_cgroup, which is parent to everyone else even
> without use_hierarchy set.  I changed it to read like this:
> 
> 	if (!root)
> 		root = root_mem_cgroup;
> 	if (!root->use_hierarchy && root != root_mem_cgroup)
> 		return root;
> 	/* actually iterate hierarchy */
> 
> Does that make more sense?

It does, sorry for misparsing it.  The thing that I really hated was
the conditional assignment of root.  Can we clean this up somehow
by making the caller pass root_mem_cgroup in the case where it
passes root right now, or at least always pass NULL when it means
root_mem_cgroup.

Note really that important in the end, it just irked me when I looked
over it, especially the conditional assigned of root to root_mem_cgroup,
and then a little later checking for the equality of the two.

Thinking about it it's probably better left as-is for now to not
complicate the series, and maybe revisit it later once things have
settled a bit.

> > It actually is the per-memcg shrinker now, and thus should be called
> > shrink_memcg.
> 
> Per-zone per-memcg, actually.  shrink_zone_memcg?

Sounds fine to me.

> I have gcc version 4.6.0 20110530 (Red Hat 4.6.0-9) (GCC) on this
> machine, and it manages to optimize the loop away completely.

Ok, good enough.

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