linux-kernel - Re: [PATCH 1/7] Fix mem_cgroup_hierarchical_reclaim() to do stable hierarchy walk.

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Date:	Wed, 22 Jun 2011 20:33:31 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"bsingharora@...il.com" <bsingharora@...il.com>,
	Ying Han <yinghan@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>
Subject: Re: [PATCH 1/7] Fix mem_cgroup_hierarchical_reclaim() to do stable
 hierarchy walk.

On Wed 22-06-11 17:15:00, Michal Hocko wrote:
> On Thu 16-06-11 12:51:41, KAMEZAWA Hiroyuki wrote:
> [...]
> > @@ -1667,41 +1668,28 @@ static int mem_cgroup_hierarchical_recla
> >  	if (!check_soft && root_mem->memsw_is_minimum)
> >  		noswap = true;
> >  
> > -	while (1) {
> > +again:
> > +	if (!shrink) {
> > +		visit = 0;
> > +		for_each_mem_cgroup_tree(victim, root_mem)
> > +			visit++;
> > +	} else {
> > +		/*
> > +		 * At shrinking, we check the usage again in caller side.
> > +		 * so, visit children one by one.
> > +		 */
> > +		visit = 1;
> > +	}
> > +	/*
> > +	 * We are not draining per cpu cached charges during soft limit reclaim
> > +	 * because global reclaim doesn't care about charges. It tries to free
> > +	 * some memory and  charges will not give any.
> > +	 */
> > +	if (!check_soft)
> > +		drain_all_stock_async(root_mem);
> > +
> > +	while (visit--) {
> 
> This is racy, isn't it? What prevents some groups to disapear in the
> meantime? We would reclaim from those that are left more that we want.
> 
> Why cannot we simply do something like (totally untested):
> 
> Index: linus_tree/mm/memcontrol.c
> ===================================================================
> --- linus_tree.orig/mm/memcontrol.c	2011-06-22 17:11:54.000000000 +0200
> +++ linus_tree/mm/memcontrol.c	2011-06-22 17:13:05.000000000 +0200
> @@ -1652,7 +1652,7 @@ static int mem_cgroup_hierarchical_recla
>  						unsigned long reclaim_options,
>  						unsigned long *total_scanned)
>  {
> -	struct mem_cgroup *victim;
> +	struct mem_cgroup *victim, *first_victim = NULL;
>  	int ret, total = 0;
>  	int loop = 0;
>  	bool noswap = reclaim_options & MEM_CGROUP_RECLAIM_NOSWAP;
> @@ -1669,6 +1669,11 @@ static int mem_cgroup_hierarchical_recla
>  
>  	while (1) {
>  		victim = mem_cgroup_select_victim(root_mem);
> +		if (!first_victim)
> +			first_victim = victim;
> +		else if (first_victim == victim)
> +			break;

this will obviously need css_get and css_put to make sure that the group
doesn't disappear in the meantime.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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