Re: [PATCH 1/7] Fix mem_cgroup_hierarchical_reclaim() to do stablehierarchy walk.

[KAMEZAWA Hiroyuki]

On Wed, 22 Jun 2011 20:33:31 +0200
Michal Hocko <mhocko@xxxxxxx> wrote:

> 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.
> 

I forgot why we didn't this. Hmm, ok, I'll use this style.

Thanks,
-Kame




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