linux-kernel - Re: [PATCH] Cgroup: Fix memory accounting scalability in shrink_page

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Message-ID: <20120720163115.GA20765@tiehlicka.suse.cz>
Date:	Fri, 20 Jul 2012 18:31:15 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	"andi.kleen" <andi.kleen@...el.com>, linux-mm <linux-mm@...ck.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Cgroup: Fix memory accounting scalability in
 shrink_page_list

On Fri 20-07-12 17:12:16, Johannes Weiner wrote:
> On Fri, Jul 20, 2012 at 04:38:48PM +0200, Michal Hocko wrote:
> > On Fri 20-07-12 16:16:25, Johannes Weiner wrote:
> > > On Fri, Jul 20, 2012 at 03:53:29PM +0200, Michal Hocko wrote:
> > > > On Thu 19-07-12 16:34:26, Tim Chen wrote:
> > > > [...]
> > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > index 33dc256..aac5672 100644
> > > > > --- a/mm/vmscan.c
> > > > > +++ b/mm/vmscan.c
> > > > > @@ -779,6 +779,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > > > >  
> > > > >  	cond_resched();
> > > > >  
> > > > > +	mem_cgroup_uncharge_start();
> > > > >  	while (!list_empty(page_list)) {
> > > > >  		enum page_references references;
> > > > >  		struct address_space *mapping;
> > > > 
> > > > Is this safe? We have a scheduling point few lines below. What prevents
> > > > from task move while we are in the middle of the batch?
> > > 
> > > The batch is accounted in task_struct, so moving a batching task to
> > > another CPU shouldn't be a problem.
> > 
> > But it could also move to a different group, right?
> 
> The batch-uncharging task will remember the memcg of the first page it
> processes, then pile every subsequent page belonging to the same memcg
> on top.  It doesn't matter which group the task is in.

Ahh, you are right. I have missed if (batch->memcg != memcg) at the end
of mem_cgroup_do_uncharge.
Thanks!

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