linux-kernel - Re: [PATCH] cgroup: use an ordered workqueue for cgroup destruction

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Date:	Fri, 7 Feb 2014 16:28:49 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Filipe Brandenburger <filbranden@...gle.com>,
	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Markus Blank-Burian <burian@...nster.de>,
	Shawn Bohrer <shawn.bohrer@...il.com>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] cgroup: use an ordered workqueue for cgroup destruction

On Fri 07-02-14 10:13:41, Tejun Heo wrote:
> Hello, Michal.
> 
> On Fri, Feb 07, 2014 at 03:37:40PM +0100, Michal Hocko wrote:
> > Hmm, this is a bit tricky. We cannot use memcg iterators to reach
> > children because css_tryget would fail on them. We can use cgroup
> > iterators instead, alright, and reparent pages from leafs but this all
> > sounds like a lot of complications.
> 
> Hmmm... I think we're talking past each other here.  Why would the
> parent need to reach down to the children?  Just bail out if it can't
> make things down to zero and let the child when it finishes its own
> cleaning walk up the tree propagating changes.  ->parent is always
> accessible.  Would that be complicated too?

This would be basically the option #2 bellow.

> > Another option would be weakening css_offline reparenting and do not
> > insist on having 0 charges. We want to get rid of as many charges as
> > possible but do not need to have all of them gone
> > (http://marc.info/?l=linux-kernel&m=139161412932193&w=2). The last part
> > would be reparenting to the upmost parent which is still online.
> > 
> > I guess this is implementable but I would prefer Hugh's fix for now and
> > for stable.
> 
> Yeah, for -stable, I think Hugh's patch is good but I really don't
> want to keep it long term.

Based on our recent discussion regarding css_offline semantic we want to
do some changes in that area. I thought we would simply update comments
but considering this report css_offline needs some changes as well. I
will look at it. The idea is to split mem_cgroup_reparent_charges into
two parts. The core one which drains LRUs and would be called from
mem_cgroup_css_offline and one which loops until all charges are gone
for mem_cgroup_css_free. mem_cgroup_move_parent will need an update as
well. It would have to go up the hierarchy to the first alive parent.

-- 
Michal Hocko
SUSE Labs
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