From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752628AbaBMAZo (ORCPT ); Wed, 12 Feb 2014 19:25:44 -0500 Received: from mail-qc0-f170.google.com ([209.85.216.170]:33388 "EHLO mail-qc0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751480AbaBMAZm (ORCPT ); Wed, 12 Feb 2014 19:25:42 -0500 Date: Wed, 12 Feb 2014 19:25:38 -0500 From: Tejun Heo To: Hugh Dickins Cc: Michal Hocko , Johannes Weiner , Filipe Brandenburger , Li Zefan , Andrew Morton , Greg Thelen , Michel Lespinasse , Markus Blank-Burian , Shawn Bohrer , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/2] memcg: reparent charges of children before processing parent Message-ID: <20140213002538.GB2916@htj.dyndns.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Wed, Feb 12, 2014 at 03:03:31PM -0800, Hugh Dickins wrote: > From: Filipe Brandenburger > > Sometimes the cleanup after memcg hierarchy testing gets stuck in > mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0. > > There may turn out to be several causes, but a major cause is this: the > workitem to offline parent can get run before workitem to offline child; > parent's mem_cgroup_reparent_charges() circles around waiting for the > child's pages to be reparented to its lrus, but it's holding cgroup_mutex > which prevents the child from reaching its mem_cgroup_reparent_charges(). > > Further testing showed that an ordered workqueue for cgroup_destroy_wq > is not always good enough: percpu_ref_kill_and_confirm's call_rcu_sched > stage on the way can mess up the order before reaching the workqueue. > > Instead, when offlining a memcg, call mem_cgroup_reparent_charges() on > all its children (and grandchildren, in the correct order) to have their > charges reparented first. > > Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction") > Signed-off-by: Filipe Brandenburger > Signed-off-by: Hugh Dickins > Cc: stable@vger.kernel.org # v3.10+ (but will need extra care) > --- > Or, you may prefer my alternative cgroup.c approach in 2/2: > there's no need for both. Please note that neither of these patches > attempts to handle the unlikely case of racy charges made to child > after its offline, but parent's offline coming before child's free: > mem_cgroup_css_free()'s backstop call to mem_cgroup_reparent_charges() > cannot help in that case, with or without these patches. Fixing that > would have to be a separate effort - Michal's? I've changed my mind several times now but I think it'd be a better idea to stick to this patch, at least for now. This one is easier for -stable backport and it looks like the requirements for ordering ->css_offline() might go away depending on how reparenting changes work out. Reviewed-by: Tejun Heo Michal, Johannes, can you guys please ack this one if you guys agree? Thanks. -- tejun