From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qa0-f52.google.com (mail-qa0-f52.google.com [209.85.216.52]) by kanga.kvack.org (Postfix) with ESMTP id B4E236B0035 for ; Wed, 12 Feb 2014 19:28:57 -0500 (EST) Received: by mail-qa0-f52.google.com with SMTP id j15so14922183qaq.25 for ; Wed, 12 Feb 2014 16:28:57 -0800 (PST) Received: from mail-qa0-x22a.google.com (mail-qa0-x22a.google.com [2607:f8b0:400d:c00::22a]) by mx.google.com with ESMTPS id o46si13216qgo.158.2014.02.12.16.28.56 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 12 Feb 2014 16:28:57 -0800 (PST) Received: by mail-qa0-f42.google.com with SMTP id k4so15242868qaq.15 for ; Wed, 12 Feb 2014 16:28:56 -0800 (PST) Date: Wed, 12 Feb 2014 19:28:53 -0500 From: Tejun Heo Subject: Re: [PATCH 2/2] cgroup: bring back kill_cnt to order css destruction Message-ID: <20140213002853.GC2916@htj.dyndns.org> References: <20140207164321.GE6963@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: 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 Hello, Hugh. On Wed, Feb 12, 2014 at 03:06:26PM -0800, Hugh Dickins wrote: > 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 bring back v3.11's css kill_cnt, repurposing it to make sure > that offline_css() is not called for parent before it has been called > for all children. > > Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction") > Signed-off-by: Hugh Dickins > Reviewed-by: Filipe Brandenburger > Cc: stable@vger.kernel.org # v3.10+ (but will need extra care) > --- > This is an alternative to Filipe's 1/2: there's no need for both, > but each has its merits. I prefer Filipe's, which is much easier to > understand: this one made more sense in v3.11, when it was just a matter > of extending the use of css_kill_cnt; but might be preferred if offlining > children before parent is thought to be a good idea generally. Not that your implementation is bad or anything but the patch itself somehow makes me cringe a bit. It's probably just because it has to add to the already overly complicated offline path. Guaranteeing strict offline ordering might be a good idea but at least for the immediate bug fix, I agree that the memcg specific fix seems better suited. Let's apply that one and reconsider this one if it turns out we do need strict offline reordering. Thanks a lot! -- tejun -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org