From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754790AbbAOR05 (ORCPT ); Thu, 15 Jan 2015 12:26:57 -0500 Received: from mail-we0-f176.google.com ([74.125.82.176]:35674 "EHLO mail-we0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752254AbbAOR04 (ORCPT ); Thu, 15 Jan 2015 12:26:56 -0500 Date: Thu, 15 Jan 2015 18:26:52 +0100 From: Michal Hocko To: Tejun Heo Cc: Vladimir Davydov , "Suzuki K. Poulose" , Johannes Weiner , linux-mm@kvack.org, "linux-kernel@vger.kernel.org" , Will Deacon Subject: Re: [PATCH cgroup/for-3.19-fixes] cgroup: implement cgroup_subsys->unbind() callback Message-ID: <20150115172652.GF7008@dhcp22.suse.cz> References: <54B01335.4060901@arm.com> <20150110085525.GD2110@esperanza> <20150110214316.GF25319@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150110214316.GF25319@htj.dyndns.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 10-01-15 16:43:16, Tejun Heo wrote: > Currently, if a hierarchy doesn't have any live children when it's > unmounted, the hierarchy starts dying by killing its refcnt. The > expectation is that even if there are lingering dead children which > are lingering due to remaining references, they'll be put in a finite > amount of time. When the children are finally released, the hierarchy > is destroyed and all controllers bound to it also are released. > > However, for memcg, the premise that the lingering refs will be put in > a finite amount time is not true. In the absense of memory pressure, > dead memcg's may hang around indefinitely pinned by its pages. This > unfortunately may lead to indefinite hang on the next mount attempt > involving memcg as the mount logic waits for it to get released. > > While we can change hierarchy destruction logic such that a hierarchy > is only destroyed when it's not mounted anywhere and all its children, > live or dead, are gone, this makes whether the hierarchy gets > destroyed or not to be determined by factors opaque to userland. > Userland may or may not get a new hierarchy on the next mount attempt. > Worse, if it explicitly wants to create a new hierarchy with different > options or controller compositions involving memcg, it will fail in an > essentially arbitrary manner. > > We want to guarantee that a hierarchy is destroyed once the > conditions, unmounted and no visible children, are met. To aid it, > this patch introduces a new callback cgroup_subsys->unbind() which is > invoked right before the hierarchy a subsystem is bound to starts > dying. memcg can implement this callback and initiate draining of > remaining refs so that the hierarchy can eventually be released in a > finite amount of time. > > Signed-off-by: Tejun Heo > Cc: Li Zefan > Cc: Johannes Weiner > Cc: Michal Hocko > Cc: Vladimir Davydov Ohh, I have missed this one as I wasn't on the CC list. FWIW this approach makes sense to me. I just think that we should have a way to fail. E.g. kmem pages are impossible to reclaim because there might be some objects lingering somewhere not bound to a task context and reparenting is hard as Vladimir has pointed out several times already. Normal LRU pages should be reclaimable or reparented to the root easily. I cannot judge the implementation but I agree with the fact that memcg controller should be the one to take an action. -- Michal Hocko SUSE Labs