From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753287AbaBCQSa (ORCPT ); Mon, 3 Feb 2014 11:18:30 -0500 Received: from zene.cmpxchg.org ([85.214.230.12]:56617 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752088AbaBCQS3 (ORCPT ); Mon, 3 Feb 2014 11:18:29 -0500 Date: Mon, 3 Feb 2014 11:18:23 -0500 From: Johannes Weiner To: Michal Hocko Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , LKML , linux-mm@kvack.org Subject: Re: [RFC 4/5] memcg: make sure that memcg is not offline when charging Message-ID: <20140203161823.GJ6963@cmpxchg.org> References: <1387295130-19771-1-git-send-email-mhocko@suse.cz> <1387295130-19771-5-git-send-email-mhocko@suse.cz> <20140130172906.GE6963@cmpxchg.org> <20140203133313.GF2495@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140203133313.GF2495@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 03, 2014 at 02:33:13PM +0100, Michal Hocko wrote: > On Thu 30-01-14 12:29:06, Johannes Weiner wrote: > > On Tue, Dec 17, 2013 at 04:45:29PM +0100, Michal Hocko wrote: > > > The current charge path might race with memcg offlining because holding > > > css reference doesn't stop css offline. As a result res counter might be > > > charged after mem_cgroup_reparent_charges (called from memcg css_offline > > > callback) and so the charge would never be freed. This has been worked > > > around by 96f1c58d8534 (mm: memcg: fix race condition between memcg > > > teardown and swapin) which tries to catch such a leaked charges later > > > during css_free. It is more optimal to heal this race in the long term > > > though. > > > > We already deal with the race, so IMO the only outstanding improvement > > is to take advantage of the teardown synchronization provided by the > > cgroup core and get rid of our one-liner workaround in .css_free. > > I am not sure I am following you here. Which teardown synchronization do > you have in mind? rcu_read_lock & css_tryget? Yes. It provides rcu synchronization between establishing new references and offlining, as long as you establish references atomically in one RCU read-side section: repeat: rcu_read_lock() css_tryget() res_counter_charge() rcu_read_unlock() if retries++ < RECLAIM_RETRIES: reclaim goto repeat > > > In order to make this raceless we would need to hold rcu_read_lock since > > > css_tryget until res_counter_charge. This is not so easy unfortunately > > > because mem_cgroup_do_charge might sleep so we would need to do drop rcu > > > lock and do css_tryget tricks after each reclaim. > > > > Yes, why not? > > Although css_tryget is cheap these days I thought that a simple flag > check would be even heaper in this hot path. Changing the patch to use > css_tryget rather than offline check is trivial if you really think it > is better? You already changed it to do css_tryget() on every single charge. Direct reclaim is invoked only from a fraction of all charges, it's already a slowpath, I don't think another percpu counter op will be the final straw that makes this path too fat.