From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f178.google.com (mail-we0-f178.google.com [74.125.82.178]) by kanga.kvack.org (Postfix) with ESMTP id 8A2AD6B0035 for ; Mon, 3 Feb 2014 08:33:16 -0500 (EST) Received: by mail-we0-f178.google.com with SMTP id q59so2156680wes.37 for ; Mon, 03 Feb 2014 05:33:16 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id cp4si3541451wib.20.2014.02.03.05.33.14 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Mon, 03 Feb 2014 05:33:15 -0800 (PST) Date: Mon, 3 Feb 2014 14:33:13 +0100 From: Michal Hocko Subject: Re: [RFC 4/5] memcg: make sure that memcg is not offline when charging Message-ID: <20140203133313.GF2495@dhcp22.suse.cz> References: <1387295130-19771-1-git-send-email-mhocko@suse.cz> <1387295130-19771-5-git-send-email-mhocko@suse.cz> <20140130172906.GE6963@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140130172906.GE6963@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , LKML , linux-mm@kvack.org 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? > > 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? Btw. I plan to repost the series as soon as Andrew releases his mmotm tree. Thanks -- Michal Hocko SUSE Labs -- 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