From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f46.google.com (mail-pa0-f46.google.com [209.85.220.46]) by kanga.kvack.org (Postfix) with ESMTP id 31D666B0031 for ; Tue, 1 Oct 2013 23:08:49 -0400 (EDT) Received: by mail-pa0-f46.google.com with SMTP id fa1so416726pad.5 for ; Tue, 01 Oct 2013 20:08:48 -0700 (PDT) Received: by mail-pb0-f51.google.com with SMTP id jt11so261055pbb.38 for ; Tue, 01 Oct 2013 20:08:46 -0700 (PDT) Date: Tue, 1 Oct 2013 20:08:43 -0700 (PDT) From: David Rientjes Subject: Re: [patch for-3.12] mm, memcg: protect mem_cgroup_read_events for cpu hotplug In-Reply-To: <20131002022227.GR856@cmpxchg.org> Message-ID: References: <20131002022227.GR856@cmpxchg.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Linus Torvalds , Andrew Morton , Michal Hocko , KAMEZAWA Hiroyuki , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On Tue, 1 Oct 2013, Johannes Weiner wrote: > On Tue, Oct 01, 2013 at 04:31:23PM -0700, David Rientjes wrote: > > for_each_online_cpu() needs the protection of {get,put}_online_cpus() so > > cpu_online_mask doesn't change during the iteration. > > There is no problem report here. > > Is there a crash? > No. > If it's just accuracy of the read, why would we care about some > inaccuracies in counters that can change before you even get the > results to userspace? And care to the point where we hold up CPU > hotplugging for this? > cpu_hotplug.lock is held while a cpu is going down, it's a coarse lock that is used kernel-wide to synchronize cpu hotplug activity. Memcg has a cpu hotplug notifier, called while there may not be any cpu hotplug refcounts, which drains per-cpu event counts to memcg->nocpu_base.events to maintain a cumulative event count as cpus disappear. Without get_online_cpus() in mem_cgroup_read_events(), it's possible to account for the event count on a dying cpu twice, and this value may be significantly large. In fact, all memcg->pcp_counter_lock use should be nested by {get,put}_online_cpus(). This fixes that issue and ensures the reported statistics are not vastly over-reported during cpu hotplug. > Also, the fact that you directly sent this to Linus suggests there is > some urgency for this fix. What's going on? > I believe users of cpu hotplug still want event counts that are approximate to the real value and that this is 3.12 material. -- 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