Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751966AbdGYMo3 (ORCPT ); Tue, 25 Jul 2017 08:44:29 -0400 Received: from mx2.suse.de ([195.135.220.15]:50347 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751127AbdGYMo1 (ORCPT ); Tue, 25 Jul 2017 08:44:27 -0400 Date: Tue, 25 Jul 2017 14:44:19 +0200 From: Michal Hocko To: Roman Gushchin Cc: Vladimir Davydov , linux-mm@kvack.org, Tejun Heo , Johannes Weiner , kernel-team@fb.com, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm, memcg: reset low limit during memcg offlining Message-ID: <20170725124419.GG26723@dhcp22.suse.cz> References: <20170725114047.4073-1-guro@fb.com> <20170725120537.o4kgzjhcjcjmopzc@esperanza> <20170725123113.GB12635@castle.DHCP.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725123113.GB12635@castle.DHCP.thefacebook.com> 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 Content-Length: 2698 Lines: 70 On Tue 25-07-17 13:31:13, Roman Gushchin wrote: > On Tue, Jul 25, 2017 at 03:05:37PM +0300, Vladimir Davydov wrote: > > On Tue, Jul 25, 2017 at 12:40:47PM +0100, Roman Gushchin wrote: > > > A removed memory cgroup with a defined low limit and some belonging > > > pagecache has very low chances to be freed. > > > > > > If a cgroup has been removed, there is likely no memory pressure inside > > > the cgroup, and the pagecache is protected from the external pressure > > > by the defined low limit. The cgroup will be freed only after > > > the reclaim of all belonging pages. And it will not happen until > > > there are any reclaimable memory in the system. That means, > > > there is a good chance, that a cold pagecache will reside > > > in the memory for an undefined amount of time, wasting > > > system resources. > > > > > > Fix this issue by zeroing memcg->low during memcg offlining. > > > > > > Signed-off-by: Roman Gushchin > > > Cc: Tejun Heo > > > Cc: Johannes Weiner > > > Cc: Michal Hocko > > > Cc: Vladimir Davydov > > > Cc: kernel-team@fb.com > > > Cc: cgroups@vger.kernel.org > > > Cc: linux-mm@kvack.org > > > Cc: linux-kernel@vger.kernel.org > > > --- > > > mm/memcontrol.c | 2 ++ > > > 1 file changed, 2 insertions(+) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index aed11b2d0251..2aa204b8f9fd 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -4300,6 +4300,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css) > > > } > > > spin_unlock(&memcg->event_list_lock); > > > > > > + memcg->low = 0; > > > + > > > memcg_offline_kmem(memcg); > > > wb_memcg_offline(memcg); > > > > > > > We already have that - see mem_cgroup_css_reset(). > > Hm, I see... > > But are you sure, that calling mem_cgroup_css_reset() from offlining path > is always a good idea? Well, originally I wanted to suggest the same but then I asked the very same question and couldn't answer it myself. memcg_offline_kmem feels much more generic. > As I understand, css_reset() callback is intended to _completely_ disable all > limits, as if there were no cgroup at all. And it's main purpose to be called > when controllers are detached from the hierarhy. yes, that is my understanding as well. > Offlining is different: some limits make perfect sence after offlining > (e.g. we want to limit the writeback speed), and other might be tweaked > (e.g. we can set soft limit to prioritize reclaiming of abandoned cgroups). and the writeback path was exactly the one that triggered my suspicious... -- Michal Hocko SUSE Labs