From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932301AbcKUOSa (ORCPT ); Mon, 21 Nov 2016 09:18:30 -0500 Received: from mail-wm0-f67.google.com ([74.125.82.67]:34487 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753191AbcKUOS1 (ORCPT ); Mon, 21 Nov 2016 09:18:27 -0500 Date: Mon, 21 Nov 2016 15:18:19 +0100 From: Michal Hocko To: "Paul E. McKenney" Cc: Paul Menzel , linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett , dvteam@molgen.mpg.de Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Message-ID: <20161121141818.GD18112@dhcp22.suse.cz> References: <24c226a5-1a4a-173e-8b4e-5107a2baac04@molgen.mpg.de> <28a9fabb-c9fe-c865-016a-467a4d5e2a34@molgen.mpg.de> <20161108170340.GB4127@linux.vnet.ibm.com> <6c717122-e671-b086-77ed-4b3c26398564@molgen.mpg.de> <20161108183938.GD4127@linux.vnet.ibm.com> <9f87f8f0-9d0f-f78f-8dca-993b09b19a69@molgen.mpg.de> <20161116173036.GK3612@linux.vnet.ibm.com> <20161121134130.GB18112@dhcp22.suse.cz> <20161121140122.GU3612@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161121140122.GU3612@linux.vnet.ibm.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 21-11-16 06:01:22, Paul E. McKenney wrote: > On Mon, Nov 21, 2016 at 02:41:31PM +0100, Michal Hocko wrote: [...] > > To the patch. I cannot say I would like it. cond_resched_rcu_qs sounds > > way too lowlevel for this usage. If anything cond_resched somewhere inside > > mem_cgroup_iter would be more appropriate to me. > > Like this? > > Thanx, Paul > > ------------------------------------------------------------------------ > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index ae052b5e3315..81cb30d5b2fc 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -867,6 +867,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, > out: > if (prev && prev != root) > css_put(&prev->css); > + cond_resched_rcu_qs(); I still do not understand why should we play with _rcu_qs at all and a regular cond_resched is not sufficient. Anyway I would have to double check whether we can do cond_resched in the iterator. I do not remember having users which are atomic but I might be easily wrong here. Before we touch this code, though, I would really like to understand what is actually going on here because as I've already pointed out we should have some resched points in the reclaim path. -- Michal Hocko SUSE Labs