Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755663AbcK3Mbr (ORCPT ); Wed, 30 Nov 2016 07:31:47 -0500 Received: from mx1.molgen.mpg.de ([141.14.17.9]:58523 "EHLO mx1.molgen.mpg.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751671AbcK3Mbl (ORCPT ); Wed, 30 Nov 2016 07:31:41 -0500 Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` To: "Paul E. McKenney" , Michal Hocko References: <20161121141818.GD18112@dhcp22.suse.cz> <20161121142901.GV3612@linux.vnet.ibm.com> <68025f6c-6801-ab46-b0fc-a9407353d8ce@molgen.mpg.de> <20161124101525.GB20668@dhcp22.suse.cz> <583AA50A.9010608@molgen.mpg.de> <20161128110449.GK14788@dhcp22.suse.cz> <109d5128-f3a4-4b6e-db17-7a1fcb953500@molgen.mpg.de> <29196f89-c35e-f79d-8e4d-2bf73fe930df@molgen.mpg.de> <20161130110944.GD18432@dhcp22.suse.cz> <20161130115320.GO3924@linux.vnet.ibm.com> <20161130115442.GA19271@linux.vnet.ibm.com> Cc: Donald Buczek , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett From: Paul Menzel Message-ID: Date: Wed, 30 Nov 2016 13:31:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <20161130115442.GA19271@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1965 Lines: 59 On 11/30/16 12:54, Paul E. McKenney wrote: > On Wed, Nov 30, 2016 at 03:53:20AM -0800, Paul E. McKenney wrote: >> On Wed, Nov 30, 2016 at 12:09:44PM +0100, Michal Hocko wrote: >>> [CCing Paul] >>> >>> On Wed 30-11-16 11:28:34, Donald Buczek wrote: >>> [...] >>>> shrink_active_list gets and releases the spinlock and calls cond_resched(). >>>> This should give other tasks a chance to run. Just as an experiment, I'm >>>> trying >>>> >>>> --- a/mm/vmscan.c >>>> +++ b/mm/vmscan.c >>>> @@ -1921,7 +1921,7 @@ static void shrink_active_list(unsigned long >>>> nr_to_scan, >>>> spin_unlock_irq(&pgdat->lru_lock); >>>> >>>> while (!list_empty(&l_hold)) { >>>> - cond_resched(); >>>> + cond_resched_rcu_qs(); >>>> page = lru_to_page(&l_hold); >>>> list_del(&page->lru); >>>> >>>> and didn't hit a rcu_sched warning for >21 hours uptime now. We'll see. >>> >>> This is really interesting! Is it possible that the RCU stall detector >>> is somehow confused? >> >> No, it is not confused. Again, cond_resched() is not a quiescent >> state unless it does a context switch. Therefore, if the task running >> in that loop was the only runnable task on its CPU, cond_resched() >> would -never- provide RCU with a quiescent state. >> >> In contrast, cond_resched_rcu_qs() unconditionally provides RCU >> with a quiescent state (hence the _rcu_qs in its name), regardless >> of whether or not a context switch happens. >> >> It is therefore expected behavior that this change might prevent >> RCU CPU stall warnings. > > I should add... This assumes that CONFIG_PREEMPT=n. So what is > CONFIG_PREEMPT? It?s not selected. ``` # CONFIG_PREEMPT is not set ``` >>>> Is preemption disabled for another reason? >>> >>> I do not think so. I will have to double check the code but this is a >>> standard sleepable context. Just wondering what is the PREEMPT >>> configuration here? Kind regards, Paul