From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 7649B6B0038 for ; Wed, 30 Nov 2016 06:53:27 -0500 (EST) Received: by mail-pf0-f198.google.com with SMTP id c4so300728398pfb.7 for ; Wed, 30 Nov 2016 03:53:27 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id y4si64012602pgo.299.2016.11.30.03.53.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Nov 2016 03:53:25 -0800 (PST) Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uAUBoV4Y020271 for ; Wed, 30 Nov 2016 06:53:25 -0500 Received: from e32.co.us.ibm.com (e32.co.us.ibm.com [32.97.110.150]) by mx0a-001b2d01.pphosted.com with ESMTP id 271u9ea0d2-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 30 Nov 2016 06:53:24 -0500 Received: from localhost by e32.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 30 Nov 2016 04:53:24 -0700 Date: Wed, 30 Nov 2016 03:53:20 -0800 From: "Paul E. McKenney" Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Reply-To: paulmck@linux.vnet.ibm.com References: <20161121140122.GU3612@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161130110944.GD18432@dhcp22.suse.cz> Message-Id: <20161130115320.GO3924@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Donald Buczek , Paul Menzel , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett 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. Thanx, Paul > > 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? > -- > 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