From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id 803226B025E for ; Wed, 30 Nov 2016 09:31:12 -0500 (EST) Received: by mail-pg0-f69.google.com with SMTP id p66so27266089pga.4 for ; Wed, 30 Nov 2016 06:31:12 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id j63si40077767pfg.51.2016.11.30.06.31.11 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Nov 2016 06:31:11 -0800 (PST) Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uAUEUAiA141114 for ; Wed, 30 Nov 2016 09:31:11 -0500 Received: from e36.co.us.ibm.com (e36.co.us.ibm.com [32.97.110.154]) by mx0a-001b2d01.pphosted.com with ESMTP id 271wugkd83-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 30 Nov 2016 09:31:10 -0500 Received: from localhost by e36.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 30 Nov 2016 07:31:10 -0700 Date: Wed, 30 Nov 2016 06:31:06 -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: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Message-Id: <20161130143106.GT3924@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Paul Menzel Cc: Michal Hocko , Donald Buczek , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett On Wed, Nov 30, 2016 at 01:31:37PM +0100, Paul Menzel wrote: > 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? > > Ita??s not selected. > > ``` > # CONFIG_PREEMPT is not set > ``` Thank you for the info! As noted elsewhere in this thread, there are other ways to get stalls, including the long irq-disabled execution that Michal suspects. 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? > > > Kind regards, > > Paul > -- 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