Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752847AbaJ0XsR (ORCPT ); Mon, 27 Oct 2014 19:48:17 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:60002 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752369AbaJ0XsQ (ORCPT ); Mon, 27 Oct 2014 19:48:16 -0400 Date: Mon, 27 Oct 2014 16:44:25 -0700 From: "Paul E. McKenney" To: Sasha Levin Cc: Dave Jones , Linux Kernel , htejun@gmail.com, linux-mm@kvack.org Subject: Re: rcu_preempt detected stalls. Message-ID: <20141027234425.GA19438@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20141013173504.GA27955@redhat.com> <543DDD5E.9080602@oracle.com> <20141023183917.GX4977@linux.vnet.ibm.com> <54494F2F.6020005@oracle.com> <20141023195808.GB4977@linux.vnet.ibm.com> <544A45F8.2030207@oracle.com> <20141024161337.GQ4977@linux.vnet.ibm.com> <544A80B3.9070800@oracle.com> <20141027211329.GJ5718@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141027211329.GJ5718@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14102723-0029-0000-0000-000000EC0C1C Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 27, 2014 at 02:13:29PM -0700, Paul E. McKenney wrote: > On Fri, Oct 24, 2014 at 12:39:15PM -0400, Sasha Levin wrote: > > On 10/24/2014 12:13 PM, Paul E. McKenney wrote: > > > On Fri, Oct 24, 2014 at 08:28:40AM -0400, Sasha Levin wrote: > > >> > On 10/23/2014 03:58 PM, Paul E. McKenney wrote: > > >>> > > On Thu, Oct 23, 2014 at 02:55:43PM -0400, Sasha Levin wrote: > > >>>>> > >> > On 10/23/2014 02:39 PM, Paul E. McKenney wrote: > > >>>>>>> > >>> > > On Tue, Oct 14, 2014 at 10:35:10PM -0400, Sasha Levin wrote: > > >>>>>>>>> > >>>> > >> On 10/13/2014 01:35 PM, Dave Jones wrote: > > >>>>>>>>>>> > >>>>> > >>> oday in "rcu stall while fuzzing" news: > > >>>>>>>>>>> > >>>>> > >>> > > >>>>>>>>>>> > >>>>> > >>> INFO: rcu_preempt detected stalls on CPUs/tasks: > > >>>>>>>>>>> > >>>>> > >>> Tasks blocked on level-0 rcu_node (CPUs 0-3): P766 P646 > > >>>>>>>>>>> > >>>>> > >>> Tasks blocked on level-0 rcu_node (CPUs 0-3): P766 P646 > > >>>>>>>>>>> > >>>>> > >>> (detected by 0, t=6502 jiffies, g=75434, c=75433, q=0) > > >>>>>>>>> > >>>> > >> > > >>>>>>>>> > >>>> > >> I've complained about RCU stalls couple days ago (in a different context) > > >>>>>>>>> > >>>> > >> on -next. I guess whatever causing them made it into Linus's tree? > > >>>>>>>>> > >>>> > >> > > >>>>>>>>> > >>>> > >> https://lkml.org/lkml/2014/10/11/64 > > >>>>>>> > >>> > > > > >>>>>>> > >>> > > And on that one, I must confess that I don't see where the RCU read-side > > >>>>>>> > >>> > > critical section might be. > > >>>>>>> > >>> > > > > >>>>>>> > >>> > > Hmmm... Maybe someone forgot to put an rcu_read_unlock() somewhere. > > >>>>>>> > >>> > > Can you reproduce this with CONFIG_PROVE_RCU=y? > > >>>>> > >> > > > >>>>> > >> > Paul, if that was directed to me - Yes, I see stalls with CONFIG_PROVE_RCU > > >>>>> > >> > set and nothing else is showing up before/after that. > > >>> > > Indeed it was directed to you. ;-) > > >>> > > > > >>> > > Does the following crude diagnostic patch turn up anything? > > >> > > > >> > Nope, seeing stalls but not seeing that pr_err() you added. > > > OK, color me confused. Could you please send me the full dmesg or a > > > pointer to it? > > > > Attached. > > Thank you! I would complain about the FAULT_INJECTION messages, but > they don't appear to be happening all that frequently. > > The stack dumps do look different here. I suspect that this is a real > issue in the VM code. And to that end... The filemap_map_pages() function does have loop over a list of pages. I wonder if the rcu_read_lock() should be moved into the radix_tree_for_each_slot() loop. CCing linux-mm for their thoughts, though it looks to me like the current radix_tree_for_each_slot() wants to be under RCU protection. But I am not seeing anything that requires all iterations of the loop to be under the same RCU read-side critical section. Maybe something like the following patch? Thanx, Paul ------------------------------------------------------------------------ mm: Attempted fix for RCU CPU stall warning It appears that filemap_map_pages() can stay in a single RCU read-side critical section for a very long time if given a large area to map. This could result in RCU CPU stall warnings. This commit therefore breaks the read-side critical section into per-iteration critical sections, taking care to make sure that the radix_tree_for_each_slot() call itself remains in an RCU read-side critical section, as required. Reported-by: Sasha Levin Signed-off-by: Paul E. McKenney diff --git a/mm/filemap.c b/mm/filemap.c index 14b4642279f1..f78f144fb41f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2055,6 +2055,8 @@ skip: next: if (iter.index == vmf->max_pgoff) break; + rcu_read_unlock(); + rcu_read_lock(); } rcu_read_unlock(); } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/