Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966621AbaLLUe1 (ORCPT ); Fri, 12 Dec 2014 15:34:27 -0500 Received: from e39.co.us.ibm.com ([32.97.110.160]:34882 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751471AbaLLUeZ (ORCPT ); Fri, 12 Dec 2014 15:34:25 -0500 Date: Fri, 12 Dec 2014 12:34:18 -0800 From: "Paul E. McKenney" To: David Lang Cc: Linus Torvalds , Dave Jones , Chris Mason , Mike Galbraith , Ingo Molnar , Peter Zijlstra , =?iso-8859-1?Q?D=E2niel?= Fraga , Sasha Levin , Linux Kernel Mailing List Subject: Re: frequent lockups in 3.18rc4 Message-ID: <20141212203417.GE25340@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20141205171501.GA1320@redhat.com> <1417806247.4845.1@mail.thefacebook.com> <20141211145408.GB16800@redhat.com> <20141212185454.GB4716@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14121220-0033-0000-0000-000002FEE54C Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 12, 2014 at 11:58:50AM -0800, David Lang wrote: > On Fri, 12 Dec 2014, Linus Torvalds wrote: > > >I'm also not sure if the bug ever happens with preemption disabled. > >Sasha, was that you who reported that you cannot reproduce it without > >preemption? It strikes me that there's a race condition in > >__cond_resched() wrt preemption, for example: we do > > > > __preempt_count_add(PREEMPT_ACTIVE); > > __schedule(); > > __preempt_count_sub(PREEMPT_ACTIVE); > > > >and in between the __schedule() and __preempt_count_sub(), if an > >interrupt comes in and wakes up some important process, it won't > >reschedule (because preemption is active), but then we enable > >preemption again and don't check whether we should reschedule (again), > >and we just go on our merry ways. > > > >Now, I don't see how that could really matter for a long time - > >returning to user space will check need_resched, and sleeping will > >obviously force a reschedule anyway, so these kinds of races should at > >most delay things by just a tiny amount, > > If the machine has NOHZ and has a cpu bound userspace task, it could > take quite a while before userspace would trigger a reschedule (at > least if I've understood the comments on this thread properly) Dave, Sasha, if you guys are running CONFIG_NO_HZ_FULL=y and CONFIG_NO_HZ_FULL_ALL=y, please let me know. I am currently assuming that none of your CPUs are in NO_HZ_FULL mode. If this assumption is incorrect, there are some other pieces of RCU that I should be taking a hard look at. Thanx, Paul -- 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/