From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8F63CC43142 for ; Wed, 27 Jun 2018 15:55:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3D8AF2450C for ; Wed, 27 Jun 2018 15:55:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3D8AF2450C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934861AbeF0PzX (ORCPT ); Wed, 27 Jun 2018 11:55:23 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:60144 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934767AbeF0PzW (ORCPT ); Wed, 27 Jun 2018 11:55:22 -0400 Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w5RFshdu131192 for ; Wed, 27 Jun 2018 11:55:21 -0400 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0a-001b2d01.pphosted.com with ESMTP id 2jvc4xvdqq-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 27 Jun 2018 11:55:21 -0400 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 27 Jun 2018 11:55:20 -0400 Received: from b01cxnp22035.gho.pok.ibm.com (9.57.198.25) by e12.ny.us.ibm.com (146.89.104.199) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 27 Jun 2018 11:55:15 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w5RFtEjU10093158 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 27 Jun 2018 15:55:14 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BD15CB205F; Wed, 27 Jun 2018 11:55:06 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8B73CB206A; Wed, 27 Jun 2018 11:55:06 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 27 Jun 2018 11:55:06 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 204B816C5EF9; Wed, 27 Jun 2018 08:57:21 -0700 (PDT) Date: Wed, 27 Jun 2018 08:57:21 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline Reply-To: paulmck@linux.vnet.ibm.com References: <20180626002052.GA24146@linux.vnet.ibm.com> <20180626171048.2181-13-paulmck@linux.vnet.ibm.com> <20180626175119.GL2494@hirez.programming.kicks-ass.net> <20180626182950.GH3593@linux.vnet.ibm.com> <20180626202615.GA32162@linux.vnet.ibm.com> <20180626203225.GT2494@hirez.programming.kicks-ass.net> <20180626234004.GQ3593@linux.vnet.ibm.com> <20180627091106.GB7184@worktop.programming.kicks-ass.net> <20180627094633.GG2512@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180627094633.GG2512@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062715-0060-0000-0000-000002834E34 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009264; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01053124; UDB=6.00539942; IPR=6.00831048; MB=3.00021883; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-27 15:55:19 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062715-0061-0000-0000-000045983AA0 Message-Id: <20180627155721.GZ3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-27_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806270171 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 27, 2018 at 11:46:33AM +0200, Peter Zijlstra wrote: > On Wed, Jun 27, 2018 at 11:11:06AM +0200, Peter Zijlstra wrote: > > On Tue, Jun 26, 2018 at 04:40:04PM -0700, Paul E. McKenney wrote: > > > The options I have considered are as follows: > > > > > 2. Stick with the no-failsafe approach, but rely on RCU's grace-period > > > kthread to wake up later due to its timed wait during the > > > force-quiescent-state process. This would be a bit obnoxious, > > > as it requires passing a don't-wake flag (or some such) up the > > > quiescent-state reporting mechanism. It would also needlessly > > > delay grace-period ends, especially on large systems (RCU scales > > > up the FQS delay on larger systems to maintain limited CPU > > > consumption per unit time). > > > > > > 3. Stick with the no-failsafe approach, but have the quiescent-state > > > reporting code hand back a value indicating that a wakeup is needed. > > > Also a bit obnoxious, as this value would need to be threaded up > > > the reporting code's return path. Simple in theory, but a bit > > > of an ugly change, especially for the many places in the code that > > > currently expect quiescent-state reporting to be an unconditional > > > fire-and-forget operation. > > > > Here's a variant on 2+3, instead of propagating the state back, we > > completely ignore if we needed a wakeup or not, and then unconditionally > > wake the GP kthread on the managing CPU's rcutree_migrate_callbacks() > > invocation. > > > > Hotplug is rare (or should damn well be), doing a spurious wake of the > > GP thread shouldn't matter here. > > Another variant, which simply skips the wakeup whever ran on an offline > CPU, relying on the wakeup from rcutree_migrate_callbacks() right after > the CPU really is dead. Cute! ;-) And a much smaller change. However, this means that if someone indirectly and erroneously causes rcu_report_qs_rsp() to be invoked from an offline CPU, the result is an intermittent and difficult-to-debug grace-period hang. A lockdep splat whose stack trace directly implicates the culprit is much better. But your point about CPU hotplug being rare is a good one. I should at the very least move ->ofl_lock to sit right beside ->lock, ditching the ____cacheline_internodealigned_in_smp. Thanx, Paul > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 7832dd556490..417496a03259 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -104,7 +104,6 @@ struct rcu_state sname##_state = { \ > .abbr = sabbr, \ > .exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \ > .exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \ > - .ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \ > } > > RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched); > @@ -1928,13 +1927,11 @@ static bool rcu_gp_init(struct rcu_state *rsp) > */ > rsp->gp_state = RCU_GP_ONOFF; > rcu_for_each_leaf_node(rsp, rnp) { > - spin_lock(&rsp->ofl_lock); > raw_spin_lock_irq_rcu_node(rnp); > if (rnp->qsmaskinit == rnp->qsmaskinitnext && > !rnp->wait_blkd_tasks) { > /* Nothing to do on this leaf rcu_node structure. */ > raw_spin_unlock_irq_rcu_node(rnp); > - spin_unlock(&rsp->ofl_lock); > continue; > } > > @@ -1970,7 +1967,6 @@ static bool rcu_gp_init(struct rcu_state *rsp) > } > > raw_spin_unlock_irq_rcu_node(rnp); > - spin_unlock(&rsp->ofl_lock); > } > rcu_gp_slow(rsp, gp_preinit_delay); /* Races with CPU hotplug. */ > > @@ -2250,11 +2246,19 @@ static int __noreturn rcu_gp_kthread(void *arg) > static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags) > __releases(rcu_get_root(rsp)->lock) > { > + int cpu = smp_processor_id(); > + > raw_lockdep_assert_held_rcu_node(rcu_get_root(rsp)); > WARN_ON_ONCE(!rcu_gp_in_progress(rsp)); > WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS); > raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags); > - rcu_gp_kthread_wake(rsp); > + > + /* > + * When our @cpu is offline, we'll get a wakeup from > + * rcutree_migrate_callbacks. > + */ > + if (cpu_online(cpu)) > + rcu_gp_kthread_wake(rsp); > } > > /* > @@ -3768,18 +3772,15 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp) > > /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ > mask = rdp->grpmask; > - spin_lock(&rsp->ofl_lock); > raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */ > rdp->rcu_ofl_gp_seq = READ_ONCE(rsp->gp_seq); > rdp->rcu_ofl_gp_flags = READ_ONCE(rsp->gp_flags); > + rnp->qsmaskinitnext &= ~mask; > if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */ > - /* Report quiescent state -before- changing ->qsmaskinitnext! */ > rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags); > raw_spin_lock_irqsave_rcu_node(rnp, flags); > } > - rnp->qsmaskinitnext &= ~mask; > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > - spin_unlock(&rsp->ofl_lock); > } > > /* > @@ -3849,6 +3850,12 @@ void rcutree_migrate_callbacks(int cpu) > { > struct rcu_state *rsp; > > + /* > + * Just in case the outgoing CPU needed to wake the GP kthread > + * do so here. > + */ > + rcu_gp_kthread_wake(rsp); > + > for_each_rcu_flavor(rsp) > rcu_migrate_callbacks(cpu, rsp); > } > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index 4e74df768c57..8dab71838141 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -367,10 +367,6 @@ struct rcu_state { > const char *name; /* Name of structure. */ > char abbr; /* Abbreviated name. */ > struct list_head flavors; /* List of RCU flavors. */ > - > - spinlock_t ofl_lock ____cacheline_internodealigned_in_smp; > - /* Synchronize offline with */ > - /* GP pre-initialization. */ > }; > > /* Values for rcu_state structure's gp_flags field. */ >