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=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, 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 CCF38C43142 for ; Wed, 27 Jun 2018 09:47:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6049A26099 for ; Wed, 27 Jun 2018 09:47:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="vAss56/M" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6049A26099 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org 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 S1753601AbeF0Jq5 (ORCPT ); Wed, 27 Jun 2018 05:46:57 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:39268 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753091AbeF0Jq4 (ORCPT ); Wed, 27 Jun 2018 05:46:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=e/pUx4Dq22b3SlcgKFg43Y7PGf9g5mMSLFh+8lC/kok=; b=vAss56/MoFjaP+U5ts7wkKHYr TzStP5q2Em/KdmfdZbxrbIvIKnhGroekJPLw/6RML4q7kJ69VO4fLmXsVUKe1Ca1cuHrU9HejqVhl BGnysN0Uat2tOMji0Qtx60ScUyoGeaHuCfmeE/WRdE1qQpHsNeFgEFPf96wQ960hCbzd0wdAZ/v+y r0uyk65e6jXql2x0UkZcEwTLk/2wG7CPSDl/r8ucGdy6tuxuppizAtSKh0coazqIjpk52EekTMrMW knfuGzK+ZWFvchpWkAlp0V0UkXRAt4QNI/i+hBIixUNQ7fTtwTL//lH+93HhtCMgCA5LEvSMGgzAK bWP4664Kw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fY724-00062a-Gc; Wed, 27 Jun 2018 09:46:36 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A15512029F1D5; Wed, 27 Jun 2018 11:46:33 +0200 (CEST) Date: Wed, 27 Jun 2018 11:46:33 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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 Message-ID: <20180627094633.GG2512@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180627091106.GB7184@worktop.programming.kicks-ass.net> User-Agent: Mutt/1.10.0 (2018-05-17) 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 11:11:06AM +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. 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. */