From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753179Ab2BBGMr (ORCPT ); Thu, 2 Feb 2012 01:12:47 -0500 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:54068 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752044Ab2BBGMq (ORCPT ); Thu, 2 Feb 2012 01:12:46 -0500 X-Originating-IP: 217.70.178.137 X-Originating-IP: 50.43.15.19 Date: Wed, 1 Feb 2012 22:12:28 -0800 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy before declaring a CPU to be offline Message-ID: <20120202061228.GV29058@leaf> References: <20120201194131.GA10028@linux.vnet.ibm.com> <1328125319-5205-1-git-send-email-paulmck@linux.vnet.ibm.com> <1328125319-5205-39-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1328125319-5205-39-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 01, 2012 at 11:41:57AM -0800, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > The force_quiescent_state() function uses a state machine to help > force grace periods to completion. One of its responsibilities is to > detect offline CPUs, and to report quiescent states on their behalf. > However, the CPU hotplug process is not atomic, in fact, there is > significant uncertainty as to exactly when a given CPU came online or > went offline. For example, once a CPU has marked itself offline and > executed the CPU_DYING notifiers, it continues executing, entering > the scheduler and perhaps also the idle loop. > > In the old days, force_quiescent_state() was guaranteed to wait for > several jiffies before declaring a given CPU offline. This is no > longer the case, due to some of the more aggressive rcutorture tests > and the CONFIG_RCU_FAST_NO_HZ idle-entry code. Therefore, this commit > makes force_quiescent_state() explicitly wait for at least a jiffy > before declaring a CPU to be offline. This commit seems to implement behavior documented as working in patch 38. Shouldn't those bits go together? - Josh Triplett