From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756230AbaICP3n (ORCPT ); Wed, 3 Sep 2014 11:29:43 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:34061 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753751AbaICP3m (ORCPT ); Wed, 3 Sep 2014 11:29:42 -0400 Date: Wed, 3 Sep 2014 17:28:50 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.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, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, <""@rjwysocki.net>, tianyu.lan@intel.com Subject: Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods Message-ID: <20140903152850.GQ4783@worktop.ger.corp.intel.com> References: <20140828194745.GA3761@linux.vnet.ibm.com> <20140901112059.GG27892@worktop.ger.corp.intel.com> <20140901160550.GL5001@linux.vnet.ibm.com> <20140901161735.GA5806@worktop.ger.corp.intel.com> <20140902163656.GC5001@linux.vnet.ibm.com> <20140903113112.GM4783@worktop.ger.corp.intel.com> <20140903150306.GU5001@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140903150306.GU5001@linux.vnet.ibm.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 03, 2014 at 08:03:06AM -0700, Paul E. McKenney wrote: > > > Normal RCU grace periods avoid this by synchronizing on a lock acquired by > > > the RCU CPU-hotplug notifiers, but this does not work for the expedited > > > grace periods because the outgoing CPU can be running random tasks for > > > quite some time after RCU's notifier executes. So the fix is just to > > > drop back to a normal grace period when there is a CPU-hotplug operation > > > in progress. > > > > So why are we 'normally' doing an expedited call here anyhow? > > Presumably because they set either the boot parameter or > the sysfs variable that causes synchronize_sched() to so > synchronize_sched_expedited(). That's not a why but a how. Why does that option exist, why are we doing this? I cannot actually find a sysfs variable that controls this though; only the rcu_pm_notifier. It seems to favour doing an expedited call when suspending on 'small' machines. > > But those are not within hotplug bits. Also weren't we removing them? I > > thought we didn't appreciate spraying IPIs like they do? > > I hadn't heard anything about removing them, but making the > expedited primitives a bit less IPI-happy is on my list. I had some recollections of removing a fair number of expedited calls, but its was a long while ago so what do I know ;-) Making them less IPI happy would be good indeed.