From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752730AbaHCNAr (ORCPT ); Sun, 3 Aug 2014 09:00:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:9307 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751971AbaHCNAq (ORCPT ); Sun, 3 Aug 2014 09:00:46 -0400 Date: Sun, 3 Aug 2014 14:57:58 +0200 From: Oleg Nesterov 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, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks() Message-ID: <20140803125758.GA671@redhat.com> References: <20140731215445.GA21933@linux.vnet.ibm.com> <1406843709-23396-1-git-send-email-paulmck@linux.vnet.ibm.com> <20140801141144.GA30293@redhat.com> <20140801182837.GI4784@linux.vnet.ibm.com> <20140801184059.GB10718@redhat.com> <20140802230020.GD8101@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140802230020.GD8101@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/02, Paul E. McKenney wrote: > > On Fri, Aug 01, 2014 at 08:40:59PM +0200, Oleg Nesterov wrote: > > On 08/01, Paul E. McKenney wrote: > > > > > > On Fri, Aug 01, 2014 at 04:11:44PM +0200, Oleg Nesterov wrote: > > > > Not sure this makes any sense, but perhaps we can check for the new > > > > callbacks and start the next gp. IOW, the main loop roughly does > > > > > > > > for (;;) { > > > > list = rcu_tasks_cbs_head; > > > > rcu_tasks_cbs_head = NULL; > > > > > > > > if (!list) > > > > sleep(); > > > > > > > > synchronize_sched(); > > > > > > > > wait_for_rcu_tasks_holdout(); > > > > > > > > synchronize_sched(); > > > > > > > > process_callbacks(list); > > > > } > > > > > > > > we can "join" 2 synchronize_sched's and do > > > > > > > > ready_list = NULL; > > > > for (;;) { > > > > list = rcu_tasks_cbs_head; > > > > rcu_tasks_cbs_head = NULL; > > > > > > > > if (!list && !ready_list) > > > > sleep(); > > > > > > > > synchronize_sched(); > > > > > > > > if (ready_list) { > > > > process_callbacks(ready_list); > > > > ready_list = NULL; > > > > } > > > > > > > > if (!list) > > > > continue; > > > > > > > > wait_for_rcu_tasks_holdout(); > > > > ready_list = list; > > > > } > > > > > > The lack of barriers for the updates I am checking mean that I really > > > do need a synchronize_sched() on either side of the grace-period wait. > > > > Yes, > > > > > The grace period needs to guarantee that anything that happened on any > > > CPU before the start of the grace period happens before anything that > > > happens on any CPU after the end of the grace period. If I leave off > > > either synchronize_sched(), we lose this guarantee. > > > > But the 2nd variant still has synchronize_sched() on both sides? > > Your second variant above? Unless it is in wait_for_rcu_tasks_holdouts(), > I am not seeing it. I guess I probably misunderstood you from the very beginning. And now I am curious what exactly I missed... The code above doesn't do process_callbacks() after wait_for_rcu_tasks_holdout(), it does this only after another synchronize_sched(). The only difference is that we dequeue the next generation of the pending rcu_tasks_cbs_head callbacks. IOW. Lets look at the current code. Suppose that synchronize_rcu_tasks() is called when rcu_tasks_kthread() sleeps in wait_for_rcu_tasks_holdout(). In this case the new wakeme_after_rcu callback will sit in rcu_tasks_cbs_head until rcu_tasks_kthread() does the 2nd synchronize_sched() + process_callbacks(). Only after that it will be dequeued and rcu_tasks_kthread() will start another gp. This means that we have 3 synchronize_sched()'s before synchronize_rcu_tasks() returns. Do we really need this? With the 2nd variant the new callback will be dequeud right after wait_for_rcu_tasks_holdout(), and we only have 2 necessary synchronize_sched()'s around wait_for_rcu_tasks_holdout(). But it seems that I missed something else. Could you please spell? Oleg.