From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755209AbZEQWIx (ORCPT ); Sun, 17 May 2009 18:08:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754407AbZEQWIj (ORCPT ); Sun, 17 May 2009 18:08:39 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.143]:40545 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752378AbZEQWIi (ORCPT ); Sun, 17 May 2009 18:08:38 -0400 Date: Sun, 17 May 2009 15:08:35 -0700 From: "Paul E. McKenney" To: Evgeniy Polyakov Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, mingo@elte.hu, akpm@linux-foundation.org, torvalds@linux-foundation.org, davem@davemloft.net, dada1@cosmosbay.com, jeff.chua.linux@gmail.com, paulus@samba.org, laijs@cn.fujitsu.com, jengelh@medozas.de, r000n@r000n.net, benh@kernel.crashing.org, mathieu.desnoyers@polymtl.ca Subject: Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods Message-ID: <20090517220835.GF6973@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090517191141.GA25915@linux.vnet.ibm.com> <20090517200223.GA31029@ioremap.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090517200223.GA31029@ioremap.net> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 18, 2009 at 12:02:23AM +0400, Evgeniy Polyakov wrote: > Hi. Hello, Evgeniy! > On Sun, May 17, 2009 at 12:11:41PM -0700, Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > Fifth cut of "big hammer" expedited RCU grace periods. This uses per-CPU > > kthreads that are scheduled in parallel by a call to smp_call_function() > > by yet another kthread. The synchronize_sched(), synchronize_rcu(), > > and synchronize_bh() primitives wake this kthread up and then wait for > > it to force the grace period. > > I'm curious, but doesn't the fact that registered 'barrier' callback is > invoked mean grace period completion? I.e. why to bother with > rescheduling, waiting for thread to complete and so on, when we only > care in the fact that 'barrier' callback is invoked, and thus all > previous ones are completed? > Or it is done just for the simplicity, since all rescheduling machinery > already manages the rcu bits correctly, so you do not want to put it > directly into 'barrier' callback? It is a short-term expedient course of action. Longer term, I will drop rcuclassic in favor of rcutree, and then merge rcupreempt into rcutree. I will then add machinery to rcutree to handle expedited grace periods (somewhat) more naturally. Trying to expedite three very different RCU implementations seems a bit silly, hence the current off-on-the-side approach. But even then I will avoid relying on a "barrier" callback, or, indeed, any sort of callback, because we don't want expedited grace periods to have to wait on invocation of earlier RCU callbacks. There will thus not be a call_rcu_expedited(), at least not unless someone comes up with a -really- compelling reason why. But the exercise of going through several possible implementations was quite useful, as I learned a number of things that will improve the eventual rcutree implementation. Like the fact that expedited grace periods don't want to be waiting on invocation of prior callbacks. ;-) And rcutiny is, as always, as special case. Here is the implementation of synchronize_rcu_expedited() in rcutiny: void synchronize_rcu_expedited(void) { } Or even: #define synchronize_rcu_expedited synchronize_rcu ;-) Thanx, Paul