From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756759Ab1H3U7Z (ORCPT ); Tue, 30 Aug 2011 16:59:25 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58536 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756690Ab1H3U7V convert rfc822-to-8bit (ORCPT ); Tue, 30 Aug 2011 16:59:21 -0400 Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs From: Peter Zijlstra To: Frederic Weisbecker Cc: LKML , Andrew Morton , Anton Blanchard , Avi Kivity , Ingo Molnar , Lai Jiangshan , "Paul E . McKenney" , Stephen Hemminger , Thomas Gleixner , Tim Pepper , Paul Menage Date: Tue, 30 Aug 2011 22:58:38 +0200 References: <1313423549-27093-6-git-send-email-fweisbec@gmail.com> <1314627922.2816.65.camel@twins> <20110829171155.GD9748@somewhere.redhat.com> <1314640155.2816.117.camel@twins> <20110829175954.GF9748@somewhere.redhat.com> <1314641160.2816.128.camel@twins> <20110829233521.GK9748@somewhere.redhat.com> <1314703315.2799.5.camel@twins> <20110830143207.GP9748@somewhere.redhat.com> <1314717993.5812.11.camel@twins> <20110830153343.GW9748@somewhere.redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1314737918.19586.8.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2011-08-30 at 17:42 +0200, Peter Zijlstra wrote: > On Tue, 2011-08-30 at 17:33 +0200, Frederic Weisbecker wrote: > > > See all that is still kernelspace ;-) I think I know what you mean to > > > say though, but seeing as you note there is even now a known shortcoming > > > I'm not very confident its a solid construction. What will help us find > > > such holes? > > > > This: https://lkml.org/lkml/2011/6/23/744 > > > > It's in one of Paul's branches and should make it for the next merge window. > > This should detect any of such holes. I made that on purpose for the nohz cpusets > > when I saw how much error prone that can be with rcu :) > > OK, good ;-) > > > > I would much rather we not rely on such fragile things too much.. this > > > RCU stuff wants way more thought, as it stands your patch-set doesn't do > > > anything useful IMO. > > > > Not sure what you mean. Well that Rcu thing for sure is fragile but we have > > the tools ready to find the problems. > > Right that thing you linked above does catch abuse, still your current > proposal means that due to RCU it will basically never disable the tick. So how about something like: Assuming we are in rcu_nohz state; on kernel enter we leave rcu_nohz but don't start the tick, instead we assign another cpu to run our state machine. On kernel exit we 'donate' all our rcu state to a willing victim (the same that earlier was kind enough to drive our state) and undo our entire GP accounting and re-enter rcu_nohz state. If between that time we did restart the tick, we take back our rcu state and skip the donate and rcu_nohz enter on kernel exit. I really should go read all those docs Paul send me to see how insane the above is.