From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756246Ab1I3NLQ (ORCPT ); Fri, 30 Sep 2011 09:11:16 -0400 Received: from mail-bw0-f46.google.com ([209.85.214.46]:42756 "EHLO mail-bw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752062Ab1I3NLO (ORCPT ); Fri, 30 Sep 2011 09:11:14 -0400 Date: Fri, 30 Sep 2011 15:11:09 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, Dipankar Sarma , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Lai Jiangshan Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833 Message-ID: <20110930131105.GC19053@somewhere> References: <20110926092052.GD18553@somewhere> <20110926225032.GQ2399@linux.vnet.ibm.com> <20110927121648.GK18553@somewhere> <20110927180142.GD2335@linux.vnet.ibm.com> <20110928123116.GP18553@somewhere> <20110928184025.GF2383@linux.vnet.ibm.com> <20110928234633.GA3537@somewhere> <20110929005545.GT2383@linux.vnet.ibm.com> <20110929123040.GB3537@somewhere> <20110929171205.GA2362@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110929171205.GA2362@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 Thu, Sep 29, 2011 at 10:12:05AM -0700, Paul E. McKenney wrote: > On Thu, Sep 29, 2011 at 02:30:44PM +0200, Frederic Weisbecker wrote: > > I was thinking about the fact that idle is a caller of rcu_enter_nohz(). > > And there may be more callers of it in the future. So I thought it may > > be better to keep rcu_enter_nohz() idle-agnostic. > > > > But it's fine, there are other ways to call rcu_idle_enter()/rcu_idle_exit() > > from the right places other than from rcu_enter/exit_nohz(). > > We have tick_check_idle() on irq entry and tick_nohz_irq_exit(), both are called > > on the first interrupt level in idle. > > > > So I can change that easily for the nohz cpusets. > > Heh! From what I can see, we were both wrong! > > My thought at this point is to make it so that rcu_enter_nohz() and > rcu_exit_nohz() are renamed to rcu_enter_idle() and rcu_exit_idle() > respectively. I drop the per-CPU variable and the added functions > from one of my patches. These functions, along with rcu_irq_enter(), > rcu_irq_exit(), rcu_nmi_enter(), and rcu_nmi_exit(), are moved out from > under CONFIG_NO_HZ. This allows these functions to track idle state > regardless of the setting of CONFIG_NO_HZ. It also separates the state > of the scheduling-clock tick from RCU's view of CPU idleness, which > simplifies things. > > I will put something together along these lines. Should I wait for your updated patch before rebasing? > > > > > > The problem I have with this is that it is rcu_enter_nohz() that tracks > > > > > the irq nesting required to correctly decide whether or not we are going > > > > > to really go to idle state. Furthermore, there are cases where we > > > > > do enter idle but do not enter nohz, and that has to be handled correctly > > > > > as well. > > > > > > > > > > Now, it is quite possible that I am suffering a senior moment and just > > > > > failing to see how to structure this in the design where rcu_idle_enter() > > > > > invokes rcu_enter_nohz(), but regardless, I am failing to see how to > > > > > structure this so that it works correctly. > > > > > > > > > > Please feel free to enlighten me! > > > > > > > > Ah I realize that you want to call rcu_idle_exit() when we enter > > > > the first level interrupt and rcu_idle_enter() when we exit it > > > > to return to idle loop. > > > > > > > > But we use that check: > > > > > > > > if (user || > > > > (rcu_is_cpu_idle() && > > > > !in_softirq() && > > > > hardirq_count() <= (1 << HARDIRQ_SHIFT))) > > > > rcu_sched_qs(cpu); > > > > > > > > So we ensure that by the time we call rcu_check_callbacks(), we are not nesting > > > > in another interrupt. > > > > > > But I would like to enable checks for entering/exiting idle while > > > within an RCU read-side critical section. The idea is to move > > > the checks from their currently somewhat problematic location in > > > rcu_needs_cpu_quick_check() to somewhere more sensible. My current > > > thought is to move them rcu_enter_nohz() and rcu_exit_nohz() near the > > > calls to rcu_idle_enter() and rcu_idle_exit(), respectively. > > > > So, checking if we are calling rcu_idle_enter() while in an RCU > > read side critical section? > > > > But we already have checks that RCU read side API are not called in > > extended quiescent state. > > Both checks are good. The existing checks catch this kind of error: > > 1. CPU 0 goes idle, entering an RCU extended quiescent state. > 2. CPU 0 illegally enters an RCU read-side critical section. > > The new check catches this kind of error: > > 1. CPU 0 enters an RCU read-side critical section. > 2. CPU 0 goes idle, entering an RCU extended quiescent state, > but illegally so because it is still in an RCU read-side > critical section. Right. > > > > This would mean that they operated only in NO_HZ kernels with lockdep > > > enabled, but I am good with that because to do otherwise would require > > > adding nesting-level counters to the non-NO_HZ case, which I would like > > > to avoid, expecially for TINY_RCU. > > And my reworking of RCU's NO_HZ code to instead be idle code removes > the NO_HZ-only restriction. Getting rid of the additional per-CPU > variable reduces the TINY_RCU overhead to acceptable levels. > > > There can be a secondary check in rcu_read_lock_held() and friends to > > ensures that rcu_is_idle_cpu(). In the non-NO_HZ case it's useful to > > find similar issues. > > > > In fact we could remove the check for rcu_extended_qs() in read side > > APIs and check instead rcu_is_idle_cpu(). That would work in any > > config and not only NO_HZ. > > > > But I hope we can actually keep the check for RCU extended quiescent > > state so that when rcu_enter_nohz() is called from other places than > > idle, we are ready for it. > > > > I believe it's fine to have both checks in PROVE_RCU. > > Agreed, I have not yet revisited rcu_extended_qs(), but some change > might be useful. Yep. > > > OK, my current plans are to start forward-porting to -rc8, and I would > > > like to have this pair of delta patches or something like them pulled > > > into your stack. > > > > Sure I can take your patches (I'm going to merge the delta into the first). > > But if you want a rebase against -rc8, it's going to be easier if you > > do that rebase on the branch you want me to work on. Then I work on top > > of it. > > > > For example we can take your rcu/dynticks, rewind to > > "rcu: Make synchronize_sched_expedited() better at work sharing" > > 771c326f20029a9f30b9a58237c9a5d5ddc1763d, rebase on top of -rc8 > > and I rebase my patches (yours included) on top of it and I repost. > > > > Right? > > Yep! Your earlier three patches look to need some extended-quiescent-state > rework as well: > > b5566f3d: Detect illegal rcu dereference in extended quiescent state > ee05e5a4: Inform the user about dynticks-idle mode on PROVE_RCU warning > fa5d22cf: Warn when rcu_read_lock() is used in extended quiescent state > > So I will leave these out and let you rebase them. Fine. Just need to know if they need an update against a patch from you that is to come or something.