From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753892Ab1I0MQ4 (ORCPT ); Tue, 27 Sep 2011 08:16:56 -0400 Received: from mail-yx0-f174.google.com ([209.85.213.174]:63130 "EHLO mail-yx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753810Ab1I0MQz (ORCPT ); Tue, 27 Sep 2011 08:16:55 -0400 Date: Tue, 27 Sep 2011 14:16:50 +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: <20110927121648.GK18553@somewhere> References: <20110925002409.GA24220@shutemov.name> <20110925050826.GC2995@linux.vnet.ibm.com> <20110925112637.GA19298@shutemov.name> <20110925130622.GA9205@somewhere.redhat.com> <20110925164804.GD2995@linux.vnet.ibm.com> <20110926010418.GA18553@somewhere> <20110926012611.GJ2995@linux.vnet.ibm.com> <20110926092052.GD18553@somewhere> <20110926225032.GQ2399@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20110926225032.GQ2399@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 Mon, Sep 26, 2011 at 03:50:32PM -0700, Paul E. McKenney wrote: > On Mon, Sep 26, 2011 at 11:20:55AM +0200, Frederic Weisbecker wrote: > > On Sun, Sep 25, 2011 at 06:26:11PM -0700, Paul E. McKenney wrote: > > > On Mon, Sep 26, 2011 at 03:10:33AM +0200, Frederic Weisbecker wrote: > > > > 2011/9/26 Frederic Weisbecker : > > > > > On Sun, Sep 25, 2011 at 09:48:04AM -0700, Paul E. McKenney wrote: > > > > >> This is required for RCU_FAST_NO_HZ, which checks to see whether the > > > > >> current CPU can accelerate the current grace period so as to enter > > > > >> dyntick-idle mode sooner than it would otherwise.  This takes effect > > > > >> in the situation where rcu_needs_cpu() sees that there are callbacks. > > > > >> It then notes a quiescent state (which is illegal in an RCU read-side > > > > >> critical section), calls force_quiescent_state(), and so on.  For this > > > > >> to work, the current CPU must be in an RCU read-side critical section. > > > > > > > > > > You mean it must *not* be in an RCU read-side critical section (ie: in a > > > > > quiescent state)? > > > > > > > > > > That assumption at least fails anytime in idle for the RCU > > > > > sched flavour given that preemption is disabled in the idle loop. > > > > > > > > > >> If this cannot be made to work, another option is to call a new RCU > > > > >> function in the case where rcu_needs_cpu() returned false, but after > > > > >> the RCU read-side critical section has exited. > > > > > > > > > > You mean when rcu_needs_cpu() returns true (when we have callbacks > > > > > enqueued)? > > > > > > > > > >> This new RCU function > > > > >> could then attempt to rearrange RCU so as to allow the CPU to enter > > > > >> dyntick-idle mode more quickly.  It is more important for this to > > > > >> happen when the CPU is going idle than when it is executing a user > > > > >> process. > > > > >> > > > > >> So, is this doable? > > > > > > > > > > At least not when we have RCU sched callbacks enqueued, given preemption > > > > > is disabled. But that sounds plausible in order to accelerate the switch > > > > > to dyntick-idle mode when we only have rcu and/or rcu bh callbacks. > > > > > > > > But the RCU sched case could be dealt with if we embrace every use of > > > > it with rcu_read_lock_sched() and rcu_read_unlock_sched(), or some light > > > > version that just increases a local counter that rcu_needs_cpu() could check. > > > > > > > > It's an easy thing to add: we can ensure preempt is disabled when we call it > > > > and we can force rcu_dereference_sched() to depend on it. > > > > > > Or just check to see if this is the first level of interrupt from the > > > idle task after the scheduler is up. > > > > I believe it's always the case. tick_nohz_stop_sched_tick() is only called > > from the first level of interrupt in irq_exit(). > > OK, good, let me see if I really understand this... > > Case 1: The interrupt interrupted non-dyntick-idle code. In this case, > rcu_needs_cpu() can look at the dyntick-idle state and determine > that it might not be in a quiescent state. I guess by dyntick idle code you mean the fact that the RCU in is extended quiescent state? (Not just the tick is stopped) If so yeah that looks good. > > Case 2: The interrupt interrupted dyntick-idle code. In this case, > the interrupted code had better not be in an RCU read-side > critical section, and rcu_needs_cpu() should be able to > detect this as well. Yeah. We already do the appropriate debug checks from the RCU read side APIs so I guess rcu_needs_cpu() doesn't even need to do its own debugging checks here about extended qs. But indeed it can return right away if we are in extended qs. > > Case 3: The interrupt interrupted the process of transitioning to > or from dyntick-idle mode. This should be prohibited by > the local_irq_save() calls, right? Indeed. > > > There is always some race window, as it's based on preempt offset: between > > the sub_preempt_count and the softirqs begin and between softirqs end and the end > > of the interrupt. But an "idle_cpu() || in_interrupt()" check in rcu_read_lock_sched_held() > > should catch those offenders. > > But all of this stuff looks to me to be called from the context > of the idle task, so that idle_cpu() will always return "true"... I meant "idle_cpu() && !in_interrupt()" that should return false in rcu_read_lock_sched_held().