From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753430Ab1IZBl0 (ORCPT ); Sun, 25 Sep 2011 21:41:26 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:39636 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753142Ab1IZBlY (ORCPT ); Sun, 25 Sep 2011 21:41:24 -0400 Date: Sun, 25 Sep 2011 18:41:18 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker 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: <20110926014118.GA25861@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20110926012611.GJ2995@linux.vnet.ibm.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Hmmm... Is it the case that rcu_needs_cpu() gets called from within an RCU read-side critical section only when called from an interrupt that interrupted an RCU read-side critical section (keeping in mind that the idle loop is a quiescent state regardless of preemption)? If so, I should be able to do the appropriate checks within rcu_needs_cpu(). The reason I didn't think of this earlier was that I thought that rcu_needs_cpu() could be invoked from the idle notifier, which is itself in an RCU read-side critical section. Thanx, Paul