Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758524Ab2HVTBc (ORCPT ); Wed, 22 Aug 2012 15:01:32 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:43797 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755342Ab2HVTBa (ORCPT ); Wed, 22 Aug 2012 15:01:30 -0400 Date: Wed, 22 Aug 2012 12:01:09 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: LKML , Richard Henderson , Ivan Kokshaysky , Matt Turner , alpha , "3.2.x.." Subject: Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop Message-ID: <20120822190109.GH2447@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1345652628-15060-1-git-send-email-fweisbec@gmail.com> <1345652628-15060-2-git-send-email-fweisbec@gmail.com> <20120822171930.GE2447@linux.vnet.ibm.com> <20120822173542.GD13674@somewhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120822173542.GD13674@somewhere> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12082219-2356-0000-0000-000001811590 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3480 Lines: 99 On Wed, Aug 22, 2012 at 07:35:45PM +0200, Frederic Weisbecker wrote: > On Wed, Aug 22, 2012 at 10:19:30AM -0700, Paul E. McKenney wrote: > > On Wed, Aug 22, 2012 at 06:23:39PM +0200, Frederic Weisbecker wrote: > > > In the old times, the whole idle task was considered > > > as an RCU quiescent state. But as RCU became more and > > > more successful overtime, some RCU read side critical > > > section have been added even in the code of some > > > architectures idle tasks, for tracing for example. > > > > > > So nowadays, rcu_idle_enter() and rcu_idle_exit() must > > > be called by the architecture to tell RCU about the part > > > in the idle loop that doesn't make use of rcu read side > > > critical sections, typically the part that puts the CPU > > > in low power mode. > > > > > > This is necessary for RCU to find the quiescent states in > > > idle in order to complete grace periods. > > > > > > Add this missing pair of calls in the Alpha's idle loop. > > > > > > Reported-by: Paul E. McKenney > > > Signed-off-by: Frederic Weisbecker > > > Cc: Richard Henderson > > > Cc: Ivan Kokshaysky > > > Cc: Matt Turner > > > Cc: alpha > > > Cc: Paul E. McKenney > > > Cc: 3.2.x.. > > > --- > > > arch/alpha/kernel/process.c | 6 +++++- > > > 1 files changed, 5 insertions(+), 1 deletions(-) > > > > > > diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c > > > index 153d3fc..2ebf7b5 100644 > > > --- a/arch/alpha/kernel/process.c > > > +++ b/arch/alpha/kernel/process.c > > > @@ -28,6 +28,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > > > > #include > > > #include > > > @@ -50,13 +51,16 @@ cpu_idle(void) > > > { > > > set_thread_flag(TIF_POLLING_NRFLAG); > > > > > > + preempt_disable(); > > > > I don't understand the above preempt_disable() not having a matching > > preempt_enable() at exit, but the rest of the patches in this series > > look good to me. > > The current code is preemptable, at least it appears so because it calls > schedule() directly. And if I call rcu_idle_enter() in a preemptable section, > I'm in trouble because I'll schedule while in extended QS. > > Thus I need to disable preemption here at least until I call rcu_idle_exit(). > > Now this is an endless loop so there is no need to re-enable > preemption after the loop. And schedule_preempt_disabled() > takes care of enabling preemption before schedule() and redisabling > it afterward. > > > > > > Thanx, Paul > > > > > while (1) { > > > /* FIXME -- EV6 and LCA45 know how to power down > > > the CPU. */ > > > > > > + rcu_idle_enter(); > > > while (!need_resched()) > > > cpu_relax(); > > > - schedule(); > > > + rcu_idle_exit(); > > > + schedule_preempt_disabled(); > > > } Understood, but what I don't understand is why you don't need a preempt_enable() right here. Thanx, Paul > > > } > > > > > > -- > > > 1.7.5.4 > > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/