From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756384Ab1HSQf6 (ORCPT ); Fri, 19 Aug 2011 12:35:58 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:41458 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756354Ab1HSQfz (ORCPT ); Fri, 19 Aug 2011 12:35:55 -0400 Date: Fri, 19 Aug 2011 09:35:47 -0700 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, patches@linaro.org, stable@kernel.org Subject: [PATCH RFC tip/core/urgent] rcu: Avoid having just-onlined CPU resched itself when RCU is idle Message-ID: <20110819163547.GA5182@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 CPUs set rdp->qs_pending when coming online to resolve races with grace-period start. However, this means that if RCU is idle, the just-onlined CPU might needlessly send itself resched IPIs. Adjust the online-CPU initialization to avoid this, and also to correctly cause the CPU to respond to the current grace period if needed. This patch is believed to fix or otherwise suppress problems in https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the relationship is not apparent to this patch's author. That said, it is hard to argue with test results. This will be available from -rcu as soon as kernel.org's mirrors update: git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent Reported-by: Nicolas Mailhot Reported-by: Michal Jaegermann Reported-by: Josh Boyer Signed-off-by: Paul E. McKenney Tested-by: Josh Boyer rcutree.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index ba06207..39cc360 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) /* Set up local state, ensuring consistent view of global state. */ raw_spin_lock_irqsave(&rnp->lock, flags); - rdp->passed_quiesc = 0; /* We could be racing with new GP, */ - rdp->qs_pending = 1; /* so set up to respond to current GP. */ rdp->beenonline = 1; /* We have now been online. */ rdp->preemptible = preemptible; rdp->qlen_last_fqs_check = 0; @@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) rnp->qsmaskinit |= mask; mask = rnp->grpmask; if (rnp == rdp->mynode) { - rdp->gpnum = rnp->completed; /* if GP in progress... */ + /* + * If there is a grace period in progress, we will + * set up to wait for it next time we run the + * RCU core code. + */ + rdp->gpnum = rnp->completed; rdp->completed = rnp->completed; + rdp->passed_quiesc = 0; + rdp->qs_pending = 0; rdp->passed_quiesc_completed = rnp->completed - 1; } raw_spin_unlock(&rnp->lock); /* irqs already disabled. */