From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756252Ab2ICJqW (ORCPT ); Mon, 3 Sep 2012 05:46:22 -0400 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:58796 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755535Ab2ICJqV (ORCPT ); Mon, 3 Sep 2012 05:46:21 -0400 X-Originating-IP: 217.70.178.134 X-Originating-IP: 50.43.46.74 Date: Mon, 3 Sep 2012 02:46:13 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, 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, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org Subject: Re: [PATCH tip/core/rcu 22/23] rcu: Reduce synchronize_rcu_expedited() latency Message-ID: <20120903094613.GK5574@leaf> References: <20120830181811.GA29154@linux.vnet.ibm.com> <1346350718-30937-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346350718-30937-22-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1346350718-30937-22-git-send-email-paulmck@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, Aug 30, 2012 at 11:18:37AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > The synchronize_rcu_expedited() function disables interrupts across a > scan of all leaf rcu_node structures, which is not good for real-time > scheduling latency on large systems (hundreds or especially thousands > of CPUs). This commit therefore holds off CPU-hotplug operations using > get_online_cpus(), and removes the prior acquisiion of the ->onofflock > (which required disabling interrupts). > > Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett > kernel/rcutree_plugin.h | 30 ++++++++++++++++++++++-------- > 1 files changed, 22 insertions(+), 8 deletions(-) > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index 7ed45c9..f1e06f6 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -800,33 +800,47 @@ void synchronize_rcu_expedited(void) > smp_mb(); /* Above access cannot bleed into critical section. */ > > /* > + * Block CPU-hotplug operations. This means that any CPU-hotplug > + * operation that finds an rcu_node structure with tasks in the > + * process of being boosted will know that all tasks blocking > + * this expedited grace period will already be in the process of > + * being boosted. This simplifies the process of moving tasks > + * from leaf to root rcu_node structures. > + */ > + get_online_cpus(); > + > + /* > * Acquire lock, falling back to synchronize_rcu() if too many > * lock-acquisition failures. Of course, if someone does the > * expedited grace period for us, just leave. > */ > while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) { > + if (ULONG_CMP_LT(snap, > + ACCESS_ONCE(sync_rcu_preempt_exp_count))) { > + put_online_cpus(); > + goto mb_ret; /* Others did our work for us. */ > + } > if (trycount++ < 10) { > udelay(trycount * num_online_cpus()); > } else { > + put_online_cpus(); > synchronize_rcu(); > return; > } > - if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) > - goto mb_ret; /* Others did our work for us. */ > } > - if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) > + if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) { > + put_online_cpus(); > goto unlock_mb_ret; /* Others did our work for us. */ > + } > > /* force all RCU readers onto ->blkd_tasks lists. */ > synchronize_sched_expedited(); > > - raw_spin_lock_irqsave(&rsp->onofflock, flags); > - > /* Initialize ->expmask for all non-leaf rcu_node structures. */ > rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) { > - raw_spin_lock(&rnp->lock); /* irqs already disabled. */ > + raw_spin_lock_irqsave(&rnp->lock, flags); > rnp->expmask = rnp->qsmaskinit; > - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */ > + raw_spin_unlock_irqrestore(&rnp->lock, flags); > } > > /* Snapshot current state of ->blkd_tasks lists. */ > @@ -835,7 +849,7 @@ void synchronize_rcu_expedited(void) > if (NUM_RCU_NODES > 1) > sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp)); > > - raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + put_online_cpus(); > > /* Wait for snapshotted ->blkd_tasks lists to drain. */ > rnp = rcu_get_root(rsp); > -- > 1.7.8 >