Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752227Ab2H1G5P (ORCPT ); Tue, 28 Aug 2012 02:57:15 -0400 Received: from mail-pz0-f46.google.com ([209.85.210.46]:39853 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751790Ab2H1G5K (ORCPT ); Tue, 28 Aug 2012 02:57:10 -0400 MIME-Version: 1.0 In-Reply-To: <20120827184435.GA13883@linux.vnet.ibm.com> References: <1345124749.31092.2.camel@localhost.localdomain> <1345125384.29668.30.camel@twins> <1345128138.29668.42.camel@twins> <1345139199.29668.46.camel@twins> <1345454817.23018.27.camel@twins> <20120820162657.GI2435@linux.vnet.ibm.com> <20120827184435.GA13883@linux.vnet.ibm.com> Date: Tue, 28 Aug 2012 12:57:09 +0600 Message-ID: Subject: Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down. From: Rakib Mullick To: paulmck@linux.vnet.ibm.com Cc: Peter Zijlstra , mingo@kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4379 Lines: 122 Hello Paul, On 8/28/12, Paul E. McKenney wrote: > On Mon, Aug 20, 2012 at 09:26:57AM -0700, Paul E. McKenney wrote: >> On Mon, Aug 20, 2012 at 11:26:57AM +0200, Peter Zijlstra wrote: > > How about the following updated patch? > Actually, I was waiting for Peter's update. > Thanx, Paul > > ------------------------------------------------------------------------ > > sched: Fix load avg vs cpu-hotplug > > Rabik and Paul reported two different issues related to the same few > lines of code. > > Rabik's issue is that the nr_uninterruptible migration code is wrong in > that he sees artifacts due to this (Rabik please do expand in more > detail). > > Paul's issue is that this code as it stands relies on us using > stop_machine() for unplug, we all would like to remove this assumption > so that eventually we can remove this stop_machine() usage altogether. > > The only reason we'd have to migrate nr_uninterruptible is so that we > could use for_each_online_cpu() loops in favour of > for_each_possible_cpu() loops, however since nr_uninterruptible() is the > only such loop and its using possible lets not bother at all. > > The problem Rabik sees is (probably) caused by the fact that by > migrating nr_uninterruptible we screw rq->calc_load_active for both rqs > involved. > > So don't bother with fancy migration schemes (meaning we now have to > keep using for_each_possible_cpu()) and instead fold any nr_active delta > after we migrate all tasks away to make sure we don't have any skewed > nr_active accounting. > > [ paulmck: Move call to calc_load_migration to CPU_DEAD to avoid > miscounting noted by Rakib. ] > > Reported-by: Rakib Mullick > Reported-by: Paul E. McKenney > Signed-off-by: Peter Zijlstra > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index e841dfc..a8807f2 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5309,27 +5309,17 @@ void idle_task_exit(void) > } > > /* > - * While a dead CPU has no uninterruptible tasks queued at this point, > - * it might still have a nonzero ->nr_uninterruptible counter, because > - * for performance reasons the counter is not stricly tracking tasks to > - * their home CPUs. So we just add the counter to another CPU's counter, > - * to keep the global sum constant after CPU-down: > - */ > -static void migrate_nr_uninterruptible(struct rq *rq_src) > -{ > - struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask)); > - > - rq_dest->nr_uninterruptible += rq_src->nr_uninterruptible; > - rq_src->nr_uninterruptible = 0; > -} > - > -/* > - * remove the tasks which were accounted by rq from calc_load_tasks. > + * Since this CPU is going 'away' for a while, fold any nr_active delta > + * we might have. Assumes we're called after migrate_tasks() so that the > + * nr_active count is stable. > + * > + * Also see the comment "Global load-average calculations". > */ > -static void calc_global_load_remove(struct rq *rq) > +static void calc_load_migrate(struct rq *rq) > { > - atomic_long_sub(rq->calc_load_active, &calc_load_tasks); > - rq->calc_load_active = 0; > + long delta = calc_load_fold_active(rq); > + if (delta) > + atomic_long_add(delta, &calc_load_tasks); > } > > /* > @@ -5622,9 +5612,18 @@ migration_call(struct notifier_block *nfb, unsigned > long action, void *hcpu) > migrate_tasks(cpu); > BUG_ON(rq->nr_running != 1); /* the migration thread */ > raw_spin_unlock_irqrestore(&rq->lock, flags); > + break; > > - migrate_nr_uninterruptible(rq); > - calc_global_load_remove(rq); > + case CPU_DEAD: > + { > + struct rq *dest_rq; > + > + local_irq_save(flags); > + dest_rq = cpu_rq(smp_processor_id()); Use of smp_processor_id() as dest cpu isn't clear to me, this processor is about to get down, isn't it? > + raw_spin_lock(&dest_rq->lock); > + calc_load_migrate(rq); Well, calc_load_migrate() has no impact cause rq->nr_running == 1 at this point. It's been already pointed out previously. Thanks, Rakib -- 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/