Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757185Ab2HTQKV (ORCPT ); Mon, 20 Aug 2012 12:10:21 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:58906 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755498Ab2HTQKQ (ORCPT ); Mon, 20 Aug 2012 12:10:16 -0400 MIME-Version: 1.0 In-Reply-To: <1345454817.23018.27.camel@twins> 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> Date: Mon, 20 Aug 2012 22:10:15 +0600 Message-ID: Subject: Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down. From: Rakib Mullick To: Peter Zijlstra Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, paulmck 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: 5057 Lines: 134 On 8/20/12, Peter Zijlstra wrote: > On Fri, 2012-08-17 at 19:39 +0600, Rakib Mullick wrote: >> On 8/16/12, Peter Zijlstra wrote: >> > On Thu, 2012-08-16 at 21:32 +0600, Rakib Mullick wrote: >> >> And also I think migrate_nr_uninterruptible() is meaning less too. >> > >> > Hmm, I think I see a problem.. we forget to migrate the effective delta >> > created by rq->calc_load_active. >> > >> And rq->calc_load_active needs to be migrated to the proper dest_rq >> not like currently picking any random rq. > > > OK, so how about something like the below, it would also solve Paul's > issue with that code. > > > Please do double check the logic, I've had all of 4 hours sleep and its > far too warm for a brain to operate in any case. > > --- > Subject: sched: Fix load avg vs cpu-hotplug > > Rabik and Paul reported two different issues related to the same few > lines of code. > First of all, you've misspelled my name, it's Rakib not Rabik. > 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). > Okay, I was thinking about per rq->nr_uninterruptible accounting due to it's use in delta calculation at calc_load_fold_active(). So, I proposed to migrate nr_uninterruptible to the rq, where tasks were migrated as if, they gets folded from update_cpu_load(). Now, note that, delta is calculated using rq->nr_running and rq->nr_uninterruptible, so if we migrate tasks into a rq, but migrate nr_uninterruptible into another rq, its wrong and we're screwing the delta calculation. > 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. > Certainly, we don't care about the rq which is going down. But, the dest_rq. > 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. > > > Reported-by: Rakib Mullick > Reported-by: Paul E. McKenney > Signed-off-by: Peter Zijlstra > --- > kernel/sched/core.c | 31 ++++++++++--------------------- > 1 file changed, 10 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 4376c9f..06d23c6 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5338,27 +5338,17 @@ 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. > + * But after migrate_tasks(), it's likely that rq->nr_running will be 1. Then, nr_active will be screwed. No? > + * 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); > } > > /* > @@ -5652,8 +5642,7 @@ migration_call(struct notifier_block *nfb, unsigned > long action, void *hcpu) > BUG_ON(rq->nr_running != 1); /* the migration thread */ > raw_spin_unlock_irqrestore(&rq->lock, flags); > > - migrate_nr_uninterruptible(rq); > - calc_global_load_remove(rq); > + calc_load_migrate(rq); > break; > #endif > } > 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/