From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757045AbcJTH4z (ORCPT ); Thu, 20 Oct 2016 03:56:55 -0400 Received: from foss.arm.com ([217.140.101.70]:39860 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756957AbcJTH4y (ORCPT ); Thu, 20 Oct 2016 03:56:54 -0400 Date: Thu, 20 Oct 2016 08:56:49 +0100 From: Morten Rasmussen To: Vincent Guittot Cc: Peter Zijlstra , Dietmar Eggemann , Joseph Salisbury , Ingo Molnar , Linus Torvalds , Thomas Gleixner , LKML , Mike Galbraith , omer.akram@canonical.com Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes Message-ID: <20161020075649.GH7509@e105550-lin.cambridge.arm.com> References: <4e15ad55-beeb-e860-0420-8f439d076758@arm.com> <20161017131952.GR3117@twins.programming.kicks-ass.net> <94cc6deb-f93e-60ec-5834-e84a8b98e73c@arm.com> <20161018090747.GW3142@twins.programming.kicks-ass.net> <20161018103412.GT3117@twins.programming.kicks-ass.net> <20161018115651.GA20956@linaro.org> <20161019132957.GA7509@e105550-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 19, 2016 at 07:41:36PM +0200, Vincent Guittot wrote: > On 19 October 2016 at 15:30, Morten Rasmussen wrote: > > On Tue, Oct 18, 2016 at 01:56:51PM +0200, Vincent Guittot wrote: > >> Le Tuesday 18 Oct 2016 à 12:34:12 (+0200), Peter Zijlstra a écrit : > >> > On Tue, Oct 18, 2016 at 11:45:48AM +0200, Vincent Guittot wrote: > >> > > On 18 October 2016 at 11:07, Peter Zijlstra wrote: > >> > > > So aside from funny BIOSes, this should also show up when creating > >> > > > cgroups when you have offlined a few CPUs, which is far more common I'd > >> > > > think. > >> > > > >> > > The problem is also that the load of the tg->se[cpu] that represents > >> > > the tg->cfs_rq[cpu] is initialized to 1024 in: > >> > > alloc_fair_sched_group > >> > > for_each_possible_cpu(i) { > >> > > init_entity_runnable_average(se); > >> > > sa->load_avg = scale_load_down(se->load.weight); > >> > > > >> > > Initializing sa->load_avg to 1024 for a newly created task makes > >> > > sense as we don't know yet what will be its real load but i'm not sure > >> > > that we have to do the same for se that represents a task group. This > >> > > load should be initialized to 0 and it will increase when task will be > >> > > moved/attached into task group > >> > > >> > Yes, I think that makes sense, not sure how horrible that is with the > >> > >> That should not be that bad because this initial value is only useful for > >> the few dozens of ms that follow the creation of the task group > > > > IMHO, it doesn't make much sense to initialize empty containers, which > > group sched_entities really are, to 1024. It is meant to represent what > > is in it, and a creation it is empty, so in my opinion initializing it > > to zero make sense. > > > >> > current state of things, but after your propagate patch, that > >> > reinstates the interactivity hack that should work for sure. > > > > It actually works on mainline/tip as well. > > > > As I see it, the fundamental problem is keeping group entities up to > > date. Because the load_weight and hence se->avg.load_avg each per-cpu > > group sched_entity depends on the group cfs_rq->tg_load_avg_contrib for > > all cpus (tg->load_avg), including those that might be empty and > > therefore not enqueued, we must ensure that they are updated some other > > way. Most naturally as part of update_blocked_averages(). > > > > To guarantee that, it basically boils down to making sure: > > Any cfs_rq with a non-zero tg_load_avg_contrib must be on the > > leaf_cfs_rq_list. > > > > We can do that in different ways: 1) Add all cfs_rqs to the > > leaf_cfs_rq_list at task group creation, or 2) initialize group > > sched_entity contributions to zero and make sure that they are added to > > leaf_cfs_rq_list as soon as a sched_entity (task or group) is enqueued > > on it. > > > > Vincent patch below gives us the second option. > > > >> kernel/sched/fair.c | 9 ++++++++- > >> 1 file changed, 8 insertions(+), 1 deletion(-) > >> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index 8b03fb5..89776ac 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -690,7 +690,14 @@ void init_entity_runnable_average(struct sched_entity *se) > >> * will definitely be update (after enqueue). > >> */ > >> sa->period_contrib = 1023; > >> - sa->load_avg = scale_load_down(se->load.weight); > >> + /* > >> + * Tasks are intialized with full load to be seen as heavy task until > >> + * they get a chance to stabilize to their real load level. > >> + * group entity are intialized with null load to reflect the fact that > >> + * nothing has been attached yet to the task group. > >> + */ > >> + if (entity_is_task(se)) > >> + sa->load_avg = scale_load_down(se->load.weight); > >> sa->load_sum = sa->load_avg * LOAD_AVG_MAX; > >> /* > >> * At this point, util_avg won't be used in select_task_rq_fair anyway > > > > I would suggest adding a comment somewhere stating that we need to keep > > group cfs_rqs up to date: > > > > ----- > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index abb3763dff69..2b820d489be0 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6641,6 +6641,11 @@ static void update_blocked_averages(int cpu) > > if (throttled_hierarchy(cfs_rq)) > > continue; > > > > + /* > > + * Note that _any_ leaf cfs_rq with a non-zero tg_load_avg_contrib > > + * _must_ be on the leaf_cfs_rq_list to ensure that group shares > > + * are updated correctly. > > + */ > > As discussed on IRC, the point is that even if the leaf cfs_rq is > added to the leaf_cfs_rq_list, it doesn't ensure that it will be > updated correctly for unplugged CPUs Agreed. We have to ensure that tg_load_avg_contrib is zeroed for leaf cfs_rqs belonging to unplugged cpus. And if modify the above to say leaf_cfs_rq_list of an online cpu, then we should be covered I think.