Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

[Joonwoo Park]

On 10/18/2016 04:56 AM, 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 <peterz@xxxxxxxxxxxxx> 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
>
> > current state of things, but after your propagate patch, that
> > reinstates the interactivity hack that should work for sure.
>
> The patch below fixes the issue on my platform:
>
> Dietmar, Omer can you confirm that this fix the problem of your platform too ?

I just noticed this thread after posting 
https://lkml.org/lkml/2016/10/18/719...
Noticed this bug while a ago and had the patch above at least a week but 
unfortunately didn't have time to post...
I think Omer had same problem I was trying to fix and I believe patch I 
post should address it.

Vincent, your version fixes my test case as well.
This is sched_stat from the same test case I had in my changelog.
Note dd-2030 which is in root cgroup had same runtime as dd-2033 which 
is in child cgroup.

dd (2030, #threads: 1)
-------------------------------------------------------------------
se.exec_start                                :        275700.024137
se.vruntime                                  :         10589.114654
se.sum_exec_runtime                          :          1576.837993
se.nr_migrations                             :                    0
nr_switches                                  :                  159
nr_voluntary_switches                        :                    0
nr_involuntary_switches                      :                  159
se.load.weight                               :              1048576
se.avg.load_sum                              :             48840575
se.avg.util_sum                              :             19741820
se.avg.load_avg                              :                 1022
se.avg.util_avg                              :                  413
se.avg.last_update_time                      :         275700024137
policy                                       :                    0
prio                                         :                  120
clock-delta                                  :                   34
dd (2033, #threads: 1)
-------------------------------------------------------------------
se.exec_start                                :        275710.037178
se.vruntime                                  :          2383.802868
se.sum_exec_runtime                          :          1576.547591
se.nr_migrations                             :                    0
nr_switches                                  :                  162
nr_voluntary_switches                        :                    0
nr_involuntary_switches                      :                  162
se.load.weight                               :              1048576
se.avg.load_sum                              :             48316646
se.avg.util_sum                              :             21235249
se.avg.load_avg                              :                 1011
se.avg.util_avg                              :                  444
se.avg.last_update_time                      :         275710037178
policy                                       :                    0
prio                                         :                  120
clock-delta                                  :                   36

Thanks,
Joonwoo

> ---
>  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