Re: [PATCH 1/1] sched/fair: Fix unfairness caused by missing load decay

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 01/05/2021 16:41, Odin Ugedal wrote:
> Hi,
> 
>> I think what I see on my Juno running the unfairness_missing_load_decay.sh script is
>> in sync which what you discussed here:
> 
> Thanks for taking a look!
> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 794c2cb945f8..7214e6e89820 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10854,6 +10854,8 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
>>                         break;
>>
>>                 update_load_avg(cfs_rq, se, UPDATE_TG);
>> +               if (!cfs_rq_is_decayed(cfs_rq))
>> +                       list_add_leaf_cfs_rq(cfs_rq);
>>         }
>> }
> 
> This might however lead to "loss" at /slice/cg-2/sub and
> slice/cg-1/sub in this particular case tho, since
> propagate_entity_cfs_rq skips one cfs_rq
> by taking the parent of the provided se. The loss in that case would
> however not be equally big, but will still often contribute to some
> unfairness.

Yeah, that's true.

By moving stopped `stress` tasks into

 /sys/fs/cgroup/cpu/slice/cg-{1,2}/sub

and then into

 /sys/fs/cgroup/cpuset/A

which has a cpuset.cpus {0-1,4-5} not containing the cpus the `stress`
tasks attached {2,3} to and then restart the `stress` tasks again I get:

cfs_rq[1]:/slice/cg-1/sub
  .load_avg                      : 1024
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 1024  <---
  .tg_load_avg                   : 2047  <---
  .se->avg.load_avg              : 511
cfs_rq[1]:/slice/cg-1
  .load_avg                      : 512
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 512  <---
  .tg_load_avg                   : 1022 <---
  .se->avg.load_avg              : 512
cfs_rq[1]:/slice
  .load_avg                      : 513
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 513
  .tg_load_avg                   : 1024
  .se->avg.load_avg              : 512
cfs_rq[5]:/slice/cg-1/sub
  .load_avg                      : 1024
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 1023 <---
  .tg_load_avg                   : 2047 <---
  .se->avg.load_avg              : 511
cfs_rq[5]:/slice/cg-1
  .load_avg                      : 512
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 510  <---
  .tg_load_avg                   : 1022 <---
  .se->avg.load_avg              : 511
cfs_rq[5]:/slice
  .load_avg                      : 512
  .removed.load_avg              : 0
  .tg_load_avg_contrib           : 511
  .tg_load_avg                   : 1024
  .se->avg.load_avg              : 510

I saw that your v2 patch takes care of that.