Subject: [PATCH v4 01/12] sched: fix imbalance flag reset

From: [email-masked] (Wanpeng Li)
Date: Tue, 25 Nov 2014 18:13:42 +0800

Hi Vincent,
On 11/25/14, 5:04 PM, Vincent Guittot wrote:
> On 25 November 2014 at 00:47, Wanpeng Li <[email-masked]> wrote:
>> Hi Vincent,
>> On 7/29/14, 1:51 AM, Vincent Guittot wrote:
>>> The imbalance flag can stay set whereas there is no imbalance.
>>>
>>> Let assume that we have 3 tasks that run on a dual cores /dual cluster
>>> system.
>>> We will have some idle load balance which are triggered during tick.
>>> Unfortunately, the tick is also used to queue background work so we can
>>> reach
>>> the situation where short work has been queued on a CPU which already runs
>>> a
>>> task. The load balance will detect this imbalance (2 tasks on 1 CPU and an
>>> idle
>>> CPU) and will try to pull the waiting task on the idle CPU. The waiting
>>> task is
>>> a worker thread that is pinned on a CPU so an imbalance due to pinned task
>>> is
>>> detected and the imbalance flag is set.
>>> Then, we will not be able to clear the flag because we have at most 1 task
>>> on
>>> each CPU but the imbalance flag will trig to useless active load balance
>>> between the idle CPU and the busy CPU.
>>>
>>> We need to reset of the imbalance flag as soon as we have reached a
>>> balanced
>>> state. If all tasks are pinned, we don't consider that as a balanced state
>>> and
>>> let the imbalance flag set.
>>>
>>> Reviewed-by: Preeti U Murthy <[email-masked]>
>>> Signed-off-by: Vincent Guittot <[email-masked]>
>>> ---
>>>    kernel/sched/fair.c | 23 +++++++++++++++++++----
>>>    1 file changed, 19 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 923fe32..7eb9126 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -6672,10 +6672,8 @@ static int load_balance(int this_cpu, struct rq
>>> *this_rq,
>>>                  if (sd_parent) {
>>>                          int *group_imbalance =
>>> &sd_parent->groups->sgc->imbalance;
>>>    -                     if ((env.flags & LBF_SOME_PINNED) && env.imbalance
>>>> 0) {
>>> +                       if ((env.flags & LBF_SOME_PINNED) && env.imbalance
>>>> 0)
>>>                                  *group_imbalance = 1;
>>> -                       } else if (*group_imbalance)
>>> -                               *group_imbalance = 0;
>>
>> As you mentioned above " We need to reset of the imbalance flag as soon as
>> we have reached a balanced state. " I think the codes before your patch have
>> already do this, where I miss? Great thanks for your patient. ;-)
> The previous code was called only when busiest->nr_running > 1.  The
> background activity will be on the rq only 1 tick per few seconds and
> we will set qroup_imbalance when the background activity is on the rq.
> Then, during the next load balances, the qroup_imbalance is still set
> but we can't clear qroup_imbalance  because we have only 1 task per rq

There is no load balance I think since busiest->nr_running > 1 is not 
true even if the patch is not applied.

Regards,
Wanpeng Li

>
> Regards,
> Vincent
>
>> Regards,
>> Wanpeng Li
>>
>>
>>>                  }
>>>                  /* All tasks on this runqueue were pinned by CPU affinity
>>> */
>>> @@ -6686,7 +6684,7 @@ static int load_balance(int this_cpu, struct rq
>>> *this_rq,
>>>                                  env.loop_break = sched_nr_migrate_break;
>>>                                  goto redo;
>>>                          }
>>> -                       goto out_balanced;
>>> +                       goto out_all_pinned;
>>>                  }
>>>          }
>>>    @@ -6760,6 +6758,23 @@ static int load_balance(int this_cpu, struct rq
>>> *this_rq,
>>>          goto out;
>>>      out_balanced:
>>> +       /*
>>> +        * We reach balance although we may have faced some affinity
>>> +        * constraints. Clear the imbalance flag if it was set.
>>> +        */
>>> +       if (sd_parent) {
>>> +               int *group_imbalance = &sd_parent->groups->sgc->imbalance;
>>> +
>>> +               if (*group_imbalance)
>>> +                       *group_imbalance = 0;
>>> +       }
>>> +
>>> +out_all_pinned:
>>> +       /*
>>> +        * We reach balance because all tasks are pinned at this level so
>>> +        * we can't migrate them. Let the imbalance flag set so parent
>>> level
>>> +        * can try to migrate them.
>>> +        */
>>>          schedstat_inc(sd, lb_balanced[idle]);
>>>          sd->nr_balance_failed = 0;
>>