Re: [patch v3 6/8] sched: consider runnable load average in move_tasks

[Vincent Guittot]

On 9 April 2013 10:05, Alex Shi <alex.shi@xxxxxxxxx> wrote:
> On 04/09/2013 03:08 PM, Vincent Guittot wrote:
>> On 2 April 2013 05:23, Alex Shi <alex.shi@xxxxxxxxx> wrote:
>>> Except using runnable load average in background, move_tasks is also
>>> the key functions in load balance. We need consider the runnable load
>>> average in it in order to the apple to apple load comparison.
>>>
>>> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
>>> ---
>>>  kernel/sched/fair.c | 11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 1f9026e..bf4e0d4 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -3966,6 +3966,15 @@ static unsigned long task_h_load(struct task_struct *p);
>>>
>>>  static const unsigned int sched_nr_migrate_break = 32;
>>>
>>> +static unsigned long task_h_load_avg(struct task_struct *p)
>>> +{
>>> +       u32 period = p->se.avg.runnable_avg_period;
>>> +       if (!period)
>>> +               return 0;
>>> +
>>> +       return task_h_load(p) * p->se.avg.runnable_avg_sum / period;
>>
>> How do you ensure that runnable_avg_period and runnable_avg_sum are
>> coherent ? an update of the statistic can occur in the middle of your
>> sequence.
>
> Thanks for your question, Vincent!
> the runnable_avg_period and runnable_avg_sum, only updated in
> __update_entity_runnable_avg().
> Yes, I didn't see some locks to ensure the coherent of them. but they
> are updated closely, and it is not big deal even a little incorrect to
> their value. These data are collected periodically, don't need very very
> precise at every time.
> Am I right? :)

The problem mainly appears during starting phase (the 1st 345ms) when
runnable_avg_period has not reached the max value yet so you can have
avg.runnable_avg_sum greater than avg.runnable_avg_period. In a worst
case, runnable_avg_sum could be twice runnable_avg_period

Vincent

>
> --
> Thanks Alex
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