Re: [PATCH v3] sched: fix first task of a task group is attached twice

[Vincent Guittot <vincent.guittot@linaro.org>]

On 1 June 2016 at 17:31, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
> On 30/05/16 16:52, Vincent Guittot wrote:
>> The cfs_rq->avg.last_update_time is initialize to 0 with the main effect
>> that the 1st sched_entity that will be attached, will keep its
>
> attached in .task_move_group ?
>
> I'm not sure if we can have a .switched_to() directly followed by a
> .enqueue_task() into a cfs_rq with avg.last_update_time = 0.

I think it can happen as well if the task is not queued during the
change of scheduling class because when we attach the task in
switched_to, the task->se.avg.last_update_time will stay to 0. So when
the task will be enqueued, it will be attached one more time

>
>> last_update_time set to 0 and will attached once again during the
>> enqueue.
>> Initialize cfs_rq->avg.last_update_time to 1 instead.
>
> Maybe worth mentioning in the header:
>
> This double se attaching for the first task which moves into the task
> group owning this cfs_rq (.task_move_group() and .enqueue_task()) can
> (obviously) only happen if CONFIG_FAIR_GROUP_SCHED is set.
>
> The reason for this is that we set 'se->avg.last_update_time =
> cfs_rq->avg.last_update_time' in attach_entity_load_avg() and use
> 'migrated = !sa->last_update_time' as a flag in
> enqueue_entity_load_avg() to decide if we call attach_entity_load_avg()
> (again) or only update se->avg .
>
> Tested-by: Dietmar Eggemann <Dietmar.Eggemann@arm.com>

Thanks

>
>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>> ---
>>
>> v3:
>> - add initialization of load_last_update_time_copy for not 64bits system
>> - move init into init_cfs_rq
>>
>> v2:
>> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held
>>
>>  kernel/sched/fair.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 218f8e8..86be9c1 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8459,6 +8459,16 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>>       cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
>>  #endif
>>  #ifdef CONFIG_SMP
>> +     /*
>> +      * Set last_update_time to something different from 0 to make
>> +      * sure the 1st sched_entity will not be attached twice: once
>> +      * when attaching the task to the group and one more time when
>> +      * enqueueing the task.
>> +      */
>> +     cfs_rq->avg.last_update_time = 1;
>> +#ifndef CONFIG_64BIT
>> +     cfs_rq->load_last_update_time_copy = 1;
>> +#endif
>>       atomic_long_set(&cfs_rq->removed_load_avg, 0);
>>       atomic_long_set(&cfs_rq->removed_util_avg, 0);
>>  #endif