linux-kernel - Re: [PATCH v2] sched: fix first task of a task group is attached twice

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Message-ID: <5748B063.4070009@arm.com>
Date:	Fri, 27 May 2016 21:38:59 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Yuyang Du <yuyang.du@...el.com>
Subject: Re: [PATCH v2] sched: fix first task of a task group is attached
 twice

On 27/05/16 18:16, Vincent Guittot wrote:
> On 27 May 2016 at 17:48, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 25/05/16 16:01, 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
>>> last_update_time set to 0 and will attached once again during the
>>> enqueue.
>>> Initialize cfs_rq->avg.last_update_time to 1 instead.
>>>
>>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>>> ---
>>>
>>> v2:
>>> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held
>>>
>>>  kernel/sched/fair.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 218f8e8..3724656 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -8586,6 +8586,14 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
>>>               se->depth = parent->depth + 1;
>>>       }
>>>
>>> +     /*
>>> +      * 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.
>>> +      */
>>> +     tg->cfs_rq[cpu]->avg.last_update_time = 1;
>>> +

Couldn't you not just set the value in init_cfs_rq():

@@ -8482,6 +8482,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
        cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
 #endif
 #ifdef CONFIG_SMP
+       cfs_rq->avg.last_update_time = 1;
        atomic_long_set(&cfs_rq->removed_load_avg, 0);
        atomic_long_set(&cfs_rq->removed_util_avg, 0);
 #endif

>>>       se->my_q = cfs_rq;
>>>       /* guarantee group entities always have weight */
>>>       update_load_set(&se->load, NICE_0_LOAD);
>>
>> So why not setting the last_update_time value for those cfs_rq's when
>> we have the lock? E.g. in task_move_group_fair() or attach_task_cfs_rq().
> 
> I'm not sure that it's worth adding this init in functions that are
> then used often only for the init of it.

Yeah, there will be this if condition overhead.

> If you are concerned by the update of the load of the 1st task that
> will be attached, it can still have elapsed  a long time between the
> creation of the group and the 1st enqueue of a task. This was the case
> for the test i did when i found this issue.

Understood, but for me, creation of the task group is
cpu_cgroup_css_alloc ->  sched_create_group() -> ... -> init_cfs_rq(),
init_tg_cfs_entry(), ...

and the functions which are called when the first task is put into the
task group are cpu_cgroup_attach() and cpu_cgroup_fork() and they whould
trigger the initial setup of the cfs_rq->avg.last_update_time.

> 
> Beside this point, I have to send a new version to set
> load_last_update_time_copy for not 64 bits system. Fengguang points me
> the issue

OK.

[...]
>>
>> +       if (!cfs_rq->avg.last_update_time)
>> +               cfs_rq->avg.last_update_time = rq_clock_task(rq_of(cfs_rq));
>> +
>>         /* Synchronize task with its cfs_rq */
>>         attach_entity_load_avg(cfs_rq, se);
>