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

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Message-ID: <CAKfTPtDwPmjCV41TcBVURfHvM-7Qpx0n57i9h3Xzm90UHLbM1Q@mail.gmail.com>
Date:	Thu, 16 Jun 2016 09:12:58 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v3] sched: fix first task of a task group is attached twice

On 15 June 2016 at 21:19, Yuyang Du <yuyang.du@...el.com> wrote:
> On Mon, May 30, 2016 at 05:52:20PM +0200, 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>
>> ---
>>
>> 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
>
> Then, when enqueued, both cfs_rq and task will be decayed to 0, due to
> a large gap between 1 and now, no?

yes, like it is done currently (but 1ns later) .