linux-kernel - Re: broken behavior in cfs when moving threads between cgroups

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Date:	Tue, 28 Sep 2010 16:23:21 -0700
From:	Dima Zavin <dmitriyz@...gle.com>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Arve Hjønnevåg <arve@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: broken behavior in cfs when moving threads between cgroups

Actually, that won't work either as it can allow a thread to gain
vruntime by being moved out and then immediately back into a group.

--Dima

2010/9/28 Dima Zavin <dmitriyz@...gle.com>:
> Mike,
>
> Thanks for the reply.
>
>> I don't really see the relevance of an entity's lag in it's previous
>> group, so would tend toward entry at parity.
>>
>> Maybe the move _should_ be treated as a fork, for the same reason we do
>> START_DEBIT, but if so, seems to me it's irrelevant whether the task is
>> currently sleeping or not, it's going to run for the first time in it's
>> new home just as any runnable task, and may do so very soon.  OTOH,
>> frequent moves with that hefty START_DEBIT price tag could hurt very
>> badly, and makes caring about previous lag seem pointless.
>>
>>        -Mike
>>
>> (untested)
>>
>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>> index 9b5b4f8..4dc6b2f 100644
>> --- a/kernel/sched_fair.c
>> +++ b/kernel/sched_fair.c
>> @@ -3827,10 +3827,14 @@ static void set_curr_task_fair(struct rq *rq)
>>  static void moved_group_fair(struct task_struct *p, int on_rq)
>>  {
>>        struct cfs_rq *cfs_rq = task_cfs_rq(p);
>> +       struct sched_entity *se = &p->se;
>> +       u64 vruntime = 0;
>>
>>        update_curr(cfs_rq);
>>        if (!on_rq)
>> -               place_entity(cfs_rq, &p->se, 1);
>> +               vruntime = cfs_rq->min_vruntime;
>> +
>> +       se->vruntime = vruntime;
>>  }
>>  #endif
>
> Hmm. Would we really want to give the sleeping task such a bump in the
> new group? Why not just start it off at min_vruntime and let
> place_entity do it's thing? Something like...
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index a7be83c..4656231 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -3622,10 +3622,15 @@ static void set_curr_task_fair(struct rq *rq)
>  static void moved_group_fair(struct task_struct *p, int on_rq)
>  {
>        struct cfs_rq *cfs_rq = task_cfs_rq(p);
> +       struct sched_entity *se = &p->se;
>
>        update_curr(cfs_rq);
> -       if (!on_rq)
> +       if (on_rq) {
> +               se->vruntime = 0;
> +       } else {
> +               se->vruntime = cfs_rq->min_vruntime;
>                place_entity(cfs_rq, &p->se, 1);
> +       }
>  }
>  #endif
>
--
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