linux-kernel - Re: [PATCH 1/2] sched: normalize sleeper's vruntime during group change

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Message-ID: <AANLkTimF+_okjY7n878f_D=HKbRn=W0=RMQ_WphADL1s@mail.gmail.com>
Date:	Wed, 29 Sep 2010 12:02:33 -0700
From:	Dima Zavin <dima@...roid.com>
To:	Mike Galbraith <efault@....de>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Arve Hjønnevåg <arve@...roid.com>
Subject: Re: [PATCH 1/2] sched: normalize sleeper's vruntime during group change

On Wed, Sep 29, 2010 at 1:13 AM, Mike Galbraith <efault@....de> wrote:
> On Tue, 2010-09-28 at 23:46 -0700, Dima Zavin wrote:
>> This adds a new callback, prep_move_task, to struct sched_class
>> to give sched_fair the opportunity to adjust the task's vruntime
>> just before setting its new group.
>>
>> This allows us to properly normalize a sleeping task's vruntime
>> when moving it between different cgroups.
>
>
>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>> index db3f674..008fe57 100644
>> --- a/kernel/sched_fair.c
>> +++ b/kernel/sched_fair.c
>> @@ -3827,10 +3827,23 @@ 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 += cfs_rq->min_vruntime;
>>               place_entity(cfs_rq, &p->se, 1);
>> +     }
>> +}
>
> I'm still missing the place_entity() rationale.  Why is a sleeping task
> eligible for pain therapy that a runnable task is spared?  That state
> can change a usec from now.

I see your point, and I can't think of a reason why we'd want to
penalize the sleeper with START_DEBIT,
but now I am wondering why it was there in the first place.

--Dima
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