linux-kernel - Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to

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Message-ID: <BLU437-SMTP40DF7F2F5C048B7C3CBF8780530@phx.gbl>
Date:	Tue, 8 Sep 2015 13:38:08 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Byungchul Park <byungchul.park@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com
Subject: Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair()

On 9/8/15 1:28 PM, Byungchul Park wrote:
> On Tue, Sep 08, 2015 at 11:46:01AM +0800, Wanpeng Li wrote:
>> On 9/7/15 10:02 PM, Peter Zijlstra wrote:
>>> Please always Cc at least the person who wrote the lines you modify.
>>>
>>> On Mon, Sep 07, 2015 at 05:45:20PM +0800, Wanpeng Li wrote:
>>>> The sleeper task will be normalized when moved from fair_sched_class, in
>>>> order that vruntime will be adjusted either the task is running or sleeping
>>>> when moved back. The nomalization in switch_to_fair for sleep task will
>>>> result in lose fair sleeper bonus in place_entity() once the vruntime -
>>>> cfs_rq->min_vruntime is big when moved from fair_sched_class.
>>>>
>>>> This patch fix it by adjusting vruntime just during migrating as original
>>>> codes since the vruntime of the task has usually NOT been normalized in
>>>> this case.
>>> Sorry, I cannot follow that at all. Maybe its me being sleep deprived,
>>> but could you try that again?
>> When changing away from the fair class while sleeping, relative
>> vruntime is calculated to handle the case sleep when moved from
>> fair_sched_class and running when moved to fair_sched_class. The
> i don't think relative vruntime is calculated to handle the special case
> you mentioned. i think the calculation is necessary for all cases detaching

Please refer why the relative vruntime caculation is introduced to 
switched_from_fair(): https://lkml.org/lkml/2011/1/17/129

> a task from a cfs_rq.
>
>> absolute vruntime will be calculated in enqueue_entity() either the
>> task is running or sleeping when moved back. The fair sleeper bonus
> i think absolute vruntime is calculated in enqueue_entuty() only when the
> task is on rq. therefore in the case that the task is not on rq,
> switched_to_fair() has to calculate the absolute vruntime instread.

Absolute vruntime is caculated in place_entity() which is called by 
enqueue_entity() for DEQUEUE_SLEEP task.

Regards,
Wanpeng Li

>
>> should be gained in place_entity() if the task is still sleeping.
>> However, after recent commit ( 23ec30ddd7c1306: 'sched: add two
>> functions for att(det)aching a task to(from) a cfs_rq'), the
>> absolute vruntime will be calculated in switched_to_fair(), so the
>> max_vruntime() which is called in place_entity() will select the
>> absolute vruntime which is calculated in switched_to_fair() as the
>> se->vruntime and lose the fair sleeper bonus.
> please refer my another reply, and let me know if i missed something.
>
> thanks,
> byungchul
>
>> Regards,
>> Wanpeng Li
>>
>>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>>> ---
>>>>   kernel/sched/fair.c |   11 +++++++----
>>>>   1 files changed, 7 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index d26d3b7..eb9aa35 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -8005,9 +8005,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
>>>>   	/* Synchronize task with its cfs_rq */
>>>>   	attach_entity_load_avg(cfs_rq, se);
>>>> -
>>>> -	if (!vruntime_normalized(p))
>>>> -		se->vruntime += cfs_rq->min_vruntime;
>>>>   }
>>>>   static void switched_from_fair(struct rq *rq, struct task_struct *p)
>>>> @@ -8066,14 +8063,20 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>>>>   #ifdef CONFIG_FAIR_GROUP_SCHED
>>>>   static void task_move_group_fair(struct task_struct *p)
>>>>   {
>>>> +	struct sched_entity *se = &p->se;
>>>> +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>>> +
>>>>   	detach_task_cfs_rq(p);
>>>>   	set_task_rq(p, task_cpu(p));
>>>>   #ifdef CONFIG_SMP
>>>>   	/* Tell se's cfs_rq has been changed -- migrated */
>>>> -	p->se.avg.last_update_time = 0;
>>>> +	se->avg.last_update_time = 0;
>>>>   #endif
>>>>   	attach_task_cfs_rq(p);
>>>> +
>>>> +	if (!vruntime_normalized(p))
>>>> +		se->vruntime += cfs_rq->min_vruntime;
>>>>   }
>>>>   void free_fair_sched_group(struct task_group *tg)
>>>> -- 
>>>> 1.7.1
>>>>
>> --
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