linux-kernel - Re: [PATCH v4 1/6] sched/fair: Fix reschedule which is generated on throttled cfs

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Date:	Fri, 24 Oct 2014 17:32:06 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	tkhai@...dex.ru
CC:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, pjt@...gle.com, oleg@...hat.com,
	rostedt@...dmis.org, umgwanakikbuti@...il.com,
	tim.c.chen@...ux.intel.com, mingo@...nel.org,
	nicolas.pitre@...aro.org
Subject: Re: [PATCH v4 1/6] sched/fair: Fix reschedule which is generated
 on throttled cfs_rq

Hi Kirill,
10/24/14, 2:01 PM, Kirill Tkhai:
> Hi, Wanpeng,
>
> the commit commentary confuses, I'm agree. Really it's just a cleanup.
>
> On Пт, 2014-10-24 at 07:27 +0800, Wanpeng Li wrote:
>> Hi Kirill,
>> 8/6/14, 4:06 PM, Kirill Tkhai:
>>> (sched_entity::on_rq == 1) does not guarantee the task is pickable;
>>> changes on throttled cfs_rq must not lead to reschedule.
>> Why (sched_entity::on_rq == 1) doesn't guarantee the task is pickable
>> since entity will be dequeued during throttling cfs_rq?
> Because one of task's (grand)parents in hierarhy may be throtthed and
> dequeued.
>
> But task_struct::on_rq check doesn't guarantee this too. So, just ignore
> commit commentary; the commentary is wrong.
>
>>> Check for task_struct::on_rq instead.
>> Do you mean task_struct::on_rq will be cleared during throttling cfs_rq?
>> I can't find codes do this.
> No, it not cleared. The commit commentary should be:
> "sched: Cleanup. Check task_struct::on_rq instead of sched_entity::on_rq,
> because it is the same for a task"

IIUR, for fair class, sched_entity::on_rq will be set/clear during 
enqueue/dequeue, task_struct::on_rq will changed during task migration, 
I'm not sure why they are the same.

Regards,
Wanpeng Li

>
>
>> Regards,
>> Wanpeng Li
>>
>>> Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
>>> ---
>>>    kernel/sched/fair.c |    6 +++---
>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index bfa3c86..6f0ce2b 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -7465,7 +7465,7 @@ static void task_fork_fair(struct task_struct *p)
>>>    static void
>>>    prio_changed_fair(struct rq *rq, struct task_struct *p, int oldprio)
>>>    {
>>> -	if (!p->se.on_rq)
>>> +	if (!p->on_rq)
>>>    		return;
>>>    
>>>    	/*
>>> @@ -7521,15 +7521,15 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
>>>     */
>>>    static void switched_to_fair(struct rq *rq, struct task_struct *p)
>>>    {
>>> -	struct sched_entity *se = &p->se;
>>>    #ifdef CONFIG_FAIR_GROUP_SCHED
>>> +	struct sched_entity *se = &p->se;
>>>    	/*
>>>    	 * Since the real-depth could have been changed (only FAIR
>>>    	 * class maintain depth value), reset depth properly.
>>>    	 */
>>>    	se->depth = se->parent ? se->parent->depth + 1 : 0;
>>>    #endif
>>> -	if (!se->on_rq)
>>> +	if (!p->on_rq)
>>>    		return;
>>>    
>>>    	/*
>>>
>>>
>>>
>>> --
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