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

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Date:	Tue, 8 Sep 2015 14:23:56 +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 2:14 PM, Byungchul Park wrote:
> On Tue, Sep 08, 2015 at 01:38:08PM +0800, Wanpeng Li wrote:
>> 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
> hello,
>
> it is just a bug caused by not calculating a relative vruntime when
> detached a task from cfs_rq, which is necessary though.

Refer to Peterz's comments:

| There is also a case where it was moved from fair_sched_class when it
| was in a blocked state and moved back while it is running.

>
>>> 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.
> as you may know, place_entity() is not for calculating an absolute
> vruntime though.. anyway the important thing here is that, when a
> sleeping task is moved back to fair class, enqueue_entity() for
> DEQUEUE_SLEEP task won't be called.

The enqueue_entity() for DEQUEUE_SLEEP task will be called when the task 
is wake up.

Regards,
Wanpeng Li
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