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

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Message-ID: <BLU436-SMTP167667091149D5A84C4D27880530@phx.gbl>
Date:	Tue, 8 Sep 2015 15:30:08 +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 3:11 PM, Byungchul Park wrote:
> On Tue, Sep 08, 2015 at 02:42:52PM +0800, Wanpeng Li wrote:
>> On 9/8/15 2:32 PM, Byungchul Park wrote:
>>> On Tue, Sep 08, 2015 at 03:14:26PM +0900, 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.
>>>>
>>>>>> 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.
>>> you may talk about calling enqueue_entity() when the task is woken up,
>>> not just when it is moved back. right?
>> Exactly.
>>
>>> even if yes, i think place_entity() should not be used directly for
>>> calculating an absolute vruntime. it should be called after non/normalizing
>>> operations.
>> The se->vruntime += cfs_rq->min_vruntime(in your switched_to_fair())
>> which means that se->vruntime is bigger than cfs_rq->min_vruntime,
> it is not always true since se->vruntime can have a negative value (even
> though it is a unsigned type.. i think it can be another problem) by
> se->vruntime -= cfs_rq->min_vruntime in detach_task_cfs_rq().

Yeah, it can be negative.

>
>> however, fair sleeper bonus is min_vuntime - sysctl_sched_latency/2,
>> which means that max_vruntime() will select the absolute vruntime
>> which is caculated in your switched_to_fair() as the se->vruntime,
> since se->vruntime can have a negative value, max_vruntime() may select
> the fair sleeper bonused value.
>
> by the way, this logic is unchanged by my patch. which part of my patch
> changed this kind of logic?

However, if se->vruntime -= cfs_rq->min_vruntime is positive, the 
behavior is different after your patch. e.g. se->vruntime(the relative 
vruntime in switched_to_fair()) < min_vruntime - sysctl_sched_latency/2

before your patch:

se->vruntime = min_vruntime - sysctl_sched_latency/2 (place_entity())

after your patch:

se->vruntime += cfs->min_vruntime  (switched_to_fair())
se->vruntime = se->vruntime  (place_entity())


Regards,
Wanpeng Li

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