Message-ID: <BLU436-SMTP1223932233483C845BE508E80530@phx.gbl>
Date:	Tue, 8 Sep 2015 16:04:49 +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 3:57 PM, Byungchul Park wrote:
> On Tue, Sep 08, 2015 at 03:30:08PM +0800, Wanpeng Li wrote:
>> 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())
> my patch is based on ff277d4 commit at tip/sched/core.
>
> there's no change between before and after.
>
> check it please.
>
> and this logic seems to be no problem to me. :(

Your logic will lose fair sleeper bonus in the scenario which I pointed out.

>
>> after your patch:
>>
>> se->vruntime += cfs->min_vruntime  (switched_to_fair())
>> se->vruntime = se->vruntime  (place_entity())
>>
>>
>> Regards,
>> Wanpeng Li
>>
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