Message-ID: <20150908064553.GH6455@byungchulpark-X58A-UD3R>
Date:	Tue, 8 Sep 2015 15:45:53 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Wanpeng Li <wanpeng.li@...mail.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 Tue, Sep 08, 2015 at 02:23:56PM +0800, Wanpeng Li wrote:
> 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.

now, i see what it means "enqueue_entity() for DEQUEUE_SLEEP task".

> 
> Regards,
> Wanpeng Li
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