Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair()

[Byungchul Park]

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. :(

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