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

[Wanpeng Li]

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

> 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.

Regards,
Wanpeng Li

> > should be gained in place_entity() if the task is still sleeping.
> > However, after recent commit ( 23ec30ddd7c1306: 'sched: add two
> > functions for att(det)aching a task to(from) a cfs_rq'), the
> > absolute vruntime will be calculated in switched_to_fair(), so the
> > max_vruntime() which is called in place_entity() will select the
> > absolute vruntime which is calculated in switched_to_fair() as the
> > se->vruntime and lose the fair sleeper bonus.
> please refer my another reply, and let me know if i missed something.
>
> thanks,
> byungchul
>
> > Regards,
> > Wanpeng Li
> >
> > > > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > > > ---
> > > >   kernel/sched/fair.c |   11 +++++++----
> > > >   1 files changed, 7 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index d26d3b7..eb9aa35 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -8005,9 +8005,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
> > > >   	/* Synchronize task with its cfs_rq */
> > > >   	attach_entity_load_avg(cfs_rq, se);
> > > > -
> > > > -	if (!vruntime_normalized(p))
> > > > -		se->vruntime += cfs_rq->min_vruntime;
> > > >   }
> > > >   static void switched_from_fair(struct rq *rq, struct task_struct *p)
> > > > @@ -8066,14 +8063,20 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
> > > >   #ifdef CONFIG_FAIR_GROUP_SCHED
> > > >   static void task_move_group_fair(struct task_struct *p)
> > > >   {
> > > > +	struct sched_entity *se = &p->se;
> > > > +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> > > > +
> > > >   	detach_task_cfs_rq(p);
> > > >   	set_task_rq(p, task_cpu(p));
> > > >   #ifdef CONFIG_SMP
> > > >   	/* Tell se's cfs_rq has been changed -- migrated */
> > > > -	p->se.avg.last_update_time = 0;
> > > > +	se->avg.last_update_time = 0;
> > > >   #endif
> > > >   	attach_task_cfs_rq(p);
> > > > +
> > > > +	if (!vruntime_normalized(p))
> > > > +		se->vruntime += cfs_rq->min_vruntime;
> > > >   }
> > > >   void free_fair_sched_group(struct task_group *tg)
> > > > --
> > > > 1.7.1
> > --
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