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

[Byungchul Park]

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? 

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.

> 
> thanks,
> byungchul
> 
> > 
> > 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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