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

[Byungchul Park]

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.

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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