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Date:	Tue, 8 Sep 2015 15:43:17 +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 02:28:17PM +0900, 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
> 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

i mean enqueue_entity(.flags=0), just when moved back.

> 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.
> 
> > 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@...mail.com>
> > >>---
> > >>  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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