linux-kernel - Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to

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Date:	Tue, 8 Sep 2015 14:28:17 +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 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
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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