linux-kernel - Re: [PATCH v4 1/6] sched/fair: Fix reschedule which is generated on throttled cfs

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Date:	Fri, 24 Oct 2014 10:01:54 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, pjt@...gle.com, oleg@...hat.com,
	rostedt@...dmis.org, umgwanakikbuti@...il.com,
	tim.c.chen@...ux.intel.com, mingo@...nel.org,
	nicolas.pitre@...aro.org
Subject: Re: [PATCH v4 1/6] sched/fair: Fix reschedule which is generated on
 throttled cfs_rq

Hi, Wanpeng,

the commit commentary confuses, I'm agree. Really it's just a cleanup.

On Пт, 2014-10-24 at 07:27 +0800, Wanpeng Li wrote:
> Hi Kirill,
> 8/6/14, 4:06 PM, Kirill Tkhai:
> > (sched_entity::on_rq == 1) does not guarantee the task is pickable;
> > changes on throttled cfs_rq must not lead to reschedule.
> 
> Why (sched_entity::on_rq == 1) doesn't guarantee the task is pickable 
> since entity will be dequeued during throttling cfs_rq?

Because one of task's (grand)parents in hierarhy may be throtthed and
dequeued.

But task_struct::on_rq check doesn't guarantee this too. So, just ignore
commit commentary; the commentary is wrong.

> > Check for task_struct::on_rq instead.
> 
> Do you mean task_struct::on_rq will be cleared during throttling cfs_rq? 
> I can't find codes do this.

No, it not cleared. The commit commentary should be:
"sched: Cleanup. Check task_struct::on_rq instead of sched_entity::on_rq,
because it is the same for a task"


> Regards,
> Wanpeng Li
> 
> >
> > Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
> > ---
> >   kernel/sched/fair.c |    6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bfa3c86..6f0ce2b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7465,7 +7465,7 @@ static void task_fork_fair(struct task_struct *p)
> >   static void
> >   prio_changed_fair(struct rq *rq, struct task_struct *p, int oldprio)
> >   {
> > -	if (!p->se.on_rq)
> > +	if (!p->on_rq)
> >   		return;
> >   
> >   	/*
> > @@ -7521,15 +7521,15 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
> >    */
> >   static void switched_to_fair(struct rq *rq, struct task_struct *p)
> >   {
> > -	struct sched_entity *se = &p->se;
> >   #ifdef CONFIG_FAIR_GROUP_SCHED
> > +	struct sched_entity *se = &p->se;
> >   	/*
> >   	 * Since the real-depth could have been changed (only FAIR
> >   	 * class maintain depth value), reset depth properly.
> >   	 */
> >   	se->depth = se->parent ? se->parent->depth + 1 : 0;
> >   #endif
> > -	if (!se->on_rq)
> > +	if (!p->on_rq)
> >   		return;
> >   
> >   	/*
> >
> >
> >
> > --
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> 


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