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

[Wanpeng Li]

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 absolute vruntime will 
be calculated in enqueue_entity() either the task is running or sleeping 
when moved back. The fair sleeper bonus 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.

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/