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

[Wanpeng Li <wanpeng.li@hotmail.com>]

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@hotmail.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
>>