linux-kernel - Re: [External] Re: [PATCH v2 09/10] sched/fair: stop load tracking when task switched_from

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Message-ID: <7eae9f41-7b33-2fc1-6b31-bbfd8d7b1364@arm.com>
Date:   Fri, 15 Jul 2022 13:15:25 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Chengming Zhou <zhouchengming@...edance.com>, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, bsegall@...gle.com, vschneid@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2 09/10] sched/fair: stop load tracking
 when task switched_from_fair()

On 14/07/2022 15:43, Chengming Zhou wrote:
> On 2022/7/14 20:33, Dietmar Eggemann wrote:
>> On 13/07/2022 06:04, Chengming Zhou wrote:

[...]

>> IMHO, a queued !fair task when switching back to fair will already be
>> enqueued (attached) as a fair task in __sched_setscheduler() prior to
>> the check_class_changed() call.
> 
> Right, this is true for a queued !fair task, it will enqueued (attached) before
> check_class_changed().
> 
> enqueue_task_fair()
>   enqueue_entity()
>     update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH)
>       if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD))  --> true
>         __update_load_avg_se(now, cfs_rq, se)  --> (1)
> check_class_changed()
>   switched_to_fair()
>     attach_task_cfs_rq()
>       attach_entity_cfs_rq()
>         update_load_avg(cfs_rq, se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD)
>           if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD))  --> true
>             __update_load_avg_se(now, cfs_rq, se)  --> (2)
> 
> 
> 1. for queued !fair: (1) delta = (now - last_update_time), last_update_time is the time
>    when switched_from_fair().
> 
> 2. for !queued !fair: (2) delta = (now - last_update_time), last_update_time is the time
>    when switched_from_fair().
> 
> The scenario in the commit message only cover !queued !fair case, I forget the queued !fair
> case, their problem is the same.

OK, that makes sense to me then.

>> I can't see how this will work with your proposed change in using
>> last_update_time=0 for fair->!fair->fair class changes?
> If we reset last_update_time=0 for !fair task, then:
> 
> 1. for queued !fair: will not do (1) since the if condition is false.
> 
> 2. for !queued !fair: will not do (2) since the if condition is false.
OK.

[...]

>>> This patch reset it's sched_avg last_update_time to 0, stop load
>>> tracking for !fair task, later in switched_to_fair() ->
>>> update_load_avg(), we can use its saved sched_avg.
>>>
>>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>>> ---
>>>  kernel/sched/fair.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 50f65a2ede32..576028f5a09e 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -11552,6 +11552,11 @@ static void attach_task_cfs_rq(struct task_struct *p)
>>>  static void switched_from_fair(struct rq *rq, struct task_struct *p)
>>>  {
>>>  	detach_task_cfs_rq(p);
>>> +
>>> +#ifdef CONFIG_SMP
>>> +	/* Stop load tracking for !fair task */

You're not really stopping p->se load tracking by doing this. We don't
do this outside fair anyway. IMHO, you freeze p->se's PELT _avg/_sum
values to be used as initial values when re-entering fair.

>>> +	p->se.avg.last_update_time = 0;
>>> +#endif
>>>  }
>>>  
>>>  static void switched_to_fair(struct rq *rq, struct task_struct *p)