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

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Message-ID: <8a54fbcc-16f5-8b0a-7254-2b14810827b1@bytedance.com>
Date:   Sat, 16 Jul 2022 00:35:30 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Dietmar Eggemann <dietmar.eggemann@....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 2022/7/15 19:15, Dietmar Eggemann wrote:
> 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.
> 

Yes, you are right, this comment is misleading and wrong, will delete it.

Thanks very much for your review!


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