linux-kernel - Re: [PATCH v2] sched/fair: Reschedule the cfs

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Message-ID: <4c285c36-6893-c0c4-1061-0d7e16445e9d@amd.com>
Date: Fri, 14 Jun 2024 08:46:54 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Chen Yu <yu.c.chen@...el.com>, Chunxin Zang <spring.cxz@...il.com>
CC: Honglei Wang <jameshongleiwang@....com>, <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, Mike Galbraith <efault@....de>,
	<linux-kernel@...r.kernel.org>, <yangchen11@...iang.com>, Jerry Zhou
	<zhouchunhua@...iang.com>, Chunxin Zang <zangchunxin@...iang.com>
Subject: Re: [PATCH v2] sched/fair: Reschedule the cfs_rq when current is
 ineligible

Hello Chenyu,

On 6/14/2024 8:41 AM, K Prateek Nayak wrote:
> Hello Chenyu,
> 
> Sorry, I'm a bit late to the thread but ...
> 
> On 6/13/2024 6:53 PM, Chen Yu wrote:
>> [..snip..]
>>>>
>>>> I wonder if we can only take care of the NO_RUN_TO_PARITY case? 
>>>> Something like this,
>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 03be0d1330a6..5e49a15bbdd3 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -745,6 +745,21 @@ int entity_eligible(struct cfs_rq *cfs_rq, 
>>>>> struct sched_entity *se)
>>>>>         return vruntime_eligible(cfs_rq, se->vruntime);
>>>>> }
>>>>>
>>>>> +static bool check_entity_need_preempt(struct cfs_rq *cfs_rq, 
>>>>> struct sched_entity *se)
>>>>> +{
>>>> if (sched_feat(RUN_TO_PARITY) || cfs_rq->nr_running <= 1 ||
>>>>      !entity_eligible(cfs_rq, se))
>>>> return false;
>>>>
>>>> return true;
>>>>
>>>> Thoughts?
>>>>
>>>
>>> This does indeed look better. In that case, do I need to make the 
>>> changes this way and send
>>> out a version 3?
>>
>> If you mean the following changes, maybe we can continue the 
>> discussion here.
>> This is just my 2 cents, not sure what others think of it. Anyway, I 
>> can launch some tests.
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 8a5b1ae0aa55..c0fdb25f0695 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -744,6 +744,15 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct 
>> sched_entity *se)
>>       return vruntime_eligible(cfs_rq, se->vruntime);
>>   }
>> +static bool check_curr_preempt(struct cfs_rq *cfs_rq, struct 
>> sched_entity *curr)
>> +{
>> +    if (sched_feat(RUN_TO_PARITY) || cfs_rq->nr_running <= 1 ||
>> +        !entity_eligible(cfs_rq, curr))
> 
> Shouldn't this return false if "entity_eligible(cfs_rq, curr)" returns 
> true since curr is still vruntime eligible on cfs_rq?
> 
> Would it better to have check_curr_preempt() as follows:
> 
>      if (sched_feat(RUN_TO_PARITY) || cfs_rq->nr_running <= 1)
>          return false;
> 
>      return entity_eligible(cfs_rq, curr);

The above return should have been:

	return !entity_eligible(cfs_rq, curr);

Which returns true once curr is not vruntime eligible on cfs_rq.
Sorry for the oversight.

> 
> which returns false if curr is ineligible and scheduler can go ahead and 
> and call schedule to evaluate the next best step?
> 
> Please let me know if I'm missing something.
> 
>> +        return false;
>> +
>> +    return true;
>> +}
>> +
>>   static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
>>   {
>>       u64 min_vruntime = cfs_rq->min_vruntime;
>> [..snip..]
> 

-- 
Thanks and Regards,
Prateek