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

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Message-ID: <9d83771f-3d12-25f0-c3f8-70dd7942e9ce@amd.com>
Date: Fri, 14 Jun 2024 08:41:58 +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,

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);

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