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Date:   Fri, 15 May 2020 12:43:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vineeth Remanan Pillai <vpillai@digitalocean.com>
Cc:     Aaron Lu <aaron.lwe@gmail.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
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        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Aaron Lu <aaron.lu@linux.alibaba.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
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Subject: Re: [PATCH updated v2] sched/fair: core wide cfs task priority
 comparison
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On Fri, May 15, 2020 at 12:38:44PM +0200, Peter Zijlstra wrote:
>   less := !((s64)(s_a - s_b) <= 0)
> 
>   (v_a - S_a) - (v_b - S_b) == v_a - v_b - S_a + S_b
>                             == v_a - (v_b - S_a + S_b)
> 

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -469,7 +469,7 @@ bool cfs_prio_less(struct task_struct *a
>  {
>  	struct sched_entity *se_a = &a->se, *se_b = &b->se;
>  	struct cfs_rq *cfs_rq_a, *cfa_rq_b;
> -	u64 vruntime_a, vruntime_b;
> +	u64 s_a, s_b, S_a, S_b;
>  
>  	while (!is_same_tg(se_a, se_b)) {
>  		int se_a_depth = se_a->depth;
> @@ -484,10 +484,16 @@ bool cfs_prio_less(struct task_struct *a
>  	cfs_rq_a = cfs_rq_of(se_a);
>  	cfs_rq_b = cfs_rq_of(se_b);
>  
> -	vruntime_a = se_a->vruntime - cfs_rq_a->core_vruntime;
> -	vruntime_b = se_b->vruntime - cfs_rq_b->core_vruntime;
> +	S_a = cfs_rq_a->core_vruntime;
> +	S_b = cfs_rq_b->core_vruntime;
>  
> -	return !((s64)(vruntime_a - vruntime_b) <= 0);
> +	if (cfs_rq_a == cfs_rq_b)
> +		S_a = S_b = cfs_rq_a->min_vruntime;
> +
> +	s_a = se_a->vruntime - S_a;
> +	s_b = se_b->vruntime - S_b;
> +
> +	return !((s64)(s_a - s_b) <= 0);
>  }

Clearly I'm not awake yet; 's/s_/l_/g', 's/v_/s_/g', IOW:

  l_a = s_a - S_a