linux-kernel - Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding actual_cpu

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Message-ID: <0763f870-e30c-46cf-aefa-b879f2ebdba4@arm.com>
Date: Fri, 7 Jun 2024 09:19:33 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org,
 mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com
Cc: rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 bristot@...hat.com, vschneid@...hat.com, vincent.donnefort@....com,
 qyousef@...alina.io, ke.wang@...soc.com, xuewen.yan94@...il.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding
 actual_cpu_capacity

On 06/06/2024 09:06, Xuewen Yan wrote:
> Because the effective_cpu_util() would return a util which
> maybe bigger than the actual_cpu_capacity, this could cause
> the pd_busy_time calculation errors.

Doesn't return effective_cpu_util() either scale or min(scale, util)
with scale = arch_scale_cpu_capacity(cpu)? So the util sum over the PD
cannot exceed eenv->cpu_cap?
Looks like this was the case with 3e8c6c9aac42 already.

> So clamp the cpu_busy_time with the eenv->cpu_cap, which is
> the actual_cpu_capacity.
> 
> Fixes: 3e8c6c9aac42 ("sched/fair: Remove task_util from effective utilization in feec()")
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/fair.c | 4 +++- 
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a5b1ae0aa55..8939d725023a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7870,7 +7870,9 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
>  	for_each_cpu(cpu, pd_cpus) {
>  		unsigned long util = cpu_util(cpu, p, -1, 0);
>  
> -		busy_time += effective_cpu_util(cpu, util, NULL, NULL);
> +		util = effective_cpu_util(cpu, util, NULL, NULL);
> +		util = min(eenv->cpu_cap, util);
> +		busy_time += util;
>  	}
>  
>  	eenv->pd_busy_time = min(eenv->pd_cap, busy_time);