linux-kernel - Re: [PATCH RFC 4/4] sched: cpufreq: Apply DVFS headroom to CFS only

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Message-ID: <7fdfff24-80ed-acbf-810f-b641570141fd@arm.com>
Date:   Mon, 21 Aug 2023 18:41:00 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH RFC 4/4] sched: cpufreq: Apply DVFS headroom to CFS only

On 20/08/2023 23:06, Qais Yousef wrote:
> RT and Deadline have exact performance requirement when running. RT runs
> at max or a specific OPP defined by uclamp_min. Deadline's OPP is
> defined by its bandwidth. Both of which are known ahead of time and
> don't require a headroom to grow into.
> 
> IRQs on the other hand have no specific performance requirement and
> cruises along at whatever the current OPP happens to be when they occur.
> 
> Now they all have PELT pressure signals that does impact frequency
> selection and task placement. The question is do they need DVFS
> headroom?
> 
> I think the answer is no because when CFS is not running at all, these
> pressure signal has no real impact on performance for RT, DL or IRQ.
> 
> If CFS util is not zero, we already add their pressure as an
> *additional* headroom to account for the lost/stolen time. So I argue
> that the pressure are headroom themselves and shouldn't need an
> additional DVFS headroom applied on top.
> 
> In summary final outcome should be:
> 
> 	CFS + DVFS headroom + (RT, DT, IRQ) pressure headroom

I assume here you want to align the difference that EAS deals with
`util_cfs` vs `capacity` whereas power deals with `util` vs
`capacity_orig`? You want that power should only apply the 1.25 to util_cfs?

> Signed-off-by: Qais Yousef (Google) <qyousef@...alina.io>
> ---
>  kernel/sched/core.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 441d433c83cd..602e369753a3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7438,10 +7438,11 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
>  	 * When there are no CFS RUNNABLE tasks, clamps are released and
>  	 * frequency will be gracefully reduced with the utilization decay.
>  	 */
> -	util = util_cfs + cpu_util_rt(rq);
>  	if (type == FREQUENCY_UTIL) {
> -		util = apply_dvfs_headroom(util);
> +		util = apply_dvfs_headroom(util_cfs) + cpu_util_rt(rq);
>  		util = uclamp_rq_util_with(rq, util, p);
> +	} else {
> +		util = util_cfs + cpu_util_rt(rq);
>  	}
>  
>  	dl_util = cpu_util_dl(rq);
> @@ -7473,12 +7474,9 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
>  	 *              max - irq
>  	 *   U' = irq + --------- * U
>  	 *                 max
> -	 *
> -	 * We only need to apply dvfs headroom to irq part since the util part
> -	 * already had it applied.
>  	 */
>  	util = scale_irq_capacity(util, irq, max);
> -	util += type ==  FREQUENCY_UTIL ? apply_dvfs_headroom(irq) : irq;
> +	util += irq;
>  
>  	/*
>  	 * Bandwidth required by DEADLINE must always be granted while, for
> @@ -7491,7 +7489,7 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
>  	 * an interface. So, we only do the latter for now.
>  	 */
>  	if (type == FREQUENCY_UTIL)
> -		util += apply_dvfs_headroom(cpu_bw_dl(rq));
> +		util += cpu_bw_dl(rq);
>  
>  	return min(max, util);
>  }