linux-kernel - Re: [PATCH] sched/uclamp: Fix getting unreasonable ucalmp

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Date:   Tue, 29 Jun 2021 11:11:17 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched/uclamp: Fix getting unreasonable ucalmp_max when rq
 is idle

+cc Qais, Quentin

Hi Dietmar,

The patch has been around for a long time, and I think it makes sense for EAS.
Do you have any suggestions or comments?

Thanks!
xuewen

On Fri, Jun 18, 2021 at 3:24 PM Xuewen Yan <xuewen.yan94@...il.com> wrote:
>
> From: Xuewen Yan <xuewen.yan@...soc.com>
>
> Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following:
> uc_rq_max = rq->uclamp[UCLAMP_MAX].value;
> uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX);
> uclamp_max = max{uc_rq_max, uc_eff_max};
>
> Consider the following scenario:
> (1)the rq is idle, the uc_rq_max is last task's UCLAMP_MAX;
> (2)the p's uc_eff_max < uc_rq_max.
>
> The result is the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable.
>
> The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX.
>
> Inserts whether the rq is idle in the uclamp_rq_util_with().
>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/sched.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a189bec13729..0feef6af89f2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2550,7 +2550,10 @@ unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
>
>         if (p) {
>                 min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> -               max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> +               if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> +                       max_util = uclamp_eff_value(p, UCLAMP_MAX);
> +               else
> +                       max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
>         }
>
>         /*
> --
> 2.25.1
>