linux-kernel - Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle

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Date:   Wed, 30 Jun 2021 15:24:42 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com
Cc:     rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        patrick.bellasi@...bug.net, qais.yousef@....com, qperret@...gle.com
Subject: Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle


On the subject: s/ucalmp/uclamp/

On 30/06/21 22:12, Xuewen Yan 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 runnable task's UCLAMP_MAX;
> (2)the p's uc_eff_max < uc_rq_max.
>
> As a result, 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.
>
> When rq has UCLAMP_FLAG_IDLE flag, enqueuing the task will lift UCLAMP_FLAG_IDLE
> and set the rq clamp as the task's via uclamp_idle_reset(). It doesn't need
> to read the rq clamp. And it can also avoid the problems described above.
>
> Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()")
>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
>

Thanks!

Reviewed-by: Valentin Schneider <valentin.schneider@....com>