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

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Message-ID: <YN70sbUGh2pViWEQ@hirez.programming.kicks-ass.net>
Date:   Fri, 2 Jul 2021 13:12:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xuewen Yan <xuewen.yan94@...il.com>
Cc:     valentin.schneider@....com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, 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 Wed, Jun 30, 2021 at 10:12:04PM +0800, 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>

Valentin, Qais, can either of you write a Changelog/comment for this, I
can't seem to make any sense of it.

Is this about wake-from-idle, where the first task's uclamp goes amis
because the rq->uclamp values haven't been updated yet?


> ---
>  kernel/sched/sched.h | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c80d42e9589b..14a41a243f7b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2818,20 +2818,27 @@ static __always_inline
>  unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
>  				  struct task_struct *p)
>  {
> -	unsigned long min_util;
> -	unsigned long max_util;
> +	unsigned long min_util = 0;
> +	unsigned long max_util = 0;
>  
>  	if (!static_branch_likely(&sched_uclamp_used))
>  		return util;
>  
> -	min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> -	max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> -
>  	if (p) {
> -		min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> -		max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> +		min_util = uclamp_eff_value(p, UCLAMP_MIN);
> +		max_util = uclamp_eff_value(p, UCLAMP_MAX);
> +
> +		/*
> +		 * Ignore last runnable task's max clamp, as this task will
> +		 * reset it. Similarly, no need to read the rq's min clamp.
> +		 */
> +		if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> +			goto out;
>  	}
>  
> +	min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value));
> +	max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value));
> +out:
>  	/*
>  	 * Since CPU's {min,max}_util clamps are MAX aggregated considering
>  	 * RUNNABLE tasks with _different_ clamps, we can end up with an
> -- 
> 2.25.1
>