linux-kernel - Re: [PATCH v2] sched/pelt: Keep UTIL_AVG_UNCHANGED flag in sync when calculating last_enqueued

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Message-ID: <20210507123528.GA106818@e120877-lin.cambridge.arm.com>
Date:   Fri, 7 May 2021 13:35:28 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>
Cc:     mingo@...hat.com, peterz@...radead.org, 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,
        zhang.lyra@...il.com, xuewyan@...mail.com
Subject: Re: [PATCH v2] sched/pelt: Keep UTIL_AVG_UNCHANGED flag in sync when
 calculating last_enqueued_diff

On Fri, May 07, 2021 at 07:20:31PM +0800, Xuewen Yan wrote:
> From: Xuewen Yan <xuewen.yan@...soc.com>
> 
> Last_enqueued_diff's meaning: "diff = util_est.enqueued(p) - task_util(p)".
> When calculating last_enqueued_diff, we add UTIL_AVG_UNCHANGED flag, which
> is the LSB, for task_util, but don't add the flag for util_est.enqueued.
> However the enqueued's flag had been cleared when the task util changed.
> As a result, we add +1 to the diff, this is therefore reducing slightly
> UTIL_EST_MARGIN.

Unless I miss something it actually depends on the situation, doesn't it?

if ue.enqueued > task_util(), we remove 1 from the diff => UTIL_EST_MARGIN + 1

if ue.enqueued < task_util(), we add 1 to the diff => UTIL_EST_MARGIN -1

> 
> Add the flag for util_est.enqueued to have a accurate computation.
> 
> Fixes: b89997aa88f0b sched/pelt: Fix task util_est update filtering
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e5e457fa9dc8..94d77b4fa601 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3996,7 +3996,7 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>  	if (ue.enqueued & UTIL_AVG_UNCHANGED)
>  		return;
>  
> -	last_enqueued_diff = ue.enqueued;
> +	last_enqueued_diff = (ue.enqueued | UTIL_AVG_UNCHANGED);
>  
>  	/*
>  	 * Reset EWMA on utilization increases, the moving average is used only
> -- 
> 2.29.0
>