linux-kernel - Re: [PATCH 1/1] sched/deadline: Fix fair

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Message-ID: <20250617085558.GN1613376@noisy.programming.kicks-ass.net>
Date: Tue, 17 Jun 2025 10:55:58 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kuyo Chang <kuyo.chang@...iatek.com>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	jstultz <jstultz@...gle.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 1/1] sched/deadline: Fix fair_server runtime calculation
 formula

On Sat, Jun 14, 2025 at 10:04:55AM +0800, Kuyo Chang wrote:
> From: kuyo chang <kuyo.chang@...iatek.com>
> 
> [Symptom]
> The calculation formula for fair_server runtime is based on
> Frequency/CPU scale-invariance.
> This will cause excessive RT latency (expect absolute time).
> 
> [Analysis]
> Consider the following case under a Big.LITTLE architecture:
> 
> Assume the runtime is : 50,000,000 ns, and FIE/CIE as below
> FIE: 100
> CIE:50
> First by FIE, the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812
> Then by CIE, it is further scaled to 4,882,812 * 50 >> 10 = 238,418.

What's this FIE/CIE stuff? Is that some ARM lingo?


> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index ad45a8fea245..8bfa846cf0dc 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1504,7 +1504,10 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
>  	if (dl_entity_is_special(dl_se))
>  		return;
>  
> -	scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
> +	if (dl_se == &rq->fair_server)
> +		scaled_delta_exec = delta_exec;
> +	else
> +		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);

Juri, the point it a bit moot atm, but is this something specific to the
fair_server in particular, or all servers?

Because if this is something all servers require then the above is
ofcourse wrong.