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

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Message-ID: <aFAkKGs5h6kfXYnO@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 16 Jun 2025 16:03:20 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Kuyo Chang <kuyo.chang@...iatek.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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

Hello,

On 14/06/25 10:04, 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.
> 
> So it will scaled to 238,418 ns.
> 
> [Solution]
> The runtime for fair_server should be absolute time
> asis RT bandwidth control.
> Fix the runtime calculation formula for the fair_server.
> 
> Signed-off-by: kuyo chang <kuyo.chang@...iatek.com>
> ---

Right, I would agree we don't actually want to scale fair_server runtime
by frequency/capacity. Your change looks good to me.

Acked-by: Juri Lelli <juri.lelli@...hat.com>

Thanks!
Juri